| Flowers |
Above and below: Passiflora, the passion flower. For many years I would say that the passion flower
was my favourite flower, but to be honest I don't think I have a favourite (it was the tulip once, then
the lupin, then the snapdragon, then the bluebell ...). Indeed, one of the great fascinations about
Nature is the diversity of forms that it generates - variety is the spice of life, as they say!
Flowers are the sexual reproductive organs of the flowering plants (angiosperms) and consist of
sterile and fertile reproductive organs borne on the swollen end of a shoot axis (receptacle). The
flower is a modified leafy (vegetative) shoot. The leaf-bearing nodes of the shoot now bear structures
derived from leaves and the internodes between these nodes are greatly shortened to bring several
whorls of these modified leaves together. Like all shoots, flowering shoots grow from buds in the axils
of leaves (axillary buds) on the parent shoot. These buds typically produce green leafy shoots, but
when the time for flowering arrives some of these buds switch to a different developmental program
and put out an inflorescence (flowering shoot) bearing one or more flowers. The leaf
accompanying this bud may become modified and is known as a bract. The bract now sits at the base
of the inflorescence. The inflorescence puts out one or more flowers, each borne on its own stalk or
pedicel and each deriving from a terminal flower bud. Once a bud switches to become a flower the
shoot ends there - no more branches will be produced from this shoot tip and it becomes destined to
become a flower and the meristem, or growing tip, ceases activity once all the flower parts have
been produced.
The various organs of the flower are arranged in whorls that encircle the stem, rather than in helixes
or spirals as are most leaves. The lowest whorl of leaf-like structures to make up the flower is called
the calyx and is made up of leaf-like sepals. Sepals are often green and photosynthetic, like
ordinary leaves, but differing in shape and with a more compact internal structure. In this case the
calyx is referred to as sepaloid. In others they turn into hard scales to protect the developing bud
and they may be shed once the flower opens. Sepals may remain attached to the flower and fall off
when the flower withers or they may remain and form part of the fruit, possibly contributing to the
flesh of the fruit. In the passion flower above, the sepals are five in number and look much like the
petals, but are greener in colour and end in hook-like awns. A calyx comprising sepals that resemble
petals is called petaloid.
was my favourite flower, but to be honest I don't think I have a favourite (it was the tulip once, then
the lupin, then the snapdragon, then the bluebell ...). Indeed, one of the great fascinations about
Nature is the diversity of forms that it generates - variety is the spice of life, as they say!
Flowers are the sexual reproductive organs of the flowering plants (angiosperms) and consist of
sterile and fertile reproductive organs borne on the swollen end of a shoot axis (receptacle). The
flower is a modified leafy (vegetative) shoot. The leaf-bearing nodes of the shoot now bear structures
derived from leaves and the internodes between these nodes are greatly shortened to bring several
whorls of these modified leaves together. Like all shoots, flowering shoots grow from buds in the axils
of leaves (axillary buds) on the parent shoot. These buds typically produce green leafy shoots, but
when the time for flowering arrives some of these buds switch to a different developmental program
and put out an inflorescence (flowering shoot) bearing one or more flowers. The leaf
accompanying this bud may become modified and is known as a bract. The bract now sits at the base
of the inflorescence. The inflorescence puts out one or more flowers, each borne on its own stalk or
pedicel and each deriving from a terminal flower bud. Once a bud switches to become a flower the
shoot ends there - no more branches will be produced from this shoot tip and it becomes destined to
become a flower and the meristem, or growing tip, ceases activity once all the flower parts have
been produced.
The various organs of the flower are arranged in whorls that encircle the stem, rather than in helixes
or spirals as are most leaves. The lowest whorl of leaf-like structures to make up the flower is called
the calyx and is made up of leaf-like sepals. Sepals are often green and photosynthetic, like
ordinary leaves, but differing in shape and with a more compact internal structure. In this case the
calyx is referred to as sepaloid. In others they turn into hard scales to protect the developing bud
and they may be shed once the flower opens. Sepals may remain attached to the flower and fall off
when the flower withers or they may remain and form part of the fruit, possibly contributing to the
flesh of the fruit. In the passion flower above, the sepals are five in number and look much like the
petals, but are greener in colour and end in hook-like awns. A calyx comprising sepals that resemble
petals is called petaloid.
The next whorl of 'leaves' along the shoot develop into petals that together constitute the corolla-
they are white in our passion flower. The internal structure of petals is much like that of green
leaves, except that they are modified to perform less (if any) photosynthesis and instead their
plastids may store pigments to give the petals colour. In green leaves these plastids develop into
chloroplasts which contain green chlorophyll, giving the leaf its green colour. Chlorophyll absorbs
light and harvests energy from the light to make sugars from carbon dioxide gas from the air and
water. From these sugars the plant will make all the building materials it needs to make its body -
fats, proteins, nucleic acids, long-chain carbohydrates, etc. The roots supply both the necessary
water (in most plants) and mineral ions from the soil. Chloroplasts also contain yellow-orange-red
pigments called carotenoids. Usually these pigments are masked by the green chlorophyll (except
in plants with reddish leaves like copper beech) but become visible in autumn when chlorophyll has
been broken down and resorbed by the plant, leaving behind the carotenoids to give the leaves
their splendid autumn colours. Pigments may also be stored inside the plant cell vacuole, principally
flavonoids like anthocyanins which are red, purple or blue depending upon the pH (acidity) of the
cell sap within the vacuole. Anthocyanins also add to the autumn colours of leaves. Many petals
also have a special microstructure to their surface tissue which comprises an epidermis (that is a
sheet of covering cells) of conical cells bearing ridges. These ridges help to focus the light and
enhance the petal's colour. The petals of some flowers remain green and photosynthetic. These
flowers tend to be wind pollinated and are usually small. Plants with large, showy, colourful flowers
are advertising for animals to come and pollinate them. Insects, birds, bats and other animals may
act as pollinators. Some flowers provide only excess pollen as food, with which to reward their
animal pollinators, but others, like the passion flower, possess nectaries that secrete a sugary
nectar that is often fine-tuned to meet the dietary needs of the flower's favourite pollinator. The
epidermal cells of the petals may secrete fragrant oils to help lure their animal pollinators.
In the passion flower some of the petals are modified into coronal filaments, forming the
spectacularly colourful 'crown of thorns'. These colourful flowers are easy for the animal to spot at a
distance and the arrangement of petals and their colours guide the animal to the food reward of
pollen and/or nectar. It is in a sense by chance that many insect-pollinated flowers are colourful to
our eyes, since although some insects, like bees, have good colour vision they are also able to see
ultraviolet light, which gives them a very different perception of colour, and to them a flower looks
very different as they see colours differently. In ultraviolet many insect-pollinated flowers bear
additional markings, often stripes that guide the insect straight to the nectar, rather as markings
guide a taxiing plane on a runway. I say, 'in a sense' because most mammals can not see colour at
all. Primates are a rare exception, they have colour vision probably to help them locate fruit in the
canopy. Since fruit possess many similar pigments and colours as flowers, we can see both.
The calyx of sepals and the corolla of petals are the sterile parts of the flower and together they
form the perianth. Sometimes the sepals and petals are very alike and hard to distinguish, in this
case they are called tepals.
they are white in our passion flower. The internal structure of petals is much like that of green
leaves, except that they are modified to perform less (if any) photosynthesis and instead their
plastids may store pigments to give the petals colour. In green leaves these plastids develop into
chloroplasts which contain green chlorophyll, giving the leaf its green colour. Chlorophyll absorbs
light and harvests energy from the light to make sugars from carbon dioxide gas from the air and
water. From these sugars the plant will make all the building materials it needs to make its body -
fats, proteins, nucleic acids, long-chain carbohydrates, etc. The roots supply both the necessary
water (in most plants) and mineral ions from the soil. Chloroplasts also contain yellow-orange-red
pigments called carotenoids. Usually these pigments are masked by the green chlorophyll (except
in plants with reddish leaves like copper beech) but become visible in autumn when chlorophyll has
been broken down and resorbed by the plant, leaving behind the carotenoids to give the leaves
their splendid autumn colours. Pigments may also be stored inside the plant cell vacuole, principally
flavonoids like anthocyanins which are red, purple or blue depending upon the pH (acidity) of the
cell sap within the vacuole. Anthocyanins also add to the autumn colours of leaves. Many petals
also have a special microstructure to their surface tissue which comprises an epidermis (that is a
sheet of covering cells) of conical cells bearing ridges. These ridges help to focus the light and
enhance the petal's colour. The petals of some flowers remain green and photosynthetic. These
flowers tend to be wind pollinated and are usually small. Plants with large, showy, colourful flowers
are advertising for animals to come and pollinate them. Insects, birds, bats and other animals may
act as pollinators. Some flowers provide only excess pollen as food, with which to reward their
animal pollinators, but others, like the passion flower, possess nectaries that secrete a sugary
nectar that is often fine-tuned to meet the dietary needs of the flower's favourite pollinator. The
epidermal cells of the petals may secrete fragrant oils to help lure their animal pollinators.
In the passion flower some of the petals are modified into coronal filaments, forming the
spectacularly colourful 'crown of thorns'. These colourful flowers are easy for the animal to spot at a
distance and the arrangement of petals and their colours guide the animal to the food reward of
pollen and/or nectar. It is in a sense by chance that many insect-pollinated flowers are colourful to
our eyes, since although some insects, like bees, have good colour vision they are also able to see
ultraviolet light, which gives them a very different perception of colour, and to them a flower looks
very different as they see colours differently. In ultraviolet many insect-pollinated flowers bear
additional markings, often stripes that guide the insect straight to the nectar, rather as markings
guide a taxiing plane on a runway. I say, 'in a sense' because most mammals can not see colour at
all. Primates are a rare exception, they have colour vision probably to help them locate fruit in the
canopy. Since fruit possess many similar pigments and colours as flowers, we can see both.
The calyx of sepals and the corolla of petals are the sterile parts of the flower and together they
form the perianth. Sometimes the sepals and petals are very alike and hard to distinguish, in this
case they are called tepals.
Finally we come to the fertile organs of the flower - the male stamens that together form the
androecium and the female carpels that form the gynoecium. Many flowers have both male and
female organs and are called perfect flowers. In some species flowers may have only the male
parts 9staminate flowers) or the female parts (carpellate or pistillate flowers) and these are called
imperfect flowers. In species with imperfect flowers, male and female flowers may occur on the
same branch, different branches or on separate plants. In the latter case the plant has distinct male
and female individuals and is said to be dioecious. Many plants, however, are far more confused
and may contain male, female and individuals that are hermaphrodite to a variable degree.
The stamens (also called microsporophylls, which literally means leaves producing microspores)
were also derived from leaves over the course of evolution. Indeed, the stamens of some plants are
still leaf-like and bear sporangia (spore producing organs) underneath, rather like ferns and other
non-flowering plants that produce spores from sporangia underneath fertile fronds. The fossil
record tells us that flowering plants appeared quite late in evolution (and genetic analysis reveals
that their dramatic speciation is in large part due to their tendency to hybridise) and some flowering
plants still bear such 'primitive' or archaic characteristics. The stamens of most flowers, however,
comprise an anther head borne on a slender stalk or filament. The anther produces pollen
(microspores) in (usually four) pollen sacs (microsporangia) within it. The pollen contain the male
sperm and when ripe the anthers dry and rupture (dehisce, dehiscence) to release their spores.
The anthers may dehisce by splitting to form a longitudinal slit (as in cotton) or a transverse slit (as
in basil) or a single pore called the stomium or a number of pores (e.g. potato) or they split into a
number of valves as in bay leaf. The layer of cells in the anther wall may contain strips that twist
upon drying to bring about rupture.
The encasement of sperm in pollen is an adaptation to life on land as the tough casing of the
pollen wall resists dry conditions and allows the sperm to be transported to a female plant by animal
or wind, over many miles if need be. Moss, ferns and cycads and other archaic lineages are less
well adapted to life on land and still produce naked sperm (as their algal ancestors did) that require
a film of moisture in which to swim.
The female carpels may be separate (an apocarpous flower) but are often fused together into a
single structure (a syncarpous flower). Sometimes the word pistil is used to describe a single
carpel in an apocarpous flower or the single compound carpel of a syncarpous flower. The carpel is
a modified folded leaf rolled up with its margins more or less fused together. The final structure has
one or more internal cavities called locules, in which future embryos may develop. The style is the
the upper elongated part of the pistil and is absent in some flowers. The style ends in the stigma,
usually a bulbous structure. The remainder of the carpel is the ovary, or main body of the carpel.
The stigma is the receptive region to which pollen adheres prior to fertilisation. The pollen grain
then grows a pollen tube down through the style to reach the ovary and then into an ovule ('little
egg'), attached by a placenta to the ovary wall inside one of the locules and then it discharges the
sperm through a pore in the ovule, called the micropore or micropyle. The sperm enter the
embryo sac (which is derived from a megaspore) within the ovule, which contains the female egg
cell (ovum) and a few other (haploid) cells. Only pollen from the right species is accepted and
even then pollen from the same plant may be rejected, thereby favouring cross-pollination and
outbreeding which keeps the genetic stock strong by preventing inbreeding.
Flowers have various other tricks to ensure cross-pollination, or to prevent self-pollination. One of
these is protandry, in which the male organs develop before the female organs on the same
flower. In the passion flower, the styles are erect when the flower opens initially, holding the stigmas
well out of the way of the anthers and any insect visitors that may brush past the anther, collecting
pollen, and then brush the stigma of the same flower. As the anthers become exhausted of pollen,
the styles bend down, finally bringing the stigmas to the same level as the anthers. Now when an
insect visits the nectaries there is no pollen left to collect, but it can easily brush the stigmas and
deposit pollen from another plant.
The passion flower is what we call an actinomorphic flower, meaning that it has 'circular' (strictly
radial) symmetry. Some flowers, like the white deadnettle, Lamium alba, have zygomorphic
flowers which have only a single plane of symmetry and distinct left and right halves and a distinct
top and bottom. These are evolved from actinomorphic flowers by assymetrical fusion of the petals
into a floral tube (the corolla). This restricts access to the nectar at the base of the floral tube to
those pollinators that can reach it. This exclusivity evolves when a plant finds certain pollinators
extremely reliable and so it rewards them by saving its nectar by not giving it to less reliable
pollinators.
The white deadnettle (White Archangel) is also protandrous. In the male stage, the style has not
yet elongated far enough to position the stigma near the entrance, but the anthers are in position
and visiting animals will brush against them to collect pollen. In the female stage, the anthers are
spent and the style has elongated to position the stigma right at the entrance to the flower, where
visiting animals can easily brush against it and deposit pollen.
androecium and the female carpels that form the gynoecium. Many flowers have both male and
female organs and are called perfect flowers. In some species flowers may have only the male
parts 9staminate flowers) or the female parts (carpellate or pistillate flowers) and these are called
imperfect flowers. In species with imperfect flowers, male and female flowers may occur on the
same branch, different branches or on separate plants. In the latter case the plant has distinct male
and female individuals and is said to be dioecious. Many plants, however, are far more confused
and may contain male, female and individuals that are hermaphrodite to a variable degree.
The stamens (also called microsporophylls, which literally means leaves producing microspores)
were also derived from leaves over the course of evolution. Indeed, the stamens of some plants are
still leaf-like and bear sporangia (spore producing organs) underneath, rather like ferns and other
non-flowering plants that produce spores from sporangia underneath fertile fronds. The fossil
record tells us that flowering plants appeared quite late in evolution (and genetic analysis reveals
that their dramatic speciation is in large part due to their tendency to hybridise) and some flowering
plants still bear such 'primitive' or archaic characteristics. The stamens of most flowers, however,
comprise an anther head borne on a slender stalk or filament. The anther produces pollen
(microspores) in (usually four) pollen sacs (microsporangia) within it. The pollen contain the male
sperm and when ripe the anthers dry and rupture (dehisce, dehiscence) to release their spores.
The anthers may dehisce by splitting to form a longitudinal slit (as in cotton) or a transverse slit (as
in basil) or a single pore called the stomium or a number of pores (e.g. potato) or they split into a
number of valves as in bay leaf. The layer of cells in the anther wall may contain strips that twist
upon drying to bring about rupture.
The encasement of sperm in pollen is an adaptation to life on land as the tough casing of the
pollen wall resists dry conditions and allows the sperm to be transported to a female plant by animal
or wind, over many miles if need be. Moss, ferns and cycads and other archaic lineages are less
well adapted to life on land and still produce naked sperm (as their algal ancestors did) that require
a film of moisture in which to swim.
The female carpels may be separate (an apocarpous flower) but are often fused together into a
single structure (a syncarpous flower). Sometimes the word pistil is used to describe a single
carpel in an apocarpous flower or the single compound carpel of a syncarpous flower. The carpel is
a modified folded leaf rolled up with its margins more or less fused together. The final structure has
one or more internal cavities called locules, in which future embryos may develop. The style is the
the upper elongated part of the pistil and is absent in some flowers. The style ends in the stigma,
usually a bulbous structure. The remainder of the carpel is the ovary, or main body of the carpel.
The stigma is the receptive region to which pollen adheres prior to fertilisation. The pollen grain
then grows a pollen tube down through the style to reach the ovary and then into an ovule ('little
egg'), attached by a placenta to the ovary wall inside one of the locules and then it discharges the
sperm through a pore in the ovule, called the micropore or micropyle. The sperm enter the
embryo sac (which is derived from a megaspore) within the ovule, which contains the female egg
cell (ovum) and a few other (haploid) cells. Only pollen from the right species is accepted and
even then pollen from the same plant may be rejected, thereby favouring cross-pollination and
outbreeding which keeps the genetic stock strong by preventing inbreeding.
Flowers have various other tricks to ensure cross-pollination, or to prevent self-pollination. One of
these is protandry, in which the male organs develop before the female organs on the same
flower. In the passion flower, the styles are erect when the flower opens initially, holding the stigmas
well out of the way of the anthers and any insect visitors that may brush past the anther, collecting
pollen, and then brush the stigma of the same flower. As the anthers become exhausted of pollen,
the styles bend down, finally bringing the stigmas to the same level as the anthers. Now when an
insect visits the nectaries there is no pollen left to collect, but it can easily brush the stigmas and
deposit pollen from another plant.
The passion flower is what we call an actinomorphic flower, meaning that it has 'circular' (strictly
radial) symmetry. Some flowers, like the white deadnettle, Lamium alba, have zygomorphic
flowers which have only a single plane of symmetry and distinct left and right halves and a distinct
top and bottom. These are evolved from actinomorphic flowers by assymetrical fusion of the petals
into a floral tube (the corolla). This restricts access to the nectar at the base of the floral tube to
those pollinators that can reach it. This exclusivity evolves when a plant finds certain pollinators
extremely reliable and so it rewards them by saving its nectar by not giving it to less reliable
pollinators.
The white deadnettle (White Archangel) is also protandrous. In the male stage, the style has not
yet elongated far enough to position the stigma near the entrance, but the anthers are in position
and visiting animals will brush against them to collect pollen. In the female stage, the anthers are
spent and the style has elongated to position the stigma right at the entrance to the flower, where
visiting animals can easily brush against it and deposit pollen.
and earlier male stage (bottom left). Bottom right is the floral diagram and floral formula. In the floral
diagram, the topmost small circle (sometimes drawn with a cross, +, inside it) indicates the position
of the axis of the flower and tells us that we are looking straight down the nd of the receptacle, that
is we are viewing the flower face-on. The black arc beneath shows us which side the bract sits on, at
the base of the flower stalk. Next we have the outermost whorl of five sepals, drawn bridged together
to show that they are joined together (as they are toward the base of the flower). When members of
the same whorl join together we say that they cohere and we have cohesion of the parts. Inside the
sepals are the five petals which have the four stamens attached to them inside. The petals are
coherent or joined to one another. The fusion of members of different whorls, such as the stamens
to the petals, is called adnation, they are adnated. Innermost are the two carpels, fused together
into a single pistil with four locules.
The floral formula describes this flower to us - the dagger symbol tells us that the flower is
zygomorphic. K represents the sepals of the calyx, and we see that there are five of them. The C is
the corolla, which is made up of five petals as indicated, but enclosed in brackets () to tell us that
the petals are fused together into a floral tube. A represents the androecium, containing 4
members. The square brackets [] around the corolla (C) and androecium (A) tell us that these two
whorls are adnate (fused to each other). Finally we have the gyboecium (G) of two carpels fused
together.
The pictures below (click to enlarge) show the snapdragon, another zygomorphic flower which is
also prontandrous.
diagram, the topmost small circle (sometimes drawn with a cross, +, inside it) indicates the position
of the axis of the flower and tells us that we are looking straight down the nd of the receptacle, that
is we are viewing the flower face-on. The black arc beneath shows us which side the bract sits on, at
the base of the flower stalk. Next we have the outermost whorl of five sepals, drawn bridged together
to show that they are joined together (as they are toward the base of the flower). When members of
the same whorl join together we say that they cohere and we have cohesion of the parts. Inside the
sepals are the five petals which have the four stamens attached to them inside. The petals are
coherent or joined to one another. The fusion of members of different whorls, such as the stamens
to the petals, is called adnation, they are adnated. Innermost are the two carpels, fused together
into a single pistil with four locules.
The floral formula describes this flower to us - the dagger symbol tells us that the flower is
zygomorphic. K represents the sepals of the calyx, and we see that there are five of them. The C is
the corolla, which is made up of five petals as indicated, but enclosed in brackets () to tell us that
the petals are fused together into a floral tube. A represents the androecium, containing 4
members. The square brackets [] around the corolla (C) and androecium (A) tell us that these two
whorls are adnate (fused to each other). Finally we have the gyboecium (G) of two carpels fused
together.
The pictures below (click to enlarge) show the snapdragon, another zygomorphic flower which is
also prontandrous.
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The Carpel
The diagram below shows the structure of a typical angiosperm carpel. The ovary is the main body of the carpel and
contains the ovule. The style is an extension of the ovary wall that holds aloft the stigma, which is a surface that is sticky
to pollen and acts to receive and trap pollen grains. The ovule is attached to the ovary wall via a stalk, called the
funiculus, which connects to a region of the ovary wall called the placenta. Vascular tissue (often a single vessel) passes
from the placenta along the funiculus.
The diagram below shows the structure of a typical angiosperm carpel. The ovary is the main body of the carpel and
contains the ovule. The style is an extension of the ovary wall that holds aloft the stigma, which is a surface that is sticky
to pollen and acts to receive and trap pollen grains. The ovule is attached to the ovary wall via a stalk, called the
funiculus, which connects to a region of the ovary wall called the placenta. Vascular tissue (often a single vessel) passes
from the placenta along the funiculus.
The ovule develops as a protuberance projecting from the ovary wall in the region of the placenta, a mass of cells
hemispherical folds grow around it (around the outer cells or nucellus), extending from the epidermis, by cell division,
lining the inside of the ovary wall and which grow to enclose the ovule in almost complete spheres, called integuments.
The integuments do not close over the ovule completely, but maintain a small opening called the micropyle. Since they
are extensions of the ovary epidermis, the integuments are each lined (on the outside and inside) by cuticle.
Inside the developing ovule, the outer cell layers are vegetative and form the nucellus. Inside the nucellus is the
sporogenous region. The chalaza is the region of tissue where funiculus, integuments and nucellus meet. The
sporogenous region begins as a large archesporial cell, which gives rise to a megaspore mother cell (MMC) either
by direct differentiation or by undergoing a mitotic division (see mitosis) into two daughter cells, a perietal cell and the
MMC. The MMC undergoes meiosis to produce four haploid megaspores (enclosed in secreted callose walls which
later dissolve). In most angiosperms, 3 of the 4 megaspores degenerate, but the remaining megaspore undergoes 3
successive mitotic divisions, doubling each time, to produce 8 haploid nuclei. These nuclei form the embryo sac, which is
the female gametophyte (gamete producing 'plant').
hemispherical folds grow around it (around the outer cells or nucellus), extending from the epidermis, by cell division,
lining the inside of the ovary wall and which grow to enclose the ovule in almost complete spheres, called integuments.
The integuments do not close over the ovule completely, but maintain a small opening called the micropyle. Since they
are extensions of the ovary epidermis, the integuments are each lined (on the outside and inside) by cuticle.
Inside the developing ovule, the outer cell layers are vegetative and form the nucellus. Inside the nucellus is the
sporogenous region. The chalaza is the region of tissue where funiculus, integuments and nucellus meet. The
sporogenous region begins as a large archesporial cell, which gives rise to a megaspore mother cell (MMC) either
by direct differentiation or by undergoing a mitotic division (see mitosis) into two daughter cells, a perietal cell and the
MMC. The MMC undergoes meiosis to produce four haploid megaspores (enclosed in secreted callose walls which
later dissolve). In most angiosperms, 3 of the 4 megaspores degenerate, but the remaining megaspore undergoes 3
successive mitotic divisions, doubling each time, to produce 8 haploid nuclei. These nuclei form the embryo sac, which is
the female gametophyte (gamete producing 'plant').
The eight haploid nuclei of the gametophyte are enclosed in 7 cells. Three haploid cells form at the pole of the embryo sac
opposite furthest from the micropyle, these are the 3 haploid antipodal cells. At the micropylar end, nearest the
micropyle, three haploid cells are situated, two haploid synergids in front of and either side of the single haploid ovum
or egg cell; together these three cells are called the egg apparatus. These six cells are surrounded by the large central
cell, which has the two remaining haploid polar nuclei inside it (these two nuclei may fuse to form a single diploid
nucleus, called the secondary endosperm nucleus, in some species). It has been shown that the integuments protect the
embryo sac and developing embryo and prevent it from being crushed as the ovule develops, by taking up compressive
stresses. The nucellus nourishes the developing embryo sac and embryo and may be partially or completely absorbed by
the time embryos sac development is complete. If the nucellus is depleted, then the inner integument may come into direct
contact with the embryo sac in which case it appears to takes over the nutritive role. The layer of cells immediately
enclosing the embryo sac and forming its wall may develop wall ingrowths as may the central cell (increasing the surface
area of their cell membranes, a characteristic that suggests they are transporting materials into or out of the embryo sac).
It is probable that nutrients are being transported into the embryo sac and central cell from surrounding tissues supplied
by vascular tissue through the placenta and funiculus.
This type of embryo sac development is the most common in angiosperms and is well-studied in Polygonum and is referred
to as monosporic development, since only one of the four megaspores produces the embryo sac. In other types,
development may be bisporic (requiring 2 of the 4 megaspores) or tetrasporic (requiring all 4 megaspores). In these types
of development not all the 8 nuclei produced need be haploid (though the ovum always is) and triploid (3n) or even
tetraploid (4n) nuclei may result.
The synergids have incomplete cell walls, covering the two-thirds of the cells that face the micropyle, leaving the naked cell
membrane to contact the central cell behind. These cell walls develop deep finger-like invaginations, forming the filiform
apparatus. The egg cell has a cell wall that also tends to be incomplete at the chalazal end (exposing naked cell
membrane for fusion with a sperm cell at fertilisation). In grasses the antipodal cells divide further by mitosis, producing a
many-celled short-lived tissue which develops wall ingrowths where they border the nucellus, suggesting that they may
serve some role in the loading of nutrients into the embryo sac.
opposite furthest from the micropyle, these are the 3 haploid antipodal cells. At the micropylar end, nearest the
micropyle, three haploid cells are situated, two haploid synergids in front of and either side of the single haploid ovum
or egg cell; together these three cells are called the egg apparatus. These six cells are surrounded by the large central
cell, which has the two remaining haploid polar nuclei inside it (these two nuclei may fuse to form a single diploid
nucleus, called the secondary endosperm nucleus, in some species). It has been shown that the integuments protect the
embryo sac and developing embryo and prevent it from being crushed as the ovule develops, by taking up compressive
stresses. The nucellus nourishes the developing embryo sac and embryo and may be partially or completely absorbed by
the time embryos sac development is complete. If the nucellus is depleted, then the inner integument may come into direct
contact with the embryo sac in which case it appears to takes over the nutritive role. The layer of cells immediately
enclosing the embryo sac and forming its wall may develop wall ingrowths as may the central cell (increasing the surface
area of their cell membranes, a characteristic that suggests they are transporting materials into or out of the embryo sac).
It is probable that nutrients are being transported into the embryo sac and central cell from surrounding tissues supplied
by vascular tissue through the placenta and funiculus.
This type of embryo sac development is the most common in angiosperms and is well-studied in Polygonum and is referred
to as monosporic development, since only one of the four megaspores produces the embryo sac. In other types,
development may be bisporic (requiring 2 of the 4 megaspores) or tetrasporic (requiring all 4 megaspores). In these types
of development not all the 8 nuclei produced need be haploid (though the ovum always is) and triploid (3n) or even
tetraploid (4n) nuclei may result.
The synergids have incomplete cell walls, covering the two-thirds of the cells that face the micropyle, leaving the naked cell
membrane to contact the central cell behind. These cell walls develop deep finger-like invaginations, forming the filiform
apparatus. The egg cell has a cell wall that also tends to be incomplete at the chalazal end (exposing naked cell
membrane for fusion with a sperm cell at fertilisation). In grasses the antipodal cells divide further by mitosis, producing a
many-celled short-lived tissue which develops wall ingrowths where they border the nucellus, suggesting that they may
serve some role in the loading of nutrients into the embryo sac.
Alternation of Generations
Plants exhibit an alternation of generations. In mammals the diploid adults (diploid means that they have 2 copies of each
chromosome (2n) one from the father and one from the mother) produce haploid gametes, eggs and sperm (haploid
means that only one copy of each chromosome is present) which fuse at fertilisation to form a diploid single-celled zygote
which develops (by mitosis) into the embryo. In plants, reproduction is a more complicated affair. Plants alternate between
separate diploid sporophyte plants and haploid gametophyte plants. The sporophytes produce spores that give rise
to the gametophytes, which produces spores that become the gametes. In ferns, these two generations are physically
separate plants, with the sporophyte being the dominant plant we know and the gametophyte small and easily overlooked,
in mosses and liverworts, the gametophyte is the dominant plant and the sporophyte grows almost parasitically from it. In
angiosperms, the sporophyte forms the dominant plant body and the gametophyte, not really now a separate plant at all,
forms a groups of cells firmly attached to the sporophyte body. The female gametophyte is the embryo sac, the male
gametophyte is the pollen grain.
Anthers and Pollen grains
The stamen is a microsporophyll - a modified leaf that produces microsopores, and consists of a filament (stalk)
bearing the anther. The anther contains pollen sacs, which are the microsporangia in which the microspores are produced
and are made up of sporogenous tissue and surrounding wall layers. For example, the anther of cotton (Gossypium
arboreum) is bilobed (has two lobes) and is bisporangiate (has two microsporangia).
The developing anther consists of an outer cell layer, the protoderm (which becomes the epidermis) and an underlying
(hypodermal) cell layer of archesporial cells. The archesporial cells undergo periclinal mitotic divisions (periclinal means
parallele to the surface of the anther) so that this single layer of cells becomes two cell layers, the outer or primary
parietal (wall) layer and the primary sporogenous cell layer. The outer parietal layer undergoes another set of
periclinal cell divisions, producing two new cell layers, the secondary parietal layers. The outer secondary parietal layer
undergoes a third periclinal division to produce two tertiary parietal layers. Thus at this stage there are three parietal
layers (the inner secondary parietal and the middle and outer tertiary parietal cell layers) and the inner sporogenous cell
layer. The sporogenous cells enlarge and transform directly (or by mitosis) into microspore mother cells (SMCs). Now,
in total the anther wall has 4 cell layers: the outer epidermis, an underlying cell layer (the outer tertiary parietal layer)
which becomes the endothecium, a middle cell layer (the inner tertiary parietal layer) and the innermost wall layer or
tapetum (the inner secondary parietal layer, which also incorporates some parenchyma cells from the central tissues of
the anther). (In some species there are two middle layers, as the inner secondary parietal layer may also divide, making 5
layers in total). The middle layer degenerates and becomes crushed between the tapetum and endothecium and is
absorbed by these cells, leaving 3 cell layers outside the sporogenous tissue in a mature anther. Inside this is the layer of
spore mother cells and then the parenchyma connective tissue that fills the centre of the anther. The endothecium (just
beneath the epidermis) develops secondary cell wall thickenings over its radial walls and its inner most tangential wall
(tangential = parallel to the surface of the anther, i.e. the back wall), except in the region of the stomium, which is the
region where the anther will open when releasing pollen.
The miscrospore mother cells (also called pollen mother cells or microsporocytes) undergo meiosis to produce 4 haploid
microspores (pollen grains in their early stage with one nucleus each). At first, the SMCs are compacted together and
joined by plasmodesmata. During meiosis, callose walls are deposited around them, the pre-existing cellulose cell walls
disintegrate, and the cells round-off, remaining connected to one another by wide cytoplasmic bridges that traverse
channels (of about 1.5 micrometre diameter) in the callose. These cytoplasmic connections allows the SMCs to
communicate rapidly with one-another, ensuring that they develop in synchrony. (In Gymnosperms, including conifers,
there are no such connections and the SMCs develop asynchronously). The connections disappear before the completion
of meiosis, so that each SMC gives rise to 4 isolated cells (a tetrad) often in a tetrahedral arrangement. The tapetum (the
innermost parietal layer) develops in synchrony with the SMCs and its cells become multinucleate or polyploid (they make
multiple copies of each chromosome) increasing the numbers of copies of each gene that they have, allowing more rapid
synthesis of proteins and they provide nutrition to the developing spores. There are two types of tapetum. Type 1 is
glandular/secretory and lyses (they burst as their cell walls rupture) after meiosis to deposit lipid-rich material, called
tryphine, over the maturing pollen grains. Type 2 lose their cell walls and becom amoeboidal, fusing together into a
multinucleate plasmodium whose cell processes protrude amongst the developing pollen and which dehydrates before
anthesis (the time of complete flower development and opening) to deposit tryphine over the maturing pollen. The end
result is essentially the same in both cases.
The microsporocytes each become enveloped in a complex cell wall with an outermost rigid layer called the exine and an
inner flexible layer called the nexine (a double layer) overlying the cell membrane. There are pores in the exine, where the
nexine thickens, with one pore being usual in monocotyledons, three in most dicotyledons. These pores may be circular or
elongated slits. The exine contains sporopollenin, a very tough and resistant polymer and sometimes also silicon for extra
hardness 9one of the few known uses of silicon in plants). The exine is more or less sculptured. In wind-pollinated flowers,
the pollen grains are usually small and smooth for easy transport, but in animal pollinated flowers they can be larger (and
so contain more food reserves) and are spikier, allowing them to stick to animal bodies better. The nexine contains pectin
and cellulose. The exine pores allow the pollen tube to emerge during pollen grain germination.
Before the pollen is shed, each grain originally containing a single microspore nucleus, undergoes mitosis to produce two
haploid nuclei, a vegetative nucleus (VN) and a generative nucleus (GN). The generative nucleus undergoes a
second mitotic division, either before or after the pollen is shed, to produce two sperm cells. The sperm cells are
spindle-shaped (microtubules form rod-like bundles inside the cells to maintain their shape), lack cell walls, and are joined
together head-to-tail and surrounded by the vegetative cell (thus each sperm cell is enclosed in a double cell membrane,
the inner cell membrane being its own and the outer belonging to the vegetative cell).
Plants exhibit an alternation of generations. In mammals the diploid adults (diploid means that they have 2 copies of each
chromosome (2n) one from the father and one from the mother) produce haploid gametes, eggs and sperm (haploid
means that only one copy of each chromosome is present) which fuse at fertilisation to form a diploid single-celled zygote
which develops (by mitosis) into the embryo. In plants, reproduction is a more complicated affair. Plants alternate between
separate diploid sporophyte plants and haploid gametophyte plants. The sporophytes produce spores that give rise
to the gametophytes, which produces spores that become the gametes. In ferns, these two generations are physically
separate plants, with the sporophyte being the dominant plant we know and the gametophyte small and easily overlooked,
in mosses and liverworts, the gametophyte is the dominant plant and the sporophyte grows almost parasitically from it. In
angiosperms, the sporophyte forms the dominant plant body and the gametophyte, not really now a separate plant at all,
forms a groups of cells firmly attached to the sporophyte body. The female gametophyte is the embryo sac, the male
gametophyte is the pollen grain.
Anthers and Pollen grains
The stamen is a microsporophyll - a modified leaf that produces microsopores, and consists of a filament (stalk)
bearing the anther. The anther contains pollen sacs, which are the microsporangia in which the microspores are produced
and are made up of sporogenous tissue and surrounding wall layers. For example, the anther of cotton (Gossypium
arboreum) is bilobed (has two lobes) and is bisporangiate (has two microsporangia).
The developing anther consists of an outer cell layer, the protoderm (which becomes the epidermis) and an underlying
(hypodermal) cell layer of archesporial cells. The archesporial cells undergo periclinal mitotic divisions (periclinal means
parallele to the surface of the anther) so that this single layer of cells becomes two cell layers, the outer or primary
parietal (wall) layer and the primary sporogenous cell layer. The outer parietal layer undergoes another set of
periclinal cell divisions, producing two new cell layers, the secondary parietal layers. The outer secondary parietal layer
undergoes a third periclinal division to produce two tertiary parietal layers. Thus at this stage there are three parietal
layers (the inner secondary parietal and the middle and outer tertiary parietal cell layers) and the inner sporogenous cell
layer. The sporogenous cells enlarge and transform directly (or by mitosis) into microspore mother cells (SMCs). Now,
in total the anther wall has 4 cell layers: the outer epidermis, an underlying cell layer (the outer tertiary parietal layer)
which becomes the endothecium, a middle cell layer (the inner tertiary parietal layer) and the innermost wall layer or
tapetum (the inner secondary parietal layer, which also incorporates some parenchyma cells from the central tissues of
the anther). (In some species there are two middle layers, as the inner secondary parietal layer may also divide, making 5
layers in total). The middle layer degenerates and becomes crushed between the tapetum and endothecium and is
absorbed by these cells, leaving 3 cell layers outside the sporogenous tissue in a mature anther. Inside this is the layer of
spore mother cells and then the parenchyma connective tissue that fills the centre of the anther. The endothecium (just
beneath the epidermis) develops secondary cell wall thickenings over its radial walls and its inner most tangential wall
(tangential = parallel to the surface of the anther, i.e. the back wall), except in the region of the stomium, which is the
region where the anther will open when releasing pollen.
The miscrospore mother cells (also called pollen mother cells or microsporocytes) undergo meiosis to produce 4 haploid
microspores (pollen grains in their early stage with one nucleus each). At first, the SMCs are compacted together and
joined by plasmodesmata. During meiosis, callose walls are deposited around them, the pre-existing cellulose cell walls
disintegrate, and the cells round-off, remaining connected to one another by wide cytoplasmic bridges that traverse
channels (of about 1.5 micrometre diameter) in the callose. These cytoplasmic connections allows the SMCs to
communicate rapidly with one-another, ensuring that they develop in synchrony. (In Gymnosperms, including conifers,
there are no such connections and the SMCs develop asynchronously). The connections disappear before the completion
of meiosis, so that each SMC gives rise to 4 isolated cells (a tetrad) often in a tetrahedral arrangement. The tapetum (the
innermost parietal layer) develops in synchrony with the SMCs and its cells become multinucleate or polyploid (they make
multiple copies of each chromosome) increasing the numbers of copies of each gene that they have, allowing more rapid
synthesis of proteins and they provide nutrition to the developing spores. There are two types of tapetum. Type 1 is
glandular/secretory and lyses (they burst as their cell walls rupture) after meiosis to deposit lipid-rich material, called
tryphine, over the maturing pollen grains. Type 2 lose their cell walls and becom amoeboidal, fusing together into a
multinucleate plasmodium whose cell processes protrude amongst the developing pollen and which dehydrates before
anthesis (the time of complete flower development and opening) to deposit tryphine over the maturing pollen. The end
result is essentially the same in both cases.
The microsporocytes each become enveloped in a complex cell wall with an outermost rigid layer called the exine and an
inner flexible layer called the nexine (a double layer) overlying the cell membrane. There are pores in the exine, where the
nexine thickens, with one pore being usual in monocotyledons, three in most dicotyledons. These pores may be circular or
elongated slits. The exine contains sporopollenin, a very tough and resistant polymer and sometimes also silicon for extra
hardness 9one of the few known uses of silicon in plants). The exine is more or less sculptured. In wind-pollinated flowers,
the pollen grains are usually small and smooth for easy transport, but in animal pollinated flowers they can be larger (and
so contain more food reserves) and are spikier, allowing them to stick to animal bodies better. The nexine contains pectin
and cellulose. The exine pores allow the pollen tube to emerge during pollen grain germination.
Before the pollen is shed, each grain originally containing a single microspore nucleus, undergoes mitosis to produce two
haploid nuclei, a vegetative nucleus (VN) and a generative nucleus (GN). The generative nucleus undergoes a
second mitotic division, either before or after the pollen is shed, to produce two sperm cells. The sperm cells are
spindle-shaped (microtubules form rod-like bundles inside the cells to maintain their shape), lack cell walls, and are joined
together head-to-tail and surrounded by the vegetative cell (thus each sperm cell is enclosed in a double cell membrane,
the inner cell membrane being its own and the outer belonging to the vegetative cell).
Possible paths of pollen tube growth shown in red. Once one
pollen tube reaches the synergid, degeneration of the synergid
removes the attractive signal and other germinating pollen tubes
cease development, so that only one pollen grain achieves
fertilisation.
pollen tube reaches the synergid, degeneration of the synergid
removes the attractive signal and other germinating pollen tubes
cease development, so that only one pollen grain achieves
fertilisation.
A fictitious flower rendered from a 3D computer model that uses
mathematical functions. Click here to see how to make flower
models in Pov-Ray.
mathematical functions. Click here to see how to make flower
models in Pov-Ray.
Fertilisation
1. Pollen attaches to the stigma of a receptive carpel, this is pollination.
2. Proteins on the pollen grain and stigma may determine compatibility in those species favouring cross-pollination, so
that only pollen from another plant will germinate.
3. The pollen grain germinates - it grows an extension of the vegetative cell, called the pollen tube, through one of
the pollen grain exine pores/apertures.
4. The VN and the two connected sperm cells (or GN if it is yet to divide) travel down the pollen tube as a single unit, with
the cell organelles accompanying them. This movement is driven primarily by actin and myosin filaments of the
cytoskeleton. The pollen tube grows near its tip and deposits a cell wall (of carbohydrate polymers) as it does so. The
older basal regions of the pollen tube may be closed off by callose plugs behind the nuclei descending along it.
5. The pollen tube grows at about 1-2 mm/h, making its way through the style tissues and into the space around the
ovule, following the ovule along the integument surface and entering via the micropyle, or sometimes working its way
along the funiculus to the chalaza. If the pollen tube needs to cross any remaining nucellus, then the nucellar cells in its
path disintegrate to provide easy passage.
6. The synergid cells attract the pollen tube (by a mechanism that is not yet understood), one more so than the other. The
pollen tube enters this synergid cell through its filiform apparatus, after which the synergid degenerates (the other
synergid degenerates also, though often at a later time). The pollen tube tip opens inside the synergid and the two sperm
cells emerge, separate and become tightly coiled. The VN also emerges and degenerates, along with the synergid
nucleus, forming X-bodies.
7. The two sperms are directed by the synergid cell (apparently by its actin cytoskeleton), one is led to the ovum, the
other to the central cell. the synergid cell membrane dissolves. The first sperm fuses with the ovum and enters it, after
which the sperm and egg nuclei fuse to produce the diploid zygote. The second sperm fuses with the two polar nuclei
(or the diploid endosperm nucleus formed by their prior fusion) to form a triploid fusion nucleus or primary endosperm
nucleus. This peculiar double fertilisation is a characteristic of angiosperms.
8. The ovule develops into the seed, the integuments forming the seed coat and the ovary forming the fruit. The zygote
develops into the plant embryo, whilst the primary endosperm nucleus gives rise to endosperm tissue which nourishes
the developing embryo/seedling after seed germination.
1. Pollen attaches to the stigma of a receptive carpel, this is pollination.
2. Proteins on the pollen grain and stigma may determine compatibility in those species favouring cross-pollination, so
that only pollen from another plant will germinate.
3. The pollen grain germinates - it grows an extension of the vegetative cell, called the pollen tube, through one of
the pollen grain exine pores/apertures.
4. The VN and the two connected sperm cells (or GN if it is yet to divide) travel down the pollen tube as a single unit, with
the cell organelles accompanying them. This movement is driven primarily by actin and myosin filaments of the
cytoskeleton. The pollen tube grows near its tip and deposits a cell wall (of carbohydrate polymers) as it does so. The
older basal regions of the pollen tube may be closed off by callose plugs behind the nuclei descending along it.
5. The pollen tube grows at about 1-2 mm/h, making its way through the style tissues and into the space around the
ovule, following the ovule along the integument surface and entering via the micropyle, or sometimes working its way
along the funiculus to the chalaza. If the pollen tube needs to cross any remaining nucellus, then the nucellar cells in its
path disintegrate to provide easy passage.
6. The synergid cells attract the pollen tube (by a mechanism that is not yet understood), one more so than the other. The
pollen tube enters this synergid cell through its filiform apparatus, after which the synergid degenerates (the other
synergid degenerates also, though often at a later time). The pollen tube tip opens inside the synergid and the two sperm
cells emerge, separate and become tightly coiled. The VN also emerges and degenerates, along with the synergid
nucleus, forming X-bodies.
7. The two sperms are directed by the synergid cell (apparently by its actin cytoskeleton), one is led to the ovum, the
other to the central cell. the synergid cell membrane dissolves. The first sperm fuses with the ovum and enters it, after
which the sperm and egg nuclei fuse to produce the diploid zygote. The second sperm fuses with the two polar nuclei
(or the diploid endosperm nucleus formed by their prior fusion) to form a triploid fusion nucleus or primary endosperm
nucleus. This peculiar double fertilisation is a characteristic of angiosperms.
8. The ovule develops into the seed, the integuments forming the seed coat and the ovary forming the fruit. The zygote
develops into the plant embryo, whilst the primary endosperm nucleus gives rise to endosperm tissue which nourishes
the developing embryo/seedling after seed germination.
Above: cross-sections through anthers of
Chrsyanthemum, showing the filament and the two
three apparent cell layers forming the walls and
containing tetrads (groups of 4).
Left: pollen grains of Winter Jasmine.
Chrsyanthemum, showing the filament and the two
three apparent cell layers forming the walls and
containing tetrads (groups of 4).
Left: pollen grains of Winter Jasmine.