1. Why are Angiosperms so Successful?
The plants are classified into 6 different phyla, namely:
- Bryophyta – moss, liverwort
- Lycopodophyta – clubmosses
- Sphemnophyta – horsetails
- Filicinophyta – ferns
- Coniferophyta – conifers
- Angiospernophyta – flowering plants
<!--Of all the plants, the angiosperms are most successful in colonization of land. Hence, they are most abundant in the terrestrial environment. Their major advantage over other plants is related to their reproduction.
1.1 Suppression of Gametophytes
The gametophytes i.e. ovules and pollen grain, are very small and protect the gametes i.e. egg and generative cell respectively, from desiccation. This allows the angiosperms to be less dependent on water for reproduction unlike other types of plants in which the gametophyte generation is dominant and conspicuous. They are highly susceptible to desiccation and become less successful than the angiosperms in colonizing the terrestrial environment.
In angiosperms, fertilization is not dependent on water. The final transfer of male gametes is after pollination.
1.2 Development of the Flower
Flowers are highly specialized reproductive organs, adapted for the entire gamut of reproductive functions:
- Gametogenesis
- Pollination – special morphological and anatomical adaptations of floral parts.
- Fertilization – transfer of male gametes by pollen tube, independent of water.
- Fruit and seed development
- Seed dispersal and germination
1.3 Development of the Embryo in a Seed or Fruit
The seed completes the process of sexual reproduction in the angiosperms and the embryo in the seed is the first stage in the life cycle of new individuals. The seed and fruit not only offer protection from desiccation and mechanical injury, it can also be modified for specific mechanisms for dispersal and colonization of new environments.
2. The Life Cycle of Flowering Plants
Sexual reproduction involves 2 processes of meiosis and fertilization. In meiosis, the chromosome number is reduced from the diploid to the haploid number. In fertilization, the nuclei of 2 gametes fuse, raising the chromosome number from haploid to diploid. These 2 activities must occur alternately if sexual reproduction is to continue.
In flowering plants or angiosperms, meiosis and fertilization divide the life cycle into 2 distinct phases.
The gametophyte generation begins with a cell produced by meiosis. The cells produced, usually the gametes, are haploid. Gametophytes become reduced to tiny structures totally contained within and dependent on their sporophyte parents
When 2 gametes fuse, the sporophyte generation begins. The sporophyte generation thus starts with a zygote. Its cells contain the diploid number of chromosomes and all the cells that derived from it by mitosis are also diploid. The sporophyte is the dominant generation in the sense that it is the conspicuous plant we see.
3. Basic Structure of Flower
The function of the flower is to facilitate the important events of gamete formation and fusion. A typical flower is in fact a compressed shoot with 4 whorls of modified leaves separated by very short internodes and attached to the receptacle. The 4 whorls are the sepals, petals, stamens and the carpel.
3.1 Sepals
They enclose the other flower parts in the bud and are generally green, thus can photosynthesize. All the sepals taken collectively constitute the calyx. The sepals are particularly important in protecting the flower bud before it blooms. In some cases, the sepals have hairs and secrete gums and toxic substances which protects the bud from predators.
3.2 Petals
The petals are usually the conspicuous, coloured attractive flower parts. Taken together, the petals constitute the corolla. Their function is to attract pollinating agents by colour, scent or production of nectar. They may also provide some protection to the inner parts of the flower. Calyx and corolla are together known as the perianth.
3.3 Stamens
They form a whorl, lying inside the corolla. Each stamen has a slender stalk or filament which supports a lobed anther, the pollen-bearing structure. When ripen the anther splits open to release the pollen grains which contains the male gametes. The whorl or grouping of stamens is called the androecium.
3.4 Carpels
It comprises the innermost whorl which is collectively known as the gynoecium. Each carpel consists of a stigma, which is sticky and sometimes hairy, and usually mounted at the tip of a style and an ovary at the base. The ovule varies from species to species.
4. Variation in Floral Morphology
4.1 Complete and Incomplete Flowers
A complete flower has all four sets of floral leaves i.e. sepals, petals, stamens and carpels attached to the receptacle.
An incomplete flower lacks one or more of the four sets. There are flowers that partially or completely lack a perianth (e.g. grasses), with carpels but no stamens (pistillate flower) or with stamens but no carpels (staminate flower).
4.2 Perfect and Imperfect Flower
A perfect flower (hermaphrodite / bisexual) has both stamens and carpels.
An imperfect flower (unisexual) lacks stamens or carpels.
4.3 Monoecius and Dioecious Plants
The plant is said to be monoecious if both the staminate or pistillate flowers are located on the same plant. Examples are corn and cucumber. All plants with bisexual / perfect flowers e.g. hibiscus, orchids, peas are monoecious.
A dioecious plant is when staminate and pistillate flowers are borne on separate male and female plants respectively.
5. Development of Pollen Grains
Each stamen consists of an anther supported by a filament. The anther usually consists of 4 elongated and connected pollen sacs. The pollen sacs are protected from mechanical injury and desiccation by the anther wall. The wall contains several layers of cells such as the epidermis, fibrous layer and tapetum. The cells of the fibrous layer have thickened cell walls which help to liberate the mature pollen grains. The tapetal cells surrounding the pollen sacs provide nourishment to the dividing cells within the pollen sacs but they degenerate before dehiscence.
Early in the development of the lily anther, each of the pollen sacs contains a large number of dividing diploid cells called microsporocytes (microspore mother cells). Each microsporocyte divides once by meiosis to form 4 haploid microspores. The four cells are grouped together initially, forming a tetrad which late separate.
The nucleus of each microspore then divides by mitosis to form a 2-celled pollen grain. The pollen grain consists of a larger tube / vegetative cell and a smaller generative cell. The tube cell produces the pollen tube which delivers male nuclei to the egg. The nucleus of the generative cell later divides mitotically once to become sperm nuclei (male gametes) for double fertilization.
Each pollen grain develops a tough protective wall around itself. The wall consists of 2 layers – an inner intine (continuous) and an outer exine (discontinuous). In places where exine is absent, the depression is called a pit which allows the pollen tube to protrude during germination.
The exine has a tough, thick, beautifully-sculptured wall. The pattern is unique to a species or genus; some may have spikes or knobs to help them stick to pollinating agents. The exine is made of an extremely resistant chemical called sporopollenin which is water-proof and enables the pollen grains to survive long periods, in some cases for millions of years.
The proteins in the exine provide a recognition system whereby a stigma can recognize pollen of its own species or even from the same plant. This is crucial in preventing self fertilization and fertilization by other species. These proteins are also responsible for the unpleasant allergic reaction called hay fever.
When the pollen grains are fully formed, the anther dries up and splits open in a process of dehiscence. They split along the stomium exposing the pollen grains on the surface. The pollen grains are not motile and depend on other agents to carry them to the receptive stigmas of a flower of the same species.
6. Misconceptions
The pollen grain is not the male gamete. The pollen grain is the gametophyte that bears the male gamete. The male gamete is the generative cell. The generative cell is also known as sperm cell. The generative sell is actually enclosed in the tube cell.
7. Development of the Ovule
The female gamete is produced inside the embryo sacs which develop within the ovule. The ovule is connected to the ovary by a stalk called the funicle. The ovary is hollow and contains one or more ovules.
The ovule starts as a small bulge of tissue called the nucellus on the inside of of the ovary wall. The 2 folds of tissue called integuments grow up and over the nucellus leaving a small pore, the micropyle, at one end. The opposite end is known as the chalaza. The nucellar cells nourish the dividing cells in the embryo sac while the integuments protects against desiccation of the female gamete and mechanical injury similar to that of the anther wall.
Inside each ovule, one large diploid cell i.e. megaspore mother cell or the megasporocyte undergoes meiotic cell division once to form 4 haploid megaspores. All but one (furthest from the micropyle) of these megaspores, degenerate. The remaining megaspore expands and its nucleus undergoes 3 successive mitotic divisions to form an immature embryo sac which is a large cell containing 8 haploid nuclei.
Inside the embryo sac, the 8 haploid nuclei become arranged in a 3:2:3 pattern. 3 nuclei migrated to the chalazal end and become surrounded by cell membranes. These 3 cells are called the antipodal cells. Three other nuclei remained at the micropylar end where they are separated from each other by cell membranes and form one egg cell (larger) and 2 smaller and similar helper or synergid cells. The remaining haploid nuclei, called the polar nuclei, are not partitioned into separate cells but share the cytoplasm of the large central cell of the embryo sac. They may fuse together to form a single diploid nucleus called the endosperm nucleus. Thus the mature embryo sac usually contains 6 haploid cells and one diploid cell.
Not every species of flowering plant produces a seven-celled embryo sac, but there is always an egg cell and an endosperm mother cell.
8. Misconceptions
The ovule is not the female gamete. The ovule is te gametophyte that bears the female gamete. The female gamete is the egg cell.
9. Pollination
Pollination refers to the transfer of pollen grains which contains the male gametes from the anther to the stigma.
Pollination is how plants have solved the problem of reproducing sexually and with diverse mates, (which promotes genetic diversity) while they remain rooted in one place. Effective means of pollination are key to success of angiosperms which depends mainly on wind and animals for this purpose.
9.1 Wind Pollination
It is effective for the gymnosperms because of where they live. If you have ever been to the taiga (a northern coniferous forest), you can understand how it would work. These vast forests are nearly a monoculture of conifers. There are many individuals of the few species found in these forests. Thus casting pollen to the wind can work as there are plenty of receivers downwind.
Wind pollination can work in angiosperms too, where almost-monocultures occur e.g. grasslands and savannahs. Here, huge strands of a relatively small number of species occur. There is plenty of opportunity for stigmas to be downwind of the anthers. Most grasses are wind pollinated. The pollen is powdery, dry (to avoid clumping and precipitation), the male flowers are produced at the top of the plant, the long pendulous filaments are thin and shake in the wind, the anthers produce vast numbers of pollen grains, the female flowers are lower on the plant and the stigmas are huge, feathery and sticky.
Angiosperms that live in a tropical rainforest cannot use wind pollination effectively. In the surrounding acre, there may be several hundred different species present. The next individual of a particular species may be quite some distance away and at some unknown angle with respect to wind direction. The plants of one species are few and far between. What is needed in such a complex biota is a magic bullet or smart bomb that can visit one flower to pick up pollen and the seek and find the next individual of that species and carry pollen to it.
9.2 Animal Vectors
They are smart enough and agile enough to do this. The plant feeds the pollinator and the pollinator accomplishes the pollen transfer for the plant. This is a kind of mutualism, We will focus on the plant-side of this mutualism.
9.3 Attraction of Specific Vectors
The plant must not just attract the vector; they need to attract the right one. Vector specificity ensures pollination as the specific vector usually visits the same species of plants. This is usually the function of the perianth. The attraction cues night be visual or olfactory.
9.3.1 Visual Cues
Showy petals or sepals with obvious shape, size and colour for the vector’s vision are important.
9.3.1.1 Colours
Butterflies and birds are attracted to red and yellow colours, and the stark contrast between a dark red and a bright yellow is important. Such strong markings help train the vector to concentrate its efforts on a particular species.
Bees have vision that is shifted toward the blue end of our visible spectrum. They do not see red colours but do see into the near ultraviolet. Yet, we find bees visiting red flowers. Typically, the red itself is attractive to other potential pollinators, but there is likely some other colour pattern that works for the bee. It may be a pattern in the UV that we cannot see which actually attracts the bee.
Moths and bats fly at night and the limited vision possibilities mean that a white or very pale colour is more observable.
Bull’s eyes, splotches and nectar guides are colour patterns that form a high-contrast exhibit to make the flowers stand out against a background of green foliage. Again all such things assists a pollinator see the flowers and begin to concentrate their visits only on those with certain colours.
9.3.1.2 Shapes and Sizes
The flower has to be designed to accommodate the vector and prevent pollen and nectar robbers from stealing the rewards. The shapes can also make a flower more attractive. Flowers pollinated by hovering vectors generally hang down and have long styles and filaments. Flowers pollinated by hungry beetles need to provide lots of structural food (and yet protect the ovules.) A beetle-pollinated flower needs to have easy entrance as beetles are clumsy in flight. Non-hovering insects and birds need perches and landing platforms as part of the flower.
The sizes and shapes of the flower parts and their alignments are critical to assure pollination when the vector does visit. A long nectar spur protects the reward from robbers and yet allows reward for the actual vector. Moreover, the body alignment while the proboscis is in the nectar is critical to the pollination event.
Some of the special once include the orchid and the pseudo-copulation by wasps, the fly bar-room of the orchids, the milkweed and its super-glue for butterfly legs and the spring-trap of the mountain laurel that brushes pollen onto the pollinator.
9.3.2 Olfactory Cues
Some vectors have limited vision but have extensive ability to find a flower by its fragrance. Flowers produce volatile chemicals that diffuse and are carried by air movements through an environment. A vector that can recognize this odour and fly up the concentration gradient of this fragrance, can easily find the next flower of a particular species. Flowers have evolved a wide array of odours to assist in attracting vectors. Humans have limited appreciation of and vocabulary for fragrance, but other animals are quite discriminating. Flowers that specialize in attracting flies are famous for their fetid aroma.
9.4 Rewards
The 2nd law of thermodynamics says there is no free lunch. It applies here too. Just getting the pollinator to land will never do. The vector is usually intelligent enough to avoid the energy waste of behaviours that do not result in some kind of reward. The plant must reward the vectors visit. This will result in training the vector to concentrate its entire efforts on visiting other flowers of the same species and achieving the pollination goal. The vector’s reward is usually one of the three things below.
9.4.1 Nectar
Somewhere in the flower is some secretory tissue called the nectary. Its location varies from species to species. This tissue is connected to phloem and secretes the sweet liquid known as nectar. This is a carbohydrate rich droplet that is used as an energy source for vectors. Hummingbirds must consume vast quantities of nectar to continue their high-energy method of flight. This is the sole energy source for most butterflies. Bees collect the nectar and evaporate it down to make honey for winter supplies. Nectar is rich in carbohydrates but is a weak source of most other nutrients. Thus a person who tells you that their kids get not sugar but you watch them pour honey all over their cereal is just plain wrong. Honey is essentially sugar just a little water added.
9.4.2 Pollen
Pollen itself is decent vegetable food. It contains protein, starch, oil and other nutrients. It is far richer than nectar in vitamins and minerals too. For bees and beetles, the consumption and collection of pollen is critical. It is their basic protein supply. Fortunately, the pollinators are not very careful in cleaning off sticky pollen that collects on their bodies each visit. Thus they arrive at a second flower with lots of pollen that collects from the first flower they visit. The position of the stigma and vector allows this pollen to be removed and stuck to the stigma.
9.4.3 Behaviour – Sex
Behaviour can also be a reward that gets a repeat visit of the vector. The orchids provide a bar-room place for the insect to het high and that is the only reward. The flies must like the experience and come back for more, achieving pollination. Another example includes the sec pheromone used by the female wasps whose mate’s virgin attempts at copulation are with the orchid flower that makes the pheromone scent. Thus the flowers attract both sexes, but only one pollinates and the other collects.
9.5 Pollination Mechanisms
There are essentially 2 different mechanisms of pollination, determined by the genetic similarity of the plants involved. Many species are capable of both self and cross pollination.
9.5.1 Self Pollination
It refers to the transfer of pollen grains to a stigma on the same flower or to a different flower on the same plant.
9.5.2 Cross Pollination
It refers to the transfer of pollen grains from one flower to a stigma of a flower on separate plants.
9.5.3 Misconceptions
Pollination is not the same as fertilization. Pollination is necessary in order to bring the male gamete in close proximity of the female gamete but it may not always result in fertilization i.e. the fusion of gametes.
9.6 Mechanisms Favouring Cross-Pollination
9.6.1 Advantages and Disadvantages of Cross-Pollination
It results in cross fertilization or out breeding. It increases genetic variation and allows greater adaptability in a changing environment thus increases it evolutionary potential. It allows for hybridization of closely linked species. Beneficial recessive alleles are unlikely to be expressed due to increased heterozygosity.
The reproductive success is not guaranteed. There is a need to synchronize flowering times with other plants of the same species so that male and female parts mature at the same time. It is dependent on environmental cues to promote flowering. It is highly dependent on pollinating agents to transfer pollen grains from plant to plant. In extreme cases, extinction of pollinators may threaten the survival of the flowering species. New and random combination of genes can destroy well-adapted genotypes.
9.6.2 Dioecious Plants
The male and female flowers are located on separate plants. Hence, self pollination is impossible.
9.6.3 Dichogamy: Protandry: Protogyny
The male and female reproductive parts of a bisexual perfect flower mature at different times. The androecium and gynoecium mature a few days apart. Therefore, the flower cannot self pollinate. It is rare in perfect flowers to have both male and female parts mature simultaneously.
The plant is said to be protogynous if the female parts mature before the anthers release the pollen. In most cases of dichogamy, there is a short overlapping period when both anthers and stigmas are mature, thus allowing selfing if crossing has been unsuccessful.
9.6.4 Hetrostyly – different arrangements of style and stamens
Distyly – 2 types of flowers – long styled (pin) and short styled (thrum) flowers. All flowers on one plant are either pin or thrum. This encourages out crossing because only pollinations between different flower morphs are successful. There are also genetic and morphological differences (e.g. thrum pollen is generally larger).
In primrose, the bee visits the flower to obtain nectar. Nectar is produced at the base of the petals and is held there because the petals are fused here forming a corolla tube so that the proboscis can reach the nectar. Any pollen attached to the proboscis after the visit to a pin flower can pollinate the thrum flower’s stigma. Pollen is deposited on the bee’s head by the thrum flower which can pollinate the stigma of a pin flower. While the insect is seeking nectar, pollen may be brushed on it or fall on to it before it leaves the flower. Such mechanisms are usually reinforced by dichogamy.
Although heterostyly apparently favours out-breeding, a much more important difference between pin-eyed and thrum –eyed primroses is a self-incompatibility mechanism which is more effective in restricting cross-pollinating to pin-eye and thrum-eye crosses. The genes that control incompatibility, style length and anther height lie close together on the same chromosome and behave as a single inheritable unit.
9.6.5 Genetic Self-Incompatibility
This means that a plant cannot produce a zygote with its own pollen. In many species, this process is controlled by multiple alleles of a single gene. Basically, if the pollen grain has the same allele as that of the stigma, then mating will not be successful. Self-fertilization will not occur.
9.6.5.1 Gametophytic Self-Incompatibility
There is the interaction between the pollen tube and the stigma / style tissues. Here, the genotypes of the pollen determine what matings will be successful. The growth of pollen tube is controlled by a single locus, which has multiple co-dominant alleles. This gene codes for a protein on the exine and stigmatic surface which is involved in cell recognition. The recognition system is such that identical proteins give an incompatible response reaction and selfing is prevented i.e. the pollen grains fail to germinate or the pollen tube grows too slowly as long as the female tissue contains the allele which is similar to itself.
9.6.5.2 Saprophytic Self-Incompatibility
There is interaction between the pollen exine and stigma / style tissues, Here, the genotype of the saprophyte determines what matings will be successful. The pollen exine contains parental tissue.
Pollen will not germinate on the stigma of a flower that contains either of the 2 alleles in the saprophyte parent that produced the pollen. This holds true even though each pollen grain (being haploid) contains only one of the alleles.
9.6.6 Monoecious Plants with Unisexual Flowers
Having separate staminate and carpellate flowers on the same plant reduces self pollinating although it does not prevent it.
9.6.7 Polygamodioecious
A few prefect flowers on a plant with imperfect flowers. This ensures pollination in the absence of cross-pollination
9.7 Mechanisms Favouring Self-Pollination
9.7.1 Advantages and Disadvantages of Self-Pollination
It results in self-fertilization or inbreeding. This is due to fusion of gametes from the same parent. It increases homozygosity / genetic uniformity and allows formation of pure-breeding plants but also allows expression of deleterious / recessive alleles. It also reduced genetic variation over time and lessen evolutionary potential to be a cause of a evolutionary dead-end. It increases survival if they are well adapted to a constant environment. However, it cannot adapt to changing environmental conditions and is only successful for a short term.
It guarantees success in reproduction and perpetuation of the species. It is more reliable if numbers of the species are widely scattered. It ensures seed set in the absence of pollinators especially for species in harsh environments where insects and other pollinators are scarce. It preserves well-adapted genotypes and a single colonizing individual is sufficient.
Self-pollination is very common and can probably occur in more than half of all flowering plant species.
9.7.2 Hermaphroditism / Bisexual Flowers
They have both male and female parts in the same flower. Stigmas should be receptive at the time of release of pollen grains. If the stigma is positioned below the stamens, the pollen grains may be shed directly onto the stigmas.
9.7.3 Monoecious Plants with Unisexual Flowers
They have both male and female parts within the same plant although the male and female parts are separated. Stigmas should be receptive at the same time as the release of pollen grains.
9.7.4 Cleistogamy
Inconspicuous, bud-like apetalous flowers concealed within the leaves are a characteristic feature. They occur after normal flowering period but never open. In these flowers, the anthers are appressed to the stigma. The pollen germinates in the anther and the pollen tube grows through the anther into the stigma.
The reproductive parts mature within the small, unopened bud. Cleistogamous flowers are the back-up plan in case pollinators are scarce in any particular year. Cross pollination may occur when the flowers bloom.
9.7.5 Apomixis (Agamospermy)
This is the production of seed without meiosis and fertilization. The embryo is genetically identical to the single parent. The egg cell is not reduced. In another words, it has full complement of the chromosomes and is not haploid.
9.8 Double Fertilization
When the process of pollination is accomplished, the next process of double fertilization begins. Genetically compatible pollen grain, will in the presence of the sugary solution produced by the stigma, absorbs water and sugars in order to germinate and a pollen tube will emerge from one of the pits.
The pollen tube secreted hydrolytic enzymes which digest the stylar tissue providing nutrients to the growing pollen tube. Styles of some flowers are hollow.
The pollen tube led by the tube cell nucleus will grow towards the micropyle in chemotatic response to usually malic acid, which the ovule secretes.
While the pollen tube is growing, the haploid generative cell within it divides by mitosis to form 2 sperm cells (male gametes) which moves down the pollen tube top be just behind the tube nucleus.
Once the tip of the pollen tube reaches the micropyle end of the ovule, the tube penetrates the layer of nucellar cells and grows into the embryo sac through the synergids which flank the egg cell.
The tip of the pollen tube ruptures and the 2 male gametes are discharged into the synergids. One haploid sperm cell fuses or fertilizes with the haploid egg nucleus, producing the diploid zygote, which will later develop into the embryo.
The second sperm cell fuses with the 2 haploid polar nuclei or diploid endosperm nucleus located in the centre of the embryo sac. It becomes the nutritive triploid endosperm tissue that will provide energy for the growth and development of the embryo.
Double fertilization refers to the union of the 2 sperm cell nuclei with the egg and endosperm cells of the embryo sac. It ensures that the endosperm develops only in the ovules where the egg has been fertilized, thereby preventing angiosperms from squandering nutrients.
9.9 Post-Fertilization Changes
The sepals, petals, stamens, style and stigma all wither off and fall. The ovary becomes the fruit wall and the ovule becomes the seed. The following describes the ovule development parts and their functions.
The integument becomes testal seed coat to protect the embryo against desiccation and injury. The micropyle is the opening to allow entry of gases (water does not enter until just before germination). The nucellus is there to provide the nutrients to the developing embryo but disintegrate when the endosperm is formed. The endosperm is the food for monocots and the food is transferred to the cotyledons in dicots.
9.9.1 Seed Development
The ovule is now known as the seed. In other words, a seed is a fertilized ovule.
The integuments become the testa or seed coat. This is a thin, tough, protective layer around the seed.
The nucellus supplies the nutrients for the initial stages of growth of the embryo and the endosperm in endospermous seeds. In order to accommodate the developing endosperm, the nucellus becomes crushed out of existence, so the embryo and endosperm may fill the whole space inside the integuments.
The triploid primary endosperm nucleus divides repeatedly by mitosis to form a multi-nucleate mass with a milky consistency. The nuclei becomes separated and surrounded by thin walls. The endosperm is rich in nutrients whist it provides to the developing embryo. Therefore the endosperm develops prior to the embryo.
Immediately after fertilization, the zygote remains quiescent for a time until there is sufficient build up of endospermous tissue. Then the zygote divides mitotically, growing and developing into the embryo. The embryo consists of a radical (root apex), plumule (shoot apex) and either one or two cotyledons (seed leaves).
The embryo is attached to the wall of the new expanding embryo sac by a row of cells called suspensor through which it may derive nourishment.
The micropyle remains as a tiny pore in the testa through which, later, oxygen and water may enter the seed during germination.
As the seed matures, the water content drops markedly from about 90% by mass to about 5 – 15 % by mass. This is in preparation for seed dormancy when metabolic activity will be much reduced. Dry seeds respire extremely slowly and can survive extended drought or cold periods.
9.10 Stages of Embryo Development
After the double fertilization stage, the zygote develops into an embryo through cell division and differentiation. There are 4 stages in the development of the embryo.
9.10.1 Globular Proembryo
The first mitotic division of the zygote is transverse, splitting the fertilized egg into a basal and a terminal cell. The basal cell will divide to form a row of cells known as the suspensor which anchors the developing embryo and transfers nutrients from the parent plant.
The terminal cell divides several times and forms a spherical proembryo attached to the suspensor. Endosperm tissue should be visible in the area not of the seed not occupied by the embryo.
9.10.2 Heart-shaped Proembryo
Cotyledons or seed leaves begin to form as bumps on the proembryo. The 2 lobes at the top of the embryo proper go on to become the 2 cotyledons in the mature embryo. Some evidence of the organization of he procambium and groundmeristem can be observed here. The procambium is the primary meristem that matures into vascular tissue. The ground meristem surrounds the procambium and matures into ground tissue. Endosperm tissue should be visible in the area not of the seed not occupied by the embryo.
9.10.3 Torpedo Proembryo
At this stage, the cotyledons have developed to the point where they are forced to curve to fit within the confines of the seed. The 3 primary meristems i.e. shoot, root and cambium are all clearly visible. Endosperm tissue should be visible in the area not of the seed not occupied by the embryo.
9.10.4 Mature Embryo
As the seed becomes mature, the embryo becomes dormant. The endosperm tissue has been completely consumed by this stage.
9.11 Fruit Development
While the seed or seeds are developing, other changes occur which result in the development of the fruit. A fruit is a matured ovary containing seeds. A true fruit is one developed solely from the ovary. If a fruit develops from the sepals, petals or receptacle as well as from the ovary, it is known as an accessory fruit.
In a typical fruit, the ovary becomes the fruit and the fruit wall is known as the pericap. The fertilized ovule produces hormones which stimulates the growth of the ovary wall. The ripening of the fruit is timed to coincide with the completion of seed development. The fruit contains the seed and the pericap is commonly modified to aid their dispersal from the parent plant by becoming fleshy or hard and dry.
9.12 Advantages and Disadvantages of Reproduction by Seed
9.12.1 Advantages
The seed protects the embryo from desiccation and mechanical injury. It enhances the survival of the species since there is less dependence on the availability of water for sexual reproduction and therefore better adapted for land environment.
The seed contains food storage (either in the cotyledons or endosperm) for embryo development upon germination. It does not depend on parental plant for supply of nutrients therefore dispersal and colonization is possible.
The seed is usually adapted for dispersal. Offspring and parent plant need not compete for space and nutrients which is common in asexual reproduction. Intra-specific competition is minimized.
The seed can remain dormant and survive adverse conditions. Staggered germination prevents extinction of species.
The seed is physiologically sensitive to favourable conditions and sometimes must undergo a period of after-ripening so that it will not germinate immediately.
The seeds are products of sexual reproduction and therefore are genetically varied. Genetic diversity enhances evolutionary potential of the species.
9.12.2 Disadvantages
Seeds are relatively large structures because of the extensive food reserves. This makes dispersal more difficult than by minute spores.
There is a large wastage of seed because the chances of survival of a given seed are limited, The parent sporophyte must therefore invest large quantities of material and energy in seed production to ensure success.
A portion of the seeds are eaten by animals as food. Plant nutrients are inevitably wasted especially if the purpose of dispersal is not accomplished.
The food supply in a seed is limited, whereas in vegetative reproduction, food is available from the parent plant until the daughter plant is fully established.
There is higher risk compared to asexual reproduction. There is a reliance on expernal agents such as wind, insects and water for pollination and dispersal. This lakes sexual reproduction risky compared to asexual reproduction in perpetuating the species. 2 individuals are required in many cases owing to gametic or self incompatibility and presence of dioecious plants.
