As all Individual plants die they have developed a way of creating new individuals. This can be derived by producing other groups of identical individuals or combine the characteristics of different individuals to produce a new generation. These differ not only amongst themselves but between themselves and their parents. The former is called asexual reproduction and the latter is sexual reproduction.
- ASEXUAL REPRODUCTION
In asexual reproduction the new individuals are exactly the same as the parent plant. There will be only one parent. Ordinary cell division is involved. An example is the potato. Seed potatoes are not seeds at all but just small tubers. The eye of the potato is a bud that grows into a new plant which will be identical as the plant from which the tuber was originally taken. Another example is the runners of grasses and strawberry plants. These types of asexual reproduction are known as Vegetative Propagation.
- SEXUAL REPRODUCTION
For a particular species to survive in a changing environment, by evolving adaptations, it is necessary that there should be a substantial degree of character variation in a species. This means that in a population of identical individuals the environment changed in such a way that these individuals could not survive, the entire population would become extinct. However, in a population of non- identical individuals some would survive. The survivors would possess some characteristic that enabled them to combat change which would be passed on to further generations. To obtain character variation in a species, one has somehow to mix the characters of different individuals to produce new combinations of characters. This cannot be done asexually, and therefore, must be done sexually.
TYPE OF CELL DIVISION INVOLVED
To obtain character variation in a population, the characters of different individuals must be mixed and future generation produced from this mixing. The characteristics of a species are contained in the Genes and are found on the Chromosomes in the cell. To mix the characters the chromosomes of two individuals must be mixed. A way of doing this would be to fuse two cells together; one from each parent and then the chromosomes would mix. However, the resulting individual would have twice the number of chromosomes of each of the parents. It is, therefore, necessary to halve the number of chromosomes in those two cells that are going to fuse. This is done during sexual reproduction by a process of cell division called meiosis. The two cells that are going to fuse are called Gametes. One is usually large and remains in one place – this is the female gamete (or egg cell), and the other is smaller and usually has to travel to meet the larger gamete – this is the male gamete or sperm.
MEIOSIS OCCURS AS FOLLOWS
As in Mitosis, the chromosomes shorten and split longitudinally but the two halves remain joined together by the centromere. However, in meiosis, the chromosomes pair up and when the cell divides, the pairs separate and migrate to the two new cells (See Figure 1 below). This gives rise to two new haploid cells (‘haploid’ means half the normal number of chromosomes. ‘Diploid’ means the full number).The chromosomes are still split and joined. However, a further division occurs during which the two halves separate and migrate to new cells. The net result in meiosis therefore, is the production of four new haploid cells containing different characters produced from one parent cell (See Figure 1). In this diagram of meiosis, the student will see that portions of different
chromosomes in a pair actually change places; this changing is called crossover and contributes to character mixing.
THE STRUCTURE OF A FLOWER
A flower contains male and female organs which produce the male and female gametes respectively. The male organ is the anther and the female the pistil. The pistil is divided into the stigma, style and ovary. The flower also often contains petals and sepals. These are not directly involved in reproduction – the whole flower sits on the receptacle. A cross section through a typical flower is shown in Figure 2 on page 4.
THE DEVELOPMENT OF FLORAL ORGANS DEVELOPMENT OF THE MALE ORGANS
The male organs are the anthers which at first contain certain cells known as microsporocytes. These cells are diploid. They undergo meiosis to produce 4 haploid cells called microspores. A microspore cell nucleus then divides to produce two nuclei in one cell. A membrane forms around one of the nuclei of this little cell called the generative cell. The other nucleus is called the tube cell or pollen cell nucleus. The whole structure is a pollen grain. (See Figure 3 below on the next page).
Figure 1: Meiosis.
Figure 2: Structure of a Flower.
Figure 3: Development of Pollen & A Mature Pollen Grain.
DEVELOPMENT OF THE FEMALE ORGAN
As can be seen from Figure 2, the ovary contains a small structure called an ovule. The development of the ovule is shown in Figure 4 below.
|Disintegrate: break up into small parts as the result of impact or decay.
The very young ovule contains one cell and it divides by meiosis to give 4 new haploid cells. Only one of these cells survives while the others disintegrate and are re-absorbed. The nucleus of the one remaining haploid cell divides by mitosis to produce 8 haploid nuclei. Three migrate to one end of the cell and become Antipodal cells. Three migrate to the lower end of the cell and form two synergid cells and one egg cell or gamete. The two remaining nuclei stay in the centre of the cell and are called the polar nuclei.
Once all the reproductive organs have developed in this way, pollination and fertilisation can then occur.
Once the pollen grains have formed inside the anther, they are released by their thousands when the anther bursts. The pollen then travels in a wind current or is carried on the legs and bodies of insects to come finally to rest on a stigma. The pollen grain then germinates and the generative cell divides to produce two sperm nuclei. The pollen grain produces a Long tube that grows down the style towards the micropyle of the ovule. See Figure 5 on the following page.
Figure 4: Development of an ovule
Figure 5: Pollination and a fertilized ovule
Figure 6: A fertilized ovule.
As the tube of the pollen grain grows through the micropyle, the two sperm cells are released; one fertilizes the egg cell producing a Zygote which develops into the embryo of the seed. The other Sperm fuses with the two polar nuclei to produce a triploid. Triploid has 1.5 times the normal number of chromosomes.
A cell wall forms around this nucleus and the new cell divides repeatedly and forms the endosperm of the new seed. See Figure 5 and 6.
- The ovary mass develops into the fruit;
- The ovule integuments develop into the testa or seed coat; and
- The endosperm develops from the fertilisation of both polar nuclei with one sperm. The embryo develops from the egg cell after it has fused with the other sperm.
TYPES OF FLOWERS
Flowers can be divided into the reproductive organs (essential organs, e.g. stamens, pistils) and the non-reproductive organs (floral parts), e.g. petals and sepals. If a flower lacks one or more of the floral parts, it is said to be incomplete. The flower is complete when all the floral parts are present.
If a flower lacks an essential organ, e.g. stamen and anther the flower is said to be imperfect and perfect if no essential organs are missing. Imperfect flowers are either male or female. Some species have both male and female flowers on the same plant, e.g. maize and on different plants, pawpaw trees. The former is called monoecious, whilst the latter is said to be dioecious.
Grasses have flowers and it is in the observation of their flowers that they are identified. A grass flower is more commonly termed as the inflorescence. The inflorescence consists of many spikelets or bunches of florets.
Each bunch of florets is protected by a Palea and a lemma. Between the palea and lemma are the normal essential organs (See Figure 7, 8 and 9 below). These figures name and also show different arrangements of spikelets.
Figure 7: Grass inflorescences