Reproduction in plants

Seed germination

A seed consists of an embryo and food reserves enclosed in a resistant seed coat called the testa. Seeds are either monocotyledonous or dicotyledonous. Monocotyledonous plants have seeds with one cotyledon and dicotyledonous plants have seeds with two cotyledons. The cotyledon (endosperm) stores food needed by the developing embryo.

A bean seed (left) and a maize seed (right)

 Seeds require three basic conditions in order to geminate water, a suitable temperature and oxygen. During germination, the seed absorbs water through the micropyle and swells in size. The water seems to ‘wake’ up the dormant seed, which now becomes active and commences enzymatic and metabolic activities. The starch of the seed is converted by the action of enzymes to simple sugars that will release energy needed by the embryo.

Then the testa opens and the hypocotyl with the radicle emerge. The radicle bends downwards to form the root system. The epicotyl, bearing the first leaves, the plumule, follows and grows upwards to form the shoot system. The new seedlings begin to elongate by the divisions of the apical meristems of the root tip and terminal buds.

 Flowering

The sepals are the outermost leaves of the flower and are usually green in colour. Inside these are the usually brightly coloured petals. The petals attract insects and birds to the flower thus aiding pollination. The sepals and petals protect the young delicate flower parts they enclose.

Parts of a flower

 Enclosed within the petal is a club-like structure called the carpel or pistil. The carpel is the female part of the flower. It consists of a sticky stigma, a style, and an ovary. The ovary contains one or more ovules which develop into seeds after fertisation.

The male part of the flower is the stamens and is made up of a filament and a thickened pollen sac, the anther. The anther produces pollen, which is the make gamete.

Pollination

Pollination involves the transfer of pollen from the anther to the stigma, aided by wind, birds and insects. There are two types of pollination, namely cross-pollination and self-pollination. In cross-pollination, birds and insects, searching for the nectar can pick up pollen grains on their bodies and transfer them to another plant.

In self-pollination, the pollen can stick onto the body of an insect or bird and become transferred to the stigma of the same plant. Wind and land can blow pollen grains on the stigma of the same flower or another flower.

(1) wind pollination                      Insect and wind pollination compared

Wind pollination	Insect pollination
Large amount of pollen produced because there is a lot of wastage	Less pollen produced because there is little wastage
Pollen grains small light and smooth	Pollen larger and heavier with projections
Often found in plants which occur in groups	Present in plants that are more or less solitary
Dull flowers, scentless and without nectar	Bright scented flowers with nectar
Stigmas protrude and above petals; stigmas often leathery or adhesive	Stigmas often deep in corolla: stigmas often small
Stamens long and protrude above petals	Stamens may be within the corolla tube

Flowers that are dispersed by wind usually have green unscented petals that are not attractive to insects and do not have nectar or have very little. The petals are also small leaving the stamens and carpel exposed. The filaments are long so that the anthers are exposed to wind. They normally produce large quantities of pollen grains that are small, light, and therefore easily airborne. Their stigmas are large and exposed to wind.

 (2) insect pollination

Insect pollinated plants are brightly coloured, which easily attracts the insects. They usually have nectar and have a nice scent. Their stamens and carpels are not exposed and are protected by the sepals. They produce small quantities of pollen that is sometimes sticky that can stick to the body of the insect. They have small-unexposed stigma and sticky style.

Seed  Fertilization

A pollen grain germinates when it is deposited on the stigma of a pistil. The stigma is sticky which helps the pollen grains to stick on it. A pollen tube grows down the style up to the micropyle. The tube nucleus divides and the two male nuclei pass down the tube

One of the nuclei divides, giving rise to two sperm cells. The pollen tube grows down the style until it enters the ovary. When the tip of the pollen tube reaches the ovule, it enters the micropyle and discharges the two sperm cells into the gametophyte or the female embryo sac. One of the sperm cells fertilises the egg cell which develops into an embryo saprophyte.

Development of the fruit

With fertilisation completed, some plants develop fruit by the enlargement of the ovary and the receptacle. The wall of the ovary then becomes the pericarp. As the fruit develops, the pericarp becomes either dry and hard or juicy and fleshy. Juicy fruits are often called succulent fruits.

After fertilisation, the ovule matures into a seed which consists of a seed coat, stored food and embryo. The seed is enveloped by the ovary and the ovary goes on to become a fruit.

The number of seeds in a fruit depends on how many ovules there were in the ovary to begin with and how many were fertilised.

The ripening of fruit in the ovary is under the action of enzymes and hormones. During the ripening of the fruit, the seed starch is broken down by enzymes to fructose, a simple sugar that gives the fruit its sweetness. Ripening involves a change of colour from green to yellow, red or orange. Unripe fruit is usually green and has a sour taste which is caused the presence of lactic acid.

Most fruits develop only after the flower has been pollinated. If pollination does not take place, the flower withers and dies. Some plants such as the banana produce fruits without being fertilised. These are parthenocarpic plants.

Mechanisms of Seed Dispersal

Seeds must be dispersed so that new plants grow in places where they have enough space and nutrients. Seed dispersal reduces competition for light, water and mineral salts. Wind, animals and water disperse seeds.

 Wind dispersal

Wind dispersal

Wind dispersal normally involves small light seeds that usually contain hairs on the surface. Such seeds can easily be blown by wind or float in the air. Other seeds such as those of a sycamore fruit are enclosed in wing-shaped pods that aid in dispersal. The coconut is dispersed by water because it is provided with air cavities that make them buoyant so that they float on the water.

Animal dispersal

Seeds dispersed by animals are of two kinds. Those that are edible and those that stick on the animal body, seeds that are not digested by the enzymes of the digestive system and seeds that have hooked spine for sticking on animal fur or human clothes. The spines of a jackfruit can stick on clothes or on the body of a grazing animal and be dispersed to other places. In peas and beans the see pod will split violently when dry and scatter the seeds. This is a form of self-dispersal.

Animal dispersal

Vegetative Propagation

In nature, some plants develop prennating such as bulbs, corms, rhizomes, and tubers that develop into stems, roots or whole plants. Their major function is to store food manufactured by the plants through photosynthesis. They usually remain dormant underground during adverse conditions such as during a draught when the leaves die and fall off. However, when a new season begins enzymes activate food reserves and new plants develop by budding.

Structure	Description	Examples
Rhizome	Underground horizontal stem always swollen with food reserves	Ginger, spear grass, couch grass.
Stem tuber	Swollen tip of underground stem	Irish potato
Stolon	Where a weak stem touches the ground its tip swells and develops roots	Strawberry.
Root tuber	Swollen fibrous roots, each one capable of developing into a new plant	Sweet potato, cassava.
Bulb	Underground stem bearing close set leaves containing stored food	Onion, garlic.
Runner	Lateral branches grow along the ground producing buds and roots at intervals	Sweet potato, pumpkin.
Sucker	Underground side branches – their ends turn up to produce buds	Banana, sugarcane.

We shall examine some of these plant organs briefly.

1. Above ground horizontal stems

These are either runners or stolons. Where their nodes touch the ground they produce adventitious roots and new plant e.g. strawberry and most creeping grasses.

 Underground horizontal stems-

These stems include rhizomes and tubers. Some rhizomes are enlarged portions as tubers that function in food storage, an example being the potato. The eyes of a potato are located at the nodes and produce new plants. Adventitious roots arise from the rhizome

 Corms-

these are short swollen or bulbous vertical underground stems. A corm consists of a swollen stem base protected by scaly leaves. They have adventitious roots.

 Vertical stems

  Swollen taproots

Included under this group are the carrots and turnips. They are food storage organs.

As we have seen, asexual reproduction results in offspring that are identical genetically to the parent. On the other hand sexual reproduction brings together two different gametes, resulting in an offspring of mixed genes that is likely to adapt better to a changing environment.

Plant Propagation

Large numbers of flowering plants are able to reproduce asexually by vegetative propagation. Agriculturalists have exploited this factor successfully and many crops are these days grown without using the seed. Vegetative reproduction is routinely practised in fruit and grain production

With plant propagation, it is easy to grow high yielding food varieties of plants in large quantities very quickly.

Propagation by cutting

When the stems of cassava and sweet potatoes are cut and planted, they grow into new plants. A mass of dividing cells called callus forms at the cut end from which adventitious roots sprout. Plant propagation can also be carried out with a cut leaf. An example of this is the violet plant. Some plants can be propagated from single leaves rather than stems. The potato can be cut into several pieces, each with an ‘eye’ that regenerates a whole plant.

Grafting

Another form of propagation is called grafting. In grafting, one plant is attached to another plant of the same species. Grafting is used to improve the quality of the yield. It may also be used to improve the resistance of a plant to disease.

 In the wine making industry, for example, twigs from French grapes usually known for their superior quality are grafted to the American stock (parent plant) known for their resistance to certain diseases.

Study Questions

1. Describe in general the structure of a monocotyledonous seed and show how it is different from that of a dicotyledonous seed.

2. Draw and a flower and describe the functions of the different parts. From which parts of the flower is the seed formed?

3. How are the insect pollinated flowers different from the wind-pollinated flowers?

4. Describe the different methods of plant propagation and their significance in agriculture.

Describe different adaptations of seed dispersal.