Results for generic sources, without interpolation

Androecium

The androecium is made up of all stamens which are the male reproductive organs of a flower. A stamen (figure 3.23) consists of an anther and a

which is attached either to the receptable as in

or to the petals as in

In a good number of flowers the filaments are free from each other, but in flowers like the

and

all the filaments unite to form a

completely enclosing the ovary and style (figure

3.23b).

stamen

anther lobe anther

filament

stamina!

tube filament

anther , lobe

pollen sac

pollen grain

Anthers are of different shapes. Some are elongated as in grasses, some are rounded as in

others are arrow shaped as in

Whatever the shape, each anther consists of two lobes and each lobe contains two pollen sacs. The pollen sacs contain pollen grains from which are the male gametes are produced. When the anther is mature, it dehisces to liberate the pollen grains which arc seen as yellowish powdery substances at the surface.

Figure 3.23

A = A Stamen; B = Stamina) tube of Hibiscus; C = Transverse section through an anther showing the pollen sacs in the anther lobes

The Gynoecium

This is the female reproductive organ consisting of an ovary, a style and a stigma (fig. 3.25). Thegynoecium is also known as pistil or carpel and forms the innermost whorl of a flower.

The ovary contains the ovules from which the female gametes are produced. The stigma receives the pollen grains. The style is a narrow tube connecting the stigma to the ovary. After pollination, a pollen tube grows through the style to carry the male gamete to the ovary.

An ovary may be monocarpous, apocarpous or syncarpous. An ovary is monocarpouus if it consists of only one chamber as in

and

stigma

style

ovary recep-tacle

Figure 3.24 The Female reproductive organs in a flower

Briefly explain the functions of the following i. Calyx

ii. Corolla

A

Shoot System-Stems, Leaves, Flower and Fruits 117

3.9 Seeds and Ft'tits Seeds

A seed is a structure which develops from the ovule after fertilisation. A seed consists of an embryo and one or two cotyledons. It may contain an endosperm or food reserve. The embryo is made up of

or future root system and

or future shoot system. The seed is therefore a potential young plant awaiting certain favourable conditions for germination. Flowering plants have been classified into two major groups according to the number of seed leaves contained in their seeds. Hence plants whose seeds contain two seed leaves are called

plants and those whose seed contain only one seed leaf are called

plant (mono = one, di = two).

During the development of a seed certain'changes take place in the ovule. These are;

1. The

develops into an endosperm which stores food for the developing embryo.

2. The

and guard cells degenerate.

3. The

become the testa or

4. The

remains as a

for absorption of water and diffusion of air during germination.

After the embryo has grown to maturity, the food reserve in the endosperm may or may not have been completely used up. Seeds in which the endosperm is completely used up during development are called non-endospermic seeds. Such seeds have no endosperm in their structure. Food used during germination is stored in their cotyledons. Many dicotyledonous seeds are non endospermic. Examples are cowpea,

and

Seeds in which the food reserve in the endosperm is not completely used up during development are called endospermic seeds. Such seeds have an endosperm in their structure which contains the food required for germination. There is no food reserve in their cotyledons.

Most monocotyledonous seeds are endospermic. Examples are

and

Seeds whether endospermic or non endospermic are important source of food for man and other animals. The food is stored as proteins, fats or carbohydrates.

Fruits

A fruit is the structure which develops from the ovary after fertilisation. Immediately after fertilisation the ovary begins to increase in size as a result of accumulation of food substances. This is followed by gradual changes in colour. The wall of the fruit develops into a

or

At maturity the pericarp is either hard and dry or soft and fleshy. A fruit can therefore be regarded as a mature ovary, containing one or more seeds.

Sometimes a fruit may be so small that it may be mistaken for a seed. Size is not an identification mark.

Fruits and seeds can be identified by the number of scars they possess. A fruit has

These are:

the remains of style at one end and the point of attachment to the receptacle at the opposite end. A seed has only one scar or

representing

of the seed

of the ovary.

Classification of fruits

There are many criteria employed in the classification of fruits, but the most popular is according to the nature of the ovary from which the fruit develops. Using this criterion, fruits can be divided into three main groups.

These are:

I.

These are fruits formed from a single flower in which the gynoecium is made up of either a single carpel or many united carpel (that is, a syncarpous ovary). Majority

fruits belong to this group.

2.

These are fruits formed from a single flower in which the gynoecium consists of

many free carpels (that is, apocarpous ovary).

pericarp black ridges

Achene of four O'clock plant

3. Multiple fruits: These are fruits formed from many flowers whose ovaries unite together, with other parts of the flower after fertilisation.