CSEC Biology: Growth, Seeds and Germination

Growth is an irreversible change in the size of cells and organisms due to cell division (mitosis) and enlargement. Every living organism has to grow or mature in some way. However, each organism has its own way of growing. The difference in manner of growth between organisms is observable in animals and plants.

As animals grow from youth, parts of their bodies grow until they reach a genetically predetermined size based on the species. Plants, however, grow throughout their entire lifespans, and they can only grow at certain parts of themselves known as meristematic tissues or meristems. These are tissues containing undifferentiated cells (meristematic cells) which are capable of cell division. These undifferentiated meristematic cells are the only ones which facilitate the plant's growth and the development of various tissues and organs, since differentiated cells cannot perform continuous cell division because they have already become specialized cells through differentiation.

You've probably noticed that plants tend to grow from their topmost point (known as an apex). There is a meristem present at the apex of every plant. These meristems are called apical meristems, and they are found at the tip of the root at the shoot (stem). Their continuous divisions and enlargements allow the shoot to grow above ground and for the roots to grow underground.

If you've ever seen a plant grow from a seed:

You will notice that the stem thickens as it grows in height. This is due to lateral meristems, which are found on the sides of the stem and roots. These meristems are responsible for increasing the thickness of the plant.

There is another type of meristem, known as intercalary meristems. The intercalary meristems are located at the internodes or the base of the leaves. The intercalary meristems help in increasing the length of the internode. This is usually seen in monocotyledonous plants.

CSEC also requires you to know the structure of a dicotyledonous seed. As you know, seeds are the matured, fertilized ovules from the flower. They consist of the embryo enclosed by the seed coat. The embryo consists of three main parts: the radicle, the embryo axis and the cotyledon(s). In the seed of a dicotelydon, there are two cotyledons.

Cotyledons are a food reserve for the developing seedling, and therefore tend to have a swollen appearance. The embryo axis has two ends. The one which forms the shoot tip is called plumule and the portion at the lower end which forms the root tip is called the radicle. The whole embryo is surrounded by a protective cover called the seed coat. The seed coat is made up of an outer layer called the testa and an inner layer called the tegmen.

Annotated diagram of the cross section of a bean seed

Germination

Germination is the process by which a new plant grows from a seed. It can also be defined as 'the initial emergence of the radicle from the seed coat.'

Germination consists of 3 stages:

1. Imbibition and Metabolism- This is the absorption of water by the dry seed. It causes the seed to rupture as the cells of the seed get rehydrated. The swelling takes place with a great force. It ruptures the seed coats and enables the radicle to come out in the form of primary root. Because water has been absorbed, metabolic activity in the seed can be resumed. Initially, the respiration may be anaerobic, but later becomes aerobic once oxygen enters the seed.

2. Digestion of Stored Food and Translocation of Nutrients- Now, the seed breaks down food that had been stored in the cotyledons during dormancy. Lipids and carbohydrates (glucose in monocots, starch in dicots) break down into sucrose (through hydrolysis), and this sucrose and broken-down protein components are shifted to a location in the seed where they can be used for energy and new proteins for the growing plant.

3. Cell Division and Growth- In the final stage, metabolism continues and the cells of the embryo divide. The plant begins to emerge, first with the roots extending down into the soil, followed by the seeding reaching up into the air to begin the process of photosynthesis.

The process of growth in germination actually comprises 5 stages:

1. The radicle bursts through the micropyle in the seed coat into the soil

2. Primary roots begin to develop and the plumule forms a hook that straightens out, pulling the cotyledons above ground.

3. The emergent seedling begins to straighten out, taking the cotyledons with it.

4. The primary leaves begin unfolding and the stem elongates.

5. The true leaves completely emerge and the cotyledons fall off.