|
There are a number of stages in the germination process. In the first place, the seed must taken in water so that the cells inside it become hydrated and capable of metabolic activity. Once metabolic activity begins, the root and the shoot begin to grow, and at this stage the cells start to divide. In order for particular functions to be brought about by specialized tissues, the cells have to differentiate. All these processes require a great deal of energy.
For the seed to grow, it needs nourishment. But the seed needs a preliminary source of food until it can obtain the required minerals from its roots. So, where does the seed find the nutrients it needs to grow?
The answer to this question lies in the construction of the seed. The seed's stored food reserves which forms together with it during the fertilization process is used by the seed until it gives off a shoot and appears above the ground. Seeds need the supplementary nutriments in their bodies until they reach the stage of being able to produce their own food.
When all of the conditions are just right, germination begins. The seed takes in water from the soil and the embryo cells start to divide. Later, the seed coat opens. First tiny roots, the beginning of the root system, appear and grow downwards in the soil. Following the development of the tiny roots, the buds which will produce the stem and leaves develop.
Germination begins under the earth, then the new little plant heads up towards the light and grows ever stronger. Once the first leaves have opened, the plant can begin to produce its own nutrition by means of photosynthesis.
When the time comes, seeds wake out of their sleep and emerge from the soil, brooking no obstacles. |
The tiny seed and its roots just half a millimetre wide come to no harm from the soil, which normally tends to rot things and destroy them. Quite the contrary, they rapidly grow and develop.
Experiments were carried out to stop seedlings reaching the daylight by closing off the escape route on top of them by various methods. The results were very surprising. The seedlings put out shoots long enough to get around any obstacle on top of them, or else created pressure where they lay and again succeeded in reaching daylight. While plants are growing they can create considerable pressure where they are. For example, a seedling growing in the cracks of a newly built road can actually open the cracks up still further. In short, they brook no obstacles as they head toward the daylight.
Shoots always grow vertically as they emerge from the soil. As they do this, they oppose the force of gravity. The roots, on the other hand, obey the force of gravity as they head downwards. This raises the question: "How is it that two organs formed on the same plant should start growing in different directions?" In order to answer this, let us have a look at some of the mechanisms in plants.
|
There are cells in the root of a germinating plant which can sense gravitational signals. In the shoot, which heads upwards, there are other, light-sensitive, cells. This sensitivity of the cells to light and gravity governs the different parts of the plant's heading in the correct direction. These two stimuli also enable the direction of growth of the root and shoot to be corrected if they are not entirely vertical.29
If we have another look at what we have already established, it will be seen that we are in the face of an extraordinary situation here. The cells which make up the plant are beginning to grow different from one another, and are changing shape to form the different parts of the plant. Furthermore, as we have seen, the shoot and the root are growing in opposite directions.
Let us now consider the root's heading down into the depth of the soil with the force of gravity, together with the shoot's heading up towards the surface. The movement of these structures, which present an image of being quite powerless, as they split the soil, will bring many questions to mind. In particular, there is an important moment of decision at this point. Who, or what, is it which establishes the moment, in other words the time the cells begin to divide, and which shows them what direction to go in? How is it that every cells acts with the knowledge of which region it is to take its place in? How is it that no confusion arises, for example, how is it that the root cells never start to head upwards?
|