Stages in Germination
Germination consists of three steps, -emergence from the seed coats, penetration of the soil, and the obtaining of first nourishment.
In getting out of the seed coats, the hypocotyl appears first, emerging by way of the micropyle. The rest of the embryo follows by various ingenious schemes, all apparently planned by Nature to enable the seedling to escape uninjured from the testa, on whose protection it has so long depended.
Penetration of the soil may be either from above or from below. When seeds are scattered on the surface of the soil they are enabled to gain a foothold in the earth by various contrivances so that the Toots may be sent down into the soil. In the case of buried (planted) seed the process of penetration not only has to do with sending down roots, but the seed must find a way out of the earth, unharmed by its passage. This latter problem is solved most often by the plantlet being started from the seed in an arched position. One end of the arched stem takes hold of the ground and sends out roots, while the other, attached to the wide cotyledons or the delicate plumule leaves, gently pulls these through the ground after the growing arch has broken away to the surface. If forced directly upward these bulky appendages would be stripped off »by soil pressure.
This arch may be caused by the weight of the cotyledons and soil (as in the case of the bean), which hold back the bulky end of the plantlet until the stem is strong enough to lift it out of the ground, or (as in the case of the pea) by the tip of the plumule being held tightly between cotyledons that are not lifted frop the ground at all. In the latter case the hold of the cotyledons weakens after its store of food has been partly exhausted and the plumule is released.
Another method of penetrating the soil is found in the corn and in general by those plants whose first leaves are long and slender. In these cases protection is secured by the leaves being tightly rolled into a point and covered by a cap, so that they pierce the soil directly, thus meeting less resistance and securing safety.
The lifting force of germinating seeds is seldom noticed, but is very great. Masses of earth a hundred times their weight are lifted by our tiny garden seedlings as they come up, forcing their way through the hardest soil.
The last and most important step in germination is the establishment of the young plant in its new environment. In describing this process it is necessary to treat of the development of each part of the embryo by itself.
The hypocotyl first penetrates the testa. Protected by its root cap and directed downward by gravitation, it begins at once the production of the primary root from its lower end. From this, in turn, the whole root system rapidly develops. The only region of growth is just back of the tip, which, protected by the cap, is safely pushed downward into the earth.
The cotyledons, as before explained, may rise above ground if the hypocotyl lengthens upward, or, if not, may remain below. In either case they act as a storage of food for the seedling.
The development of the plumule usually attracts most attention for from it arise the leaves, stem, and, later, the flowers and fruit. It constitutes the shoot of the plant.
The first organ to develop in germination is the root, because the function required by the seedling is the absorption which the root performs. Many of the statements made in this, and the preceding chapter, can be proven by simple experiments.
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