This section is from the book "Handbook Of Hardy Trees, Shrubs, And Herbaceous Plants", by W. Botting Hemsley. Also available from Amazon: Handbook of hardy trees, shrubs, and herbaceous plants.
- This is the first phase in the development of independent life in a plant from a seed. In order to accomplish this stage certain conditions are indispensable. These essential conditions are : warmth, moisture, and air. The temperature at which seeds will germinate varies considerably in different species, ranging mainly from 40° to 75° Fahrenheit. But the seeds of some hardy plants will vegetate at a lower temperature, whilst a few tropical things require a still higher degree of warmth to start them into life. There must be sufficient moisture within reach of the seed to enable it to burst its coat by absorption and feed the young embryo. And the access of air is indispensable to effect the chemical changes to which the contents of the seed are subject in germination for the use of the young plant. Unless these three conditions are united in their proper degrees, the seeds will soon perish, especially if there be an excess of humidity. In the absence of moisture, and when not exposed to deleterious atmospheric or other influences, some seeds will retain their germinating powers for many years, whilst others will not grow after the first season. Most seeds contain the nourishment required for the support of the young plant in its earliest stage. This is stored up either in the embryo itself, and chiefly in the cotyledons, or it is present in the form of starch and other ingredients, in the albumen, surrounding the embryo, and constituting in many cases the bulk of the seed. When a seed is committed to the soil, it more or less rapidly absorbs sufficient water to soften its coats and distend the tissue of the embryo, causing it to push forth its radicle or ro,otlet, which invariably turns downwards, no matter what the position of the seed may be. This is soon followed by the appearance of the plumule or growing point of the stem, emerging from between the cotyledons when there are two, or laterally when there is only one.1 Immediately water is absorbed, and, other conditions being favourable, important chemical changes are started into operation. The most important is the transformation of the insoluble starch of the perisperm or cotyledons into soluble sugar, thereby rendering it available to circulate with the imbibed water in the growing tissue. This constitutes the first food of the young plant, just as milk is the first nourishment of the young of mammiferous animals, and the white of an egg the support of the young bird during the period of incubation. The solution of the starch is gradual in its action, and, when this provision is exhausted, if due care has been taken in the selection of soil and in the supply of moisture, the young plant will be in a state to draw and to assimilate the elements it requires from the earth. In by far the greater number of plants the cotyledons are borne above the soil, as in the Scarlet Runner Bean; but there are others, like the Pea, in which they remain buried in the ground. And, again, there are others in which the cotyledon or cotyledons never become free from the seed-shell, especially of those seeds of which the albumen is of a horny nature, and in which the process of conversion into sugar is slow; the cotyledons serving in this case as conductors of the sugary matter to the young plant, according as it is developed from the albumen. So long as the cotyledons remain buried beneath the soil, they retain the white hue they had in the seed; but as soon as they are brought under the influence of light, they secrete chlorophyll, and otherwise fulfil the functions of true leaves.
The time consumed by seeds in germination varies according as the conditions are more or less favourable for the same species; but there is a greater difference in the time required by the seeds of different species. Certain seeds, those of the common Mustard (Sinapis alba) amongst others, will germinate in forty-eight hours, or even in a shorter period; whilst the majority of seeds require a week, and from that to several weeks. And lastly, there are some seeds that exhibit no sign of life until they have been in the ground one or two years. These are principally such as have hard woody or horny integuments, those of the Rose, for example. Experience has, moreover, taught that the older seeds are the longer they are in germinating. Some seeds must be sown almost immediately after they are harvested, as contact with the air causes them to decay and soon destroys their vitality; hence the difficulties encountered in introducing many desirable exotic plants. Others, again, will retain their germinating powers for a great number of years. And we may add that seeds buried too deep in the soil for atmospheric influences to reach them will preserve their vitality for a period to which we can assign no limits - perhaps thousands of years, as would appear from the plants that often spring up on newly moved soil and in fresh clearings, which are sometimes different from any previously seen in the surrounding country.
1 The germination of Ferns, as explained under that order, is a very different process; the act of impregnation not taking place till after the first stage of development of the spores.