The same spring sun which calls forth the new leaves and early flowers exerts its vivifying influence on the seeds that fell to the ground before the winter's frosts set in, and in sheltered places myriads of young seedlings of plants and trees may be found in their first stages of growth. The early history of a plant is as interesting a study as that of the mature specimen, and the young botanist will do well if he seeks out the germinating seeds and watches their development. This part of botanical study may, perhaps, be carried on more conveniently at home than in the field, for the seedlings may be grown in soil, wet sawdust, or in water alone, and the stages closely observed.
The seed is a plant in embryo. It consists of a young root, a bud, and one or two seed-leaves or cotyledons. Some seeds contain nothing but the parts just named, and when this is the case the cotyledons contain a reserve of food material sufficient to maintain the developing plant until the root is enabled to absorb sufficient nutriment from the soil, and the first foliage leaves are so far advanced that they can absorb carbonic acid gas from the air, and build up with the aid of this gas, together with the food obtained from the soil, the compounds required by the growing plant.
Other seeds contain, in addition to the embryo, a reserve of nutrient material quite distinct from it; and in such instances the cotyledons have the power of taking up this reserve, changing it to a condition suitable to the requirements of the plant, and then distributing it to the growing parts.
In some seedlings the cotyledons will remain for some time within, or partially within the seed, in order that they may continue the absorption of this reserve; and while this process is going on the seed may remain below the surface of the soil, or it may be lifted into the air by the upward growth of the cotyledons themselves.
In cases where the cotyledons contain the food reserve for the seedling they sometimes remain under the soil, but in many instances they are pushed into the air by the upward growth of that portion of the plant axis immediately below them. In either case they decay as soon as their work is accomplished. This often happens as soon as they have delivered up to the seedling their reserve of food, but frequently the cotyledons which ascend into the air expand, becoming really leaflike in general appearance, assuming a green colour through the development of chlorophyll (the green colouring matter of plants), and then perform all the functions of the ordinary foliage leaves of the plant. Such cotyledons often continue to exist long after the first foliage leaves have appeared from the bud, for, although the original food reserve has been exhausted, they are now in a position to manufacture, under the combined influence of the sun's warmth and light, compounds essential for their own growth as well as that of the other parts of the seedling. These cotyledons, however, are never of the same form as the true foliage leaves.
The student should obtain a variety of seeds or seedlings of our wild plants and forest trees in order to study these interesting early stages. Such employment will prove very valuable at a season when there is but little call for outdoor work.