=Experiment.=--Into some tumblers of moist sand put cuttings of several kinds of plants that root readily (Fig. 18), geranium, tradescantia, begonia, etc. Put cuttings of same plants into tumblers filled with clay that has been wet and stirred very thoroughly, until it is about the consistency of cake batter. Keep the sand and puddled clay moist; do not allow the clay to crack, which it will do if it dries. The cuttings in the sand will strike root and grow, while most, if not all, those in the clay will soon die. The reason for this is that the sand is well ventilated and there is sufficient air for root development, while the clay is very poorly ventilated, and there is not sufficient air for root growth.
These experiments show us that to develop and do their work roots need air or a well-ventilated soil.
We have found the conditions which are necessary for the growth and development of plant roots, namely:
A soil supplied with available plant food.
These are the most important facts about plant growth so far as the plant grower is concerned. In other words, these conditions which are necessary for root growth and development are the most important truths of agriculture, or they are the foundation truths or principles upon which all agriculture is based. Having found these conditions, the next most important step is to find out how to bring them about in the soil, or, if they already exist, how to keep them or to improve them. This brings us, then, to a study of soils.
[Illustration: FIG. 12. To show where growth in length of the root takes place. Forty hours before the photograph was taken the tip of the root was ¼ inch from the lowest thread. The glass cover was taken from this in order to get a good picture of the root.]
[Illustration: FIG. 13. Radish seeds sprouted on dark cloth. To show root hairs.]
[Illustration: FIG. 14. To show how water gets into the roots of plants. Water passed up into the egg through the skin, or membrane, and forced the contents up the glass tube until it began to overflow.]
[Illustration: FIG. 15. To show osmose (see page 19).]
The soil considered agriculturally, is that part of the earth's crust which is occupied by the roots of plants and from which they absorb food and moisture.