“An agricultural practice of supreme interest is based, at least partly, on the excessive development of certain roots. The plant is a laboratory where life converts into nutritive matter the manure from our stables and poultry-yards. A cart-load of dung becomes at the farmer’s pleasure, after passing through one sort of plant or another, a crop of peas or beans, a basket of fruit, or a loaf of bread. Hence this fertilizer is a very precious thing which nothing can replace and which must be utilized to the very utmost. The nourishment of us all depends on it. Enriched with this fertilizer, the soil produces, we will say, a first harvest of wheat. But wheat with its bunch of short and fine roots, has drawn only upon the upper layer of fertilizing material, leaving intact all that the rain has dissolved and carried down into the lower layers. It has performed its mission admirably, it is true; it has made a clean sweep and converted into wheat all the fertilizer contained in the layer of soil accessible to its roots, so that if wheat were sown a second time no harvest would be obtained. The soil, then, is exhausted on the surface, but in its underlying strata it is still rich. Well, what crop shall we choose for the utilization of these lower strata and the production of still further supplies of food? It cannot be barley, oats, or rye, since their little fibrous roots would find nothing to glean in the surface soil after the first crop of wheat. But it will be lucerne, since this plant will send down its roots, each as thick as your finger, to the depth of one, two, or even three meters, if need be, and give back the fertilizer in the form of forage, which, with the help of the animal that feeds on it, will be converted into nutritious meat, valuable dairy products, excellent wool, or, at the very least, animal power for draft service or other work. This succession of two or more different kinds of crops for the utmost utilization of a given area of prepared soil is called rotation of crops, of which there will be more to say later. “Deep roots, so admirably adapted to the utilization of the lower strata of the soil, become in other circumstances a source of serious difficulty. Suppose a tree is to be transplanted. Its long tap-root will make the operation difficult and hazardous. You must dig deep, both in pulling it out and in replanting it; and then you must be careful not to injure the root, for it is all in one piece and if it does not take hold and grow the sapling will die. In this case it would be much to the tree’s advantage to have fibrous roots running down only to a slight depth; it could then be pulled up easily, and if some roots perished in the operation enough would be left intact to insure the success of the transplanting.