Fig. 37. Grass plant, showing the many finely divided roots.
If you leave the plants you have pulled up lying in the air for an hour or two, you will find that they will wither, the leaves becoming quite limp and the whole plant drooping. Now place them with their roots only in water, and you will soon find that they are beginning to revive. They will revive fully and live a long time if their roots are kept in water. This reminds us of the second very important use of its roots to a plant, which we have already found out (see Chapter IV.), and shows us again that the roots absorb water and keep the whole plant supplied with it. Of course you know that cut flowers can drink up water with their stems, but that is only for a short time, and is not quite natural. The special part of the rootlet, which does the actual absorption, is the part near the tip which is covered with root hairs. You have already seen these root hairs in the course of your work (see pp. [13] and [15]).
There are then two chief duties of roots, to absorb water from the soil for the whole plant, and to hold it firmly in the ground. The fine fibres of the root, which are so much divided and run in the soil, serve both these purposes, as they expose a large area to contact with the soil, and so can absorb much from it, as well as getting a good hold of it.
As well as these two chief functions, there are many other pieces of work which roots may do, and according to the special work they take up, so they become modified and look different from usual roots.
Fig. 38. Tap root of Carrot, swollen with stored food.
One thing they often do is to act as storehouses of food. For example, examine the root of a carrot. The part we commonly call the carrot and which we eat, you will see is really the main axis of the tap root, and has the little side roots attached in the usual way. The unusual thickness of the main root is due to the large quantities of food which it stores. Just in the same way radishes and many other plants have their main roots very thick and packed with food, while dahlias have their side roots thickened in a similar way (see fig. 39). Such modified roots, which look quite different from ordinary ones, are called Storage roots, and if you examine many of them you will find them packed with starch (see p. [11] for iodine test).