(b) Place a similar but uninjured twig in water, and notice that the new roots are produced at the end. What is the reason for the difference? The duties of stems.—No one can study leaves without being impressed by the great importance to the plant of the work which they do. Even the casual observation that year after year thousands of fresh leaves make their appearance would indicate this. And when it is learned that throughout the day every leaf is busily engaged in [Pg 69] decomposing carbon dioxide, and joining up the carbon with the water and mineral compounds which have come up from the root—thus forming sugars, starch and a host of other valuable substances—some general idea is obtained that the trunk and spreading boughs of a tree may, after all, be of minor importance, and may exist mainly to help the leaves to perform their duties as perfectly as possible. This is the true view to take of the stem and its branches; their duties are (1) to bear the leaves and spread them out, so that these will receive as much sunshine and fresh air as possible; (2) to supply the leaves with the water and mineral substances which they require for their work; and (3) to receive from the leaves and distribute to the rest of the plant the food materials which the leaves have prepared. A stem, together with the leaves and branches which it bears, is called a shoot. It will be found that almost all the variations in the structure and habits of stems are connected with the arrangements of the leaves. For example, the weight of the mass of leaves borne by a forest tree is very great, and they offer a great resistance to the wind; the trunk and its branches, therefore, are correspondingly strong and stout. Again, when a stem is ribbed or ridged in any particular manner it will generally be found that the ridges have a definite relation to the leaves and to their points of insertion on the stem. This has been seen (p. 44) to be the case in the stem of the wallflower. The tip of a stem or branch is occupied by a bud. As the tip elongates (Fig. 40), the outer leaves of the bud become separated by internodes, and new leaves continually arise around the growing point.