The beginner in this interesting work will find some curious confirmation of the theory that leaf buds are essential to root and top growth. As an example, a leaf of some plants will root in the cutting-bench from the stem cut above the axillary bud. After rooting the leaf will increase in size and thickness and continue to live for years. A leaf of wax plant (Hoya carnosa) will throw out roots and at the end of six years it will be only a leaf enlarged and much thickened. It makes no further growth, as there is no bud from which a plant can extend upward. But if the leaf is cut so as to include the bud in the axil we soon have roots and plant growth.
Fig. 31. - Rex Begonia leaf rooting in sand. (After Bailey.)
Again, if we make a dahlia cutting without a bud at the base we can develop a flowering plant with supporting roots, but we find no tubers below. In this case we have buds for upward growth, but no bud below for tuber formation. Still again, if we root the stem of a gloxinia leaf it will continue to grow as a leaf like the Hoya carnosa, but it will develop at the lower end of the petiole a small tuber, which can be dried off, planted, and it will produce a perfect plant.
On the same principle, if the crown buds of the tubers of dahlia, peony, or rhubarb are broken or cut off we will have root enlargement and growth below the ground, but they will perish after a longer or shorter period, as they cannot develop buds for upward growth. On the other hand, we have tubers like the potato that have eyes or buds for growth at many points, and we have leaves, like the rex begonia, that when pegged down on moist sand of the cutting-bench will develop buds and make root and top growth at every point where wounded, as shown in Fig. 31.