44 MORPHOLOGY OF STEMS AND BRANCHES. [lESSON (x 



phology of the branches, — that is, in the different forms they appear 

 under, and the purposes they serve. The Potato-plant has three 

 principal forms of branches: — 1. Those that bear ordinary leaves, 

 expanded in the air, to digest what they gather from it and what 

 the roots gather from the soil, and convert it into nourishment. 

 2, After a while a second set of branches at the summit of the 

 plant bear flowers, which form fruit and seed out of a portion of the 

 nourishment which the leaves have pre{)ared. 3 But a larger part 

 of this nourishment, while in a liquid state, is carried down the stem, 

 into a third sort of branches under ground, and accumulated in the 

 form of starch at their extremities, which become tubers, or deposi- 

 tories of prepared solid food; — just as in the Turnip, Carrot, 

 Dahlia, &c. (Fig. 57-60), it is deposited in the root. The use 

 of the store of food is obvious enough. In the autumn the whole 

 plant dies, except the seeds (if it formed them) and tlie tubers; and 

 the latter are left disconnected in the ground. Just as that small 

 portion of nourishing matter which is deposited in the seed (3, and 

 Fig. 34) feeds the embryo when it germinates, so the much larger 

 portion deposited in the tuber nourishes its buds, or ^.yf^'f; when they 

 likewise grow, the next spring, info new plants. And the great 

 supply enables them to shoot with a greater vigor at the beginning, 

 and to produce a greater amount of vegetation than the seedling 

 plant could do in the same space of time ; which vegetation in turn 

 may prepare and store up, in the course of a few weeks or months, 

 the largest quantity of solid nourishing material, in a form most 

 available for food. Taking advantage of this, man has transported 

 the Potato from the cool Andes of South America to other cool cli- 

 mates, and makes it yield him a copious supply of food, especially in 

 countries where the season is too short, or the summer's heat too 

 little, for profitabl}^ cultivating the principal grain-plants. 



105. All the sorts of subterranean stems or branches distinguished 

 by botanists pass into one another by gradations. We have seen 

 how nearly related the tuber is to the rootstock, and there are many 

 cases in which it is difficult to say which is the proper name to use. 

 So likewise, 



lOG. Th? Cnrm, or Solill Bulb, like that of the Indian Turnip and 

 the Crocus (Fig. 71), is just a very short and thick rootstock; as 

 will be seen by comparing Fig. 71 with Fig. G7. Indeed, it grows 

 80 very little in length, that it is often much broader than long, as 

 in the Indian Turnip, and the Cyclamen of our greenhouses. Corms 



