SECTION 6.] 



STEMS. 



45 



has three forms of branches : 1. Those that bear ordinary leaves expamded 

 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 nour- 

 ishment which, the leaves have pre- 

 pared. . 3. But a larger part of this 

 nourishment, wliile 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 tu- 

 bers, or deposito- 

 ries of prepared 

 solid food, — just 

 as in the Turnip, 

 Carrot, and Dah- 

 lia (Fig. 83-87), 

 it is deposited in 1"^ 



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 the tubers ; 

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

 portion of nourishing matter which is deposited in the seed feeds the 

 embryo when it germinates, so the much larger portion deposited in the 

 tuber nourishes its buds, or eyes, when they likewise grow, the next 

 spring, into new plants. And the great supply enables them to shoot 

 vrith 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 Chili to other cool climates, 

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

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

 profitably cultivating the principal grain-plants. 



111. The Corm or Solid Bulb, like that of Cyclamen (Fig. 103), and 

 of Indian Turnip (Fig. 104), is a very short and thick flesliy subterranean 

 stem, often broader than high. It sends off roots from its lower end, or rather 

 face, leaves and stalks from its upper. The corm of Cyclamen goes on to 

 enlarge and to produce a succession of flowers and leaves year after year. 



PiQ. 101. Tubers of Helianthus tuberosus, called " artichokes." 

 FiQ. 102. Bulblet-like tubers, such as are occasionally formed on the stem of a 

 Potato-plant above ground. 



