202 TEXTBOOK OF BOTANY 



221. Tubers. — A potato tuber is the thickened end of an 

 underground branch. Its "eyes" are buds, each of which 

 is in the axil of a small scale leaf that may, however, be 

 rubbed off unless the tuber is very carefully removed from the 

 soil. The " skin " of the tuber is a corky layer. Most of 

 the cells within the tuber contain many grains of starch, 

 which is the form in which the potato stores most of its re- 

 serve food. The " hilling " of a potato plant induces it to 

 form more underground branches and so to increase the 

 number of tubers. If, in hilling, a green branch that has been 

 growing above ground is covered, this branch may form a 

 tuber in the same way that a branch does which has been 

 underground from the start. Sometimes, in an unusually 

 cloudy, wet year, tubers are formed on the green branches 

 that are entirely above ground. So it appears that the 

 above-ground and underground branches are alike in being 

 able to form tubers, and that the stimuli of darkness and 

 moisture determine upon which branches tubers shall appear. 

 When the other parts of the plant die, the tubers remain alive. 

 They are therefore a means by which the plant lives over the 

 winter and by which the number of plants is increased. As 

 is well known, a whole tuber is not needed to produce a new 

 potato plant ; a tuber may be cut into several pieces, each of 

 which, if it contains an eye, may give rise to a plant. Each 

 bud grows out into a stem, which in turn produces leaves, 

 adventive roots, and branches. Many other plants, less 

 well known than the potato, bear tubers ; among these are 

 the Jerusalem artichoke and the groundnut. The tuber of 

 what is improperly called a " tuberous-rooted " begonia is 

 really the base of the stem. 



222. Permanently Green Stems. — The young parts of 

 the stems of nearly all seed plants are green, and usually many 

 of the outer cells retain some chlorophjd and manufacture 

 some food for a long time — in the case of annual stems, 

 often through practically the whole life of the plant. But 



