84 PLANT PROPAGATION 



the crudest form of division, is little practiced in a com- 

 mercial way, more in home gardens. 



128. Tubers are short, thickened parts of subterranean 

 branches (Irish potato, dahlia) stored with plant food, 

 largely starch, to start new plants at the beginning of 

 the next growing season or other period of stress. An- 

 other view is that parent tubers serve as water res- 

 ervoirs, since experiment has shown that they are heavier 

 after the plant has grown several weeks than before 

 growth starts, due to accumulation of water during plant 

 growth. Often thickened roots (sweet potato) are popu- 

 lar^ classed with tubers. Botanically the distinction is 

 that true tubers have "eyes" or buds, whereas thickened 

 roots do not. In practice, true tubers (Irish potato) are 

 often used for making cuttings, whereas thickened roots 

 (sweet potato) are generally planted whole, either direct 

 in the field or in hotbeds (Figs. 72, 80), and sprouts taken 

 from them for transplanting. 



When tubers are planted whole, just as broken from the parent 

 plant, the process is called division; but when cut in pieces, each 

 bearing one or more eyes, it is called cuttage (138). From the eyes 

 shoots are developed. Roots form at the bases of these shoots 

 (Fig. 73), not from the tuber itself. As growth progresses special 

 stems are produced above the roots and swell into new tubers. 



Tubers are most frequently found in arid climates, but are by no 

 means rare in moist ones. Like bulbs, some are hardy, some tender. 

 Hardy species (Jerusalem artichoke) do best left in the open 

 ground until spring; tender ones (dahlia, potato) must be dug_in 

 fall and stored in a cool place not too dry nor too moist, otherwise 

 they will either shrivel or mold. 



When true tubers are cut (dahlia) each piece must have at least 

 one bud, because such tubers do not produce adventitious buds, but 

 when thickened roots (sweet potato) are cut and placed in a prop- 

 agating bed, adventitious buds develop and produce stems. Roots 

 almost never grow from the tubers or the cuttings themselves, but 

 from the bases of sprouts. The shoots may, therefore, be removed 

 and planted separately, as is almost always done with sweet potato 

 and often in increasing stock of new varieties of Irish potato. Other 

 shoots soon develop from the tubers, and the process may be re- 

 peated several times. Pseudo-bulbs of orchids are similarly handled. 



129. Hastening growth of potatoes ma y be done in three wavs. 

 summarized by the Rhode Island Station as the result of experiments 

 thus : a, by planting sets in pots in greenhouses and transplanting 



