[Chap. XXXV VEGETATIVE MULTIPLICATION 409 



to the highlands of tropical America, where they grow naturally as peren- 

 nials. In the latitude of central Ohio the tubers left in the field are killed 

 by low temperature in winter. But farther north, where the temperature 

 of the soil beneath deep snow may remain above freezing, the cultivated 

 potato may in some local areas live indefinitely as a perennial. 



Fig. 184. Vegetative multiplication of potato from a bud on a piece of tuber. 



Photo by J. Bushnell. 



The tubers of many kinds of plants survive the temperature of winter 

 even if snow is not deep or present throughout the cold season. Such 

 plants are classified as perennials; but the vegetative propagules, the 

 tubers, develop annually and live only through parts of two growing sea- 

 sons. Each year leafv shoots with roots at their bases develop from buds 

 in the so-called eyes of the tuber. As these shoots grow to mature 

 plants, another crop of tuber-bearing rhizomes develops. 



The buds of tubers, like those of many other stems, have a definite 

 dormant period; and when they grow, apical dominance is also evident 

 (Chapter XXVI). If the temperature of the storage bin is unsuitable 

 during this period, or if artificial treatments of the buds with thiourea 

 to break their dormancy are excessive, apical dominance may be an- 

 nulled, and inferior plants result. If the tubers are planted before the 



