158 



PLANTS AND MAN 



origin, sweet potatoes thrive best in our southern states, where 

 the continuous sunshine and high temperatures make it com- 

 mercially worth while to cultivate as a crop. Varieties grown for 

 northern consumption have light yellow, dry and mealy flesh; 

 more watery varieties, softer and sweeter, are in demand in the 

 South, where they are erroneously known as "yams." True 

 yams are Oriental plants of an entirely different family {Di- 

 oscoreaceae) related to the lilies. A native North American species 

 of the same genus is the wild potato vine of eastern United 



Fig. 94. — ^The thickened root of the sweet potato has most of the stored food in 

 the central xylem zone. 



States; this produces a huge storage root, often several feet long 

 and weighing as much as twenty pounds. It is quite edible, when 

 roasted; and was used as a food by the eastern Indians. 



Stems as Food 



Stems and leaf bases which grow entirely above ground have 

 never assumed great import? ace as food-storage organs; few 

 are important food plants exce t rhubarb, a niember of the Buck- 

 wheat Family; celery, of the P. fsley Family; endive of the Com- 

 posites; and asparagus of the Lily Family (fig. 95). The 

 underground storage stem, or tuber, of the white potato (in the 

 Nightshade Family) is the most important of all stem crops. 



