2 BULLETIN 468, U. S. DEPARTMENT OF AGRICULTURE. 
In the pages which follow attention is paid particularly to the 
potato, sweet potato, and Jerusalem artichoke; and more briefly to 
the yam, cassava, dasheen, taro, and yautia — roots commonly known 
in our island possessions and to at least a limited extent in the 
United States proper. It is almost needless to say that the white, 
or common, potato, judged both by the extent to which it is grown 
and by its food value, is by far the most important representative 
of the starchy-root group. Next in importance comes the sweet 
potato, which is well known all over the country and is a staple root 
crop in a large area of the more southern portion of the Southern 
States. What a prominent place these two together hold in the diet 
may be seen from the fact that in 376 American dietary studies they 
were found to furnish an average of 12.5 per cent, or about one-eighth 
of the total food material, and 8.3 per cent, or about one-twelfth of 
all the carbohydrates eaten. 
POTATOES. 
HISTORY AND EXTENT OF CULTIVATION. 
The potato, called in different regions white potato, Irish potato, 
English potato, or round potato, was first introduced into Europe 
from America toward the close of the sixteenth century by both 
the Spaniards and the English, in the latter case as a result of the ex- 
peditions sent by Ealeigh to the Virginia colony. It is believed to be 
a native of western South America, where wild forms are still found. 
In 1915 the potato crop in the United States totaled, in round 
numbers, 359,000,000 bushels, valued at $221,000,000. Part of the 
annual crop is used for feeding farm animals, and the poorer grades 
of tubers for manufacturing purposes, but the greater part is served 
on our tables. 
STRUCTURE AND COMPOSITION. 
The food value and cooking quality of a potato depend upon its 
structure and composition. The tuber is in reality a modified stem, 
shortened and thickened to form a storehouse for material held in 
reserve for the early growth of new plants. As in all other plant 
forms, the framework of the tuber is made up of cellulose, or crude 
fiber. This carbohydrate forms the walls of the multitude of tiny 
cells, which make up the structure of the tuber and which vary in 
shape and size in different sections of the tuber according to the part 
they play in its life. The tuber has several distinct parts not 
all equally nutritious. If a crosswise section of a raw potato be held 
