58 7 VARIOUS FOOD-PLANTS 
Fic. 56.—Sweet Potato (Ipomea Batatas, Morning-Glory Family, Con- 
volvulacee). Stem, leaves and roots. X 3. (Redrawn. )—-Plant a per- 
ennial (i. e. growing more than two years) with creeping stems be- 
coming 2-3 m. long; leaves dark green, glossy; flowers purple, closely 
resembling those of the common morning-glory; roots becoming fleshy, 
sweet, and yellow within; fruit dry. 
herbage-vegetables are found to be a good deal like earth- 
vegetables. The chief difference is that the former have, on — 
the whole, a somewhat larger percentage of water, and a 
smaller amount of digestible carbohydrate. As against these 
deficiencies, however, there is a decidedly larger proportion 
of proteid in relation to the other nutritious materials. For 
example in lettuce which has at once the most water and the 
least proteid of any of the herbage-vegetables given in the 
table, we find that about one-third of the nutritive material 
(representing nearly one-quarter of the total weight exclusive 
of water) is proteid; while in the sweet potato (which of all 
the earth-vegetables given, has the least water and next to 
the most proteid) the proportion of proteid to other nutrients 
is approximately 1 to 12 (being to the total weight of the 
material dried, nearly as 1 to 18). . 
