458 SOUTHERN FIELD CROPS 



without frost, and the yield is larger if this period is still 

 longer. While the plant has been grown north of the 

 central parts of the Gulf States, yet its cultivation is 

 scarcely practicable above the southern third of these 

 states. The plant is very sensitive to frost. Moreover, 

 its northward extension is Umited by (1) the difficulty 

 of saving through winter the stems, used as propagating 

 material, and (2) by the fact that where the ground freezes 

 it is impracticable to leave the roots in the ground until 

 they are needed for use. 



435. Uses and composition. — The only valuable por- 

 tion of the cassava plant is the root. From each plant 

 grow 4 to 8 roots in a cluster, each of which is usually 

 1 to 2|^ inches in diameter, and 2 to 3 feet long. These 

 roots are rich in starch. They are used for the manu- 

 facture of starch and of the human foods, tapioca and 

 arrowroot. Their more common use, however, in the 

 United States is as food for hogs, poultry, and other 

 live-stock. Fresh cassava roots contain 25 to 30 per cent 

 of starch, and the total dry matter averages about 34 per 

 cent, or a little more than in sweet-potatoes. As cassava 

 contains only about 1 per cent of protein (nitrogenous 

 material) the roots should be fed in connection with foods 

 rich in nitrogen, such as cowpeas, peanuts, and velvet 

 beans. 



436. Poisonous constituent. — The bitter varieties of 

 cassava, as grown in tropical countries, contain in the un- 

 cooked fresh roots appreciable amounts of the poison, 

 prussic acid. However, this poison is volatile, and is easily 

 removed by heat or by exposure for a few hours to the air. 

 The amount of this poison found in sweet cassava is too 



