136 TILLEES OF THE GEOUND CHAP. 



notice that the potato belongs to a family some 

 members of which are very poisonous. Even the 

 potato itself, if the tubers are allowed to become 

 green by being exposed to the air, becomes unwhole- 

 some and even dangerous. That is why potatoes 

 have to be " earthed up " carefully, and should 

 always be stored in dark places. 



In hot countries, our potato is replaced by a 

 plant something like it, called the sweet potato, 

 which really belongs to quite a different family, 

 the Convolvulus family. 



Other plants which in hot countries take the 

 place of our potato in supplying the necessary 

 starch, are manioc and yams. Manioc is an 

 interesting plant. It is a shrub with thick, fleshy 

 roots, which grows easily and rapidly ; so quickly, 

 indeed, that its produce is said to be six times that 

 of wheat. It belongs to the Spurge family, and 

 just as other plants of the potato family are very 

 poisonous, so plants of the Spurge family are also 

 often very poisonous. Even the manioc plant 

 itself is poisonous, and this is a curious and 

 interesting point. 



We saw before that cooking is generally more 

 necessary for plant foods than for meat, because, 

 without cooking, plant food is often very indigestible. 

 There is, however, another reason for cooking 

 vegetable food, and that is that uncooked vegetables 



