ACCUMULATION IN STEMS 193 



food and water are subsequently used to the advantage of 

 the plant; sometimes they are not used. Evidently, 

 however, plants which have the habit of making such 

 accumulations may live successfully in certain surround- 

 ings wherein plants without this habit may die out. Waste 

 products as well as food are often accumulated by plants. 



The accumulation of water in the stems of some desert 

 plants is quite striking. During rainy periods such plants 

 absorb much water which is retained and used during 

 periods of drought. The nail-keg cactus, so named from 

 its shape and size, has been used by travelersin the deserts 

 as a source of water. When split open, the "keg " may 

 yield a pint or more of free water. 



On bright days the accumulation of food occurs in 

 nearly all tissues in which photosynthesis takes place ; this 

 applies to some stems as well as to leaves. In bright sun- 

 light food is usually manufactured more rapidly than it 

 moves away. During the night this food is transformed 

 into soluble forms and moves to other parts of the 

 plant. 



Rhizomes and tubers are the forms of stems in which 

 the accumulation of food principally occurs; these, along 

 with tap-roots and seeds, form the principal organs of food 

 accumulation. The food in seeds is, of course, used by 

 the progeny of the plant. The food in underground 

 structures may be used by the progeny of the plant or it 

 may be used by the plant itself or much of it may not be 

 used at all. Thus, for example, potatoes contain much 

 more food than is used by the plant. 



C. Latex. — Reference has already been made to the 

 milky juice which many plants contain (see pages 53 and 



