STEMS 719 



mum circumferential difference being considerable. Many plants out- 

 side of arid regions have fleshy stems, as Portulaca and Begonia, and 

 succulence is a notable feature of many halophytes, as Salicornia. 

 Many underground stems accumulate water, as well as food, in large 

 amount; this is true especially of tubers and corms. 



The accumulation of foods. — General remarks. — In sunshine most 

 leaves manufacture more food than is disposed of during the day, this 

 excess commonly accumulating as starch. During the night, when 

 carbohydrate manufacture ceases, this starch is transformed into sugar 

 and migrates to other parts of the plant. Food accumulations of 

 much greater permanence occur in stems, roots, and seeds, because the 

 manufacture of food is more rapid than is its use in growth or otherwise. 



Food accumulation in aerial and aquatic stems. — In trees and shrubs, 

 food accumulates, especially in the cortex, medullary rays, and wood 

 parenchyma, and in some instances even in the central pith region 

 (medulla). The carbohydrates that accumulate in the trunk may as- 

 sume various forms, starch on the whole being the most representative. 

 In most trees, shrubs, and evergreen herbs of temperate and cold cli- 

 mates, there is in autumn a maximum of starch, which accumulates 

 chiefly in the parenchyma. Since low temperatures favor the conver- 

 sion of starch into sugar, the latter then increases at the expense of the 

 former; it is believed that this sugar is of protective value (p. 587). 

 In early spring the sugar moves toward the buds, and again is trans- 

 formed into starch, which accumulates especially in the embryonic leaves. 



In some trees carbohydrates are transformed into fats at the inception of winter, 

 while the reverse transformation takes place in spring; among the trees that ac- 

 cumulate fats are such northern trees as birches and conifers, and it has been sup- 

 posed that these fats are of protective significance during periods of low tempera- 

 ture. Sometimes there is developed in autumn in living leptome and wood cells 

 a hemicellulose layer which is dissolved the following spring. In herbaceous stems 

 the cortex and bundle sheath frequently are regions of starch accumulation, especially 

 in water plants (fig. 1017). 



Food accumulation in subterranean stems. — The chief subterranean 

 stem organs of food accumulation are tubers and corms (figs. 989, 990, 

 993), in which the food commonly is starch that is formed by leuco- 

 plasts in the cells in which it accumulates. Sometimes the starch 

 grains of underground stems are relatively large, as in the potato tuber 

 (figs. 1206, 1207), and in the rhizome of Canna, where individual grains 

 may be 0.17 mm. in length. The chief advantage associated with 



