MANUFACTURE AND UTILIZATION OF CARBOHYDRATES 169 



tissues. The starch present in the leaf accumulates there only because 

 the manufacture of sugars is proceeding more rapidly than their removal. 

 During the night the starch is digested by enzymes to maltose and the 

 maltose to glucose, which then passes out of the leaf. 



The seasonal variation in the carbohydrate supply of leaves has been 

 studied by Michel Durant.^^'' He distinguishes two stages in the life 

 of a leaf: (1) a period of carbohydrate synthesis and polymerization, 

 extending from the time the leaves begin to function until the end of 

 summer or in annual plants until the seeds begin to develop, during 

 which period carbohydrate assimilation is active and carbohydrates of all 

 types increase in amount; (2) a period of hydrolysis and simplification 

 beginning about the time when the leaves turn yellow. This is marked 

 by a decrease in the amount of compound carbohydrates and a further 

 accumulation of simple sugars. The development of the abscission layer 

 at the base of the leaves of deciduous plants is correlated with this 

 accumulation of simple sugars in the leaf blade, so that their removal 

 to the branch is soon stopped. The sugars increase until they are re- 

 spired, fermented or washed out by rain. In leaves of annual plants, a 

 larger proportion of these sugars is removed to the developing seeds and 

 fruits; consequently, accumulation of simple sugars is less pronounced 

 than in tree leaves. Nevertheless, at the end of this second period 

 there are always appreciable amounts of carbohydrates left in the leaf. 



In evergreen leaves, the accumulation of simple sugars in the fall 

 and winter is accentuated by photosynthesis which continues and pro- 

 duces appreciable effects because cold weather retards respiration more 

 than photosynthesis. Starch disappears or persists in small amounts 

 and disaccharides containing fructose, such as sucrose, are prevalent. 

 In the spring, starch is resynthesized at the expense of soluble sugars. In 

 June, the polysaccharides of the leaves decrease, being added to stores 

 in the branches or used in the development of the fruit. The carbohy- 

 drate content remains low until the end of autumn. In general, the older 

 the leaf, the greater its carbohydrate content and a maximum in poly- 

 saccharides corresponds to a minimum in simple sugars. 



It has been found that the pentosans form a larger and larger propor- 

 tion of the matter insoluble in alcohol and that the pentoses increase as 

 the season advances, the latter probably representing hydrolytic products 

 of pentosans.^- 



The entire plant depends on the assimilating function of its leaves 

 for its supply of carbohydrates and of those compounds manufactured 

 from them. The carbohydrates synthesized in the leaves are trans- 

 located as hexoses through the phloem to all parts of the plant where they 

 are either stored or utilized in ways specified later. 



Forms of Storage. — Since starch is the most common form in which 

 carbohydrates are stored, it is important to consider the structure of the 



