V] CARBOHYDRATES 71 



There is reason to believe that ciitin is an aggregate of similar 

 modifications of various "cutinogenic" acids. The suberin and cutin of one 

 plant probably differ from that of another in the kind and proportion of 

 the acids present. 



Hemi-celluloses. These are not strictly celluloses since they are built 

 up of mannans, galactans and pentosans on lines which have already been 

 considered (see pp. 61 and 62). They frequently occur united with each 

 other, for instance as galacto-, gluco- and fructomannan, galactoaraban, 

 galactoxylan, etc. They are found in the cell-walls of the tissues of 

 many seeds, and are apparently hydrolyzed by certain enzymes, termed 

 cytases, during germination. 



The Synthesis and Inter-relationships of Carbohydrates 



IN THE Plant. 



Now that the properties and characteristics of various carbohydrates 

 have been dealt with, their synthesis and their relationships, one to 

 another, may be considered. 



In the previous chapter it has been shown how the plant synthesizes 

 a sugar from carbon dioxide and water by virtue of the chemical energy 

 obtained from transformation of radiant energy by means of chlorophyll. 

 When this sugar reaches a certam concentration in the cell, in the 

 majority of plants, starch is synthesized from it by condensation with 

 elimination of water. The starch is thus the first visible product of 

 assimilation and is temporarily "stored" in an insoluble form during the 

 day, when photosynthesis is active. During the night photosynthesis 

 ceases but the sugar is still translocated from the leaf, as it was in fact 

 during the day ; thus, since the supply ceases, the concentration in the 

 cell falls, and the "stored" starch is then hydrolyzed again into sugar, 

 and the process continues until the leaf is either starch- free, or contains 

 considerably less starch. During the next day, the starch formation is 

 repeated and so forth. The process of hydrolysis of starch is carried out 

 by the enzyme, diastase, with the formation of dextrin and maltose. In 

 all probability this same enzyme controls the synthesis of starch. 



On the other hand, it has been shown that many plants do not form 

 starch at all in their leaves but only sugar. Examples are the adult 

 Mangold plant (Beta vulgaris) and many Monocotyledons {Allium, 

 Scilla). 



