THE CARBOHYDRATES 145 



pectoses. By some workers lichenin is considered to be a pecto- 

 cellulose and is classified here rather than with the dextrosans 

 (page 138). 



The cuto-celluloses, which occur in the walls of cuticularized 

 cells thus rendering them waterproof, have been supposed to be 

 cellulose impregnated with cutin, which, along with waxy sub- 

 stances, forms the cuticle. Investigations of Gilson (1890), Van 

 Wisselingh (1893), and more recent workers (Sando, unpublished 

 work) tend to show, however, that no cellulose is chemically com- 

 bined with the cutin, in which case these compounds should be 

 dropped from the category of celluloses. 



The hemi- or pseudo-celluloses are compounds which occur in the 

 seed coats of peas and beans, in coconut shells, date seeds, etc. 

 They are much more easily hydrolyzed than other celluloses and 

 give rise to mannose, galactose, and pentoses. They bear the same 

 relation to these sugars that starch does to glucose and, for this 

 reason, are practically the same as the mannosans, galactosans, and 

 pentosans already considered. They seem to function as a kind 

 of reserve material stored in the external structures of the testa 

 and shell rather than in the internal parts of the seed such as the 

 endosperm and embryo. Also, according to Murneek (1929), 

 they may act as important reserves in woody plants, notably in 

 fruit trees. 



Conclusion. — The foregoing pages have been an attempt to 

 give a brief summary of the carbohydrates found in plants, their 

 chemical relationships, and their place in the physiology of the 

 plant. They are the first products formed and from the point of 

 view of energy changes the most important. 



Why some plants should store their food in one way and some in 

 another is not entirely clear, although, in some instances, it is 

 easy to see a survival value in the method adopted. Cold weather 

 means physiological dryness for the plant, owing to the fact that 

 water in the form of ice is unavailable, and favors the storage in a 

 soluble form such as sugar rather than an insoluble form like starch. 

 In aquatic plants which hibernate at the bottom and are killed by 

 freezing weather, e. g., Elodea, the food is in the form of starch; 

 while those like Myosotis, which live through the winter, even 

 when the temperature reaches -7° C, contain sugar during the 

 cold season even though they may have starch in the summer. 

 Arctic plants contain relatively large quantities of sugar and less 



