42 



PLANT PHYSIOLOGY 



There are a-glucosides, right rotating and split by the enzymes 

 of yeast cells, and /S-glucosides, left rotating and split by emulsin, 

 which has no effect whatever on a-glucosides. The difference 

 between these two types of glucosides is explained by the fact 

 that the a-glucoside contains a more strongly rotating modifi- 

 cation of glucose, the so-called a-glucose, while in /3-glucosides 

 the more feebly rotating jS-glucose occurs. 



.>^UBAN. 



Fig. 9. — Starch grains in cells of the embryo of pea. 



Of reserve carbohydrates, the colloidal polysaccharides are 

 the most widespread in plants. Their complex molecule is 

 built up of a number of molecules of monoses. Because of the 

 large molecules, the storage of polysaccharides has the advantage 

 that its accumulation is not accompanied by an excessive increase 

 of osmotic pressure in cells, which would rise still higher with the 

 gradual drying of seeds. In this respect, disaccharides are more 

 advantageous than monosaccharides. Solutions of the same 

 percentage (on the basis of weight) contain only half the number 



