36 



PLANT PHYSIOLOGY 



either for their supply in periods unfavorable for synthetic 

 activity, e.g., in winter, or for their vegetative propagation or 

 for both purposes. Good examples of such structures are the 

 fleshy roots, tubers, bulbs, and storage roots, or the storage 

 tissues of tree trunks. 



Although the reserve substances stored by plants in the 

 various organs are quite varied in their chemical nature, three 

 major groups, the carbohydrates, fats, and proteins, usually 

 predominate. The first two are of a simple composition. Their 

 molecules contain only three elements: carbon, hydrogen, and 

 oxygen. They are, therefore, called the ''nonnitrogenous com- 

 pounds." The composition of the protein molecule, however, is 

 more complex and contains, besides the three elements already 

 mentioned, nitrogen, sulphur, and often phosphorus. In addi- 

 tion to carbohydrates, fats, and proteins, storage organs fre- 

 quently contain various glucosides, phosphatides, alkaloids, 

 organic acids, and many other substances. Their role, though 

 very little understood, appears to be of a secondary character. 

 Attention, therefore, will be centered here upon the examination 

 of the transformation of the major groups, those interested in 

 the minor substances being referred to more detailed texts of 

 plant physiology or to special books on biochemistry. 



In the seeds of various plants are found different quantitative 

 proportions of the three major groups of reserve substances. Of 

 the nonnitrogenous compounds, the fats and carbohydrates may 

 replace each other in various plants. In the seeds of some, fats 

 predominate; in others, carbohydrates, primarily starch. It is 

 customary, therefore, to divide them into two groups, the oily 

 and the starchy seeds. In the former, the fat content reaches 

 40 to 60 per cent, as the following table shows: 



Oily seeds 



Hemp 



Flax 



Sunflower . . . 



Poppy 



Almond. . . . 

 Coconut . . . . 

 Castor bean 



Fat, 

 per cent 



30 to 35 

 30 to 35 

 45 to 55 

 40 to 50 

 40 to 50 



65 

 60 to 65 



Carbohydrates, 

 per cent 



20 

 25 

 10 

 25 

 20 

 12 

 15 



