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TEXTBOOK OF PLANT PHYSIOLOGY 



very high rate, while the process of organic synthesis has not yet 

 begun. Other convenient objects of such a study are the various 

 organs containing large food reserves, which serve in part for 

 their conservation during the winter or through periods unfavor- 

 able to synthetic activity or for vegetative reproduction. Good 

 examples of such structures are the fleshy root stocks, 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 of these substances are of a simple composition. 

 Their molecules contain only three elements; carbon, hydrogen, 

 and oxygen. They are, therefore, called the "non-nitrogenous 

 compounds." The composition of the protein molecule, however, 

 is more complex. Besides the three mentioned elements we find 

 in it nitrogen, sulphur, and often phosphorus. In addition to car- 

 bohydrates, fats, and proteins, storage organs frequently contain 

 various glucosides, phosphatides, alkaloids, organic acids, and other 

 substances. Their role, though very little understood, appears to 

 be of a secondary character. Attention, therefore, will be cen- 

 tered here upon the examination of the transformation of the 

 major groups, referring those interested in the minor substances to 

 more detailed texts of plant physiology or to special books on bio- 

 chemistry. 



In the seeds of various plants are found different quantitative 

 proportions of the three major groups of reserve substances. Of 

 the non-nitrogenous 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: 



Hemp 



Flax 



Sunflower 

 Poppy. . . 

 Almond. . 

 Cocoanut 



