ORGANIC SUBSTANCES AS PLANT FOOD 231 



Not having a complete idea of the composition of the active 

 proteins of protoplasm, we cannot understand clearly the process 

 of their synthesis. At any rate their synthesis begins with the 

 end product of the hydro lyzed reserve proteins; with ammonia. 

 Besides asparagine, which releases ammonia, non-nitrogenous sub- 

 stances, especially the carbohydrates, also take part in this syn- 

 thesis. With seeds poor in carbohydrates, as lupines, germina- 

 tion experiments in darkness have definitely shown that a large 

 quantity of asparagine accumulates, but that the synthesis of 

 protein substances is at a standstill. When such sprouting seeds 

 are placed in light, a rapid synthesis of proteins is soon initiated 

 through the utilization of carbohydrates which are produced by the 

 leaves in light. 



Light really plays an indirect part in this instance. Seeds rich 

 in carbohydrates are able to synthesize proteins in darkness as well, 

 and they do not accumulate any large amounts of asparagine. Sim- 

 ilarly, the synthesis of proteins in the germinating bulbs of the 

 onion and other closely related plants will proceed in darkness. It 

 is very convenient to study protein synthesis in these bulbs, since 

 both the nitrogenous and non-nitrogenous reserves are stored in 

 the form of crystalline substances, as sugars and amino acids. In 

 germinating seeds, on the other hand conclusions about the syn- 

 thesis of proteins in the protoplasm must be founded on the basis of 

 indirect evidence, namely, by the increase in the quantity of the 

 nucleins indigestible in gastric juice or by the increase in amount of 

 purine bases. 



Such processes of hydrolysis of reserve proteins and synthesis 

 of active proteins of the protoplasm also take place at the time of 

 resumption of growth by other storage organs. Asparagine, for 

 instance, accumulates in large quantities in the underground 

 sprouts of asparagus, from which it has received its name. Con- 

 siderable quantities of it are also to be found in the unfolding leaves 

 and flower buds of trees which have been kept in darkness; but in 

 the light, synthetic processes soon become dominant, proceeding 

 at the expense of the newly produced carbohydrates. 



73. Nutrition of Saprophytic Microorganisms and Higher 

 Plants. — Saprophytes do not produce the organic substances 

 necessary for their nutrition. They secure these already prepared, 

 settling wherever there are accumulations of such substances, as in 

 soil rich in humus, on the dead parts of animals and plants, or on 



