PROTEINS 175 



over and over. The proteins inside the plant are broken down to 

 ammonia compounds and then reused the same as the nitrogen 

 that comes in through the roots or leaves (in insectivorous plants). 

 But this material is not broken down into ammonia for amino 

 acids except when needed, but instead is stored (also translocated, 

 according to some students) in the plant as the compound called 

 asparagine, which plays a very special role in the protein economy 

 of plants. Although the ultimate waste product, corresponding 

 to urea in animals, it is not excreted but is stored until the time 

 for it to be used, when it is broken down into ammonia and starts 

 again on the internal nitrogen cycle of the plant. This may be 

 diagrammed as follows: 



amino acids— ^proteins— >amino acids 

 t i 



HN0 3 -^NH 3 < asparagine< NH 3 . 



One may ask why NH 3 is formed before the asparagine, when 

 the amino acids are broken down. The reason for this is that 

 not all proteins yield asparagine directly, but the asparagine 

 must be synthesized from its components, — an amino acid 

 (aspartic) and ammonia. As Prianishnikov has stated it: " Am- 

 monia is the primary product and at the same time the end- 

 product of the transformations of nitrogenous substances within 

 the plant." 



We have thus seen the general steps in protein synthesis and 

 metabolism, but the details of the process are still among the 

 trade secrets of the plant. In some of the lower plants, notably 

 the fungi, urea plays somewhat the same role as asparagine does 

 among the higher plants. Here again one is impressed with the 

 fact that the fungi resemble animals in many of their metabolic 

 aspects more than they do green (autotrophic) plants, although 

 it should also be noted that the urea is a reserve food for the 

 fungi and not a waste product (as in animals). 



Treub's theory (1895-1907) of the synthesis of proteins from 

 hydrocyanic acid should be mentioned although it has few fol- 

 lowers at the present time. Many common plants have gluco- 

 sides which contain hydrocyanic acid (Chap. XV). When left in 

 darkness these disappear, and form again when exposed to light 

 if the plant is supplied with nitrates. Treub thought that the 

 HCN polymerizes into substances like adenine, 



