Metabolism; Digestion and Translocation 261 



of which occur in seeds and in cell-sap, such as leucosin of 

 many cereals, legumelin of legumes, and ricin of the castor- 

 bean. Such proteins are generally soluble in water. 

 Globulins are soluble in salt solutions, but practically in- 

 soluble in pure water ; such are legumin of many legumes, 

 amandin of nuts, tuberin of the potato, and many others. 



The conjugate proteins are so named because of the 

 apparent association of two substances in the molecule 

 (nucleic acid and protein in the case of nucleo-proteins) 

 or of the ready separation of the molecule into these two 

 substances. The nucleo-proteins are of much importance 

 as constituents of nuclei. 



Derived proteins, such as the proteoses and peptones, 

 are considered more particularly under digestion, and 

 may be regarded as the digested and diffusible products 

 demanded by the cell for direct use in assimilation. 



149. Amides. - -Amides are also well known in plant tis- 

 sues. Among these asparagin is of frequent occurrence. 

 As a rule these compounds are not a storage form of nitro- 

 gen. They are commonly produced, and may accumulate 

 to a considerable extent, during germination ; from 10 to 

 30 per cent of the nitrogen in this form is not infrequent 

 at that time. LeguminosaB may contain these com- 

 pounds in exceptional amount, 75 per cent of the total 

 nitrogen in vetch during germination being thus reported 

 by Schultze. It may be regarded as a degradation prod- 

 uct of protein, a product which is readily diffused, and 

 again used. 



150. Organic acids. - - Organic acids are common con- 

 stituents of plant juices. They may occur free or com- 

 bined with mineral bases. As a result of the presence of 



