X. 



THE CHEMICAL CONSTITUENTS OF PLANTS 



207 



indispensable. Plants deprived of iron are unable to produce chloro- 

 phyll. 



Sodium, though always present in the ash, does not appear to be 

 essential to the vital processes. It cannot fulfil the functions of po- 

 tassium. 



The metals mentioned above, in addition to the special functions 

 in connection with assimilation and other processes, act also an im- 

 portant part as carriers of nitric acid ; when the nitrogen is elaborated 

 into proteids, the metals unite with organic acids, which yield carbo- 

 nates on ignition. 1 It has been found that the richer a plant is in 

 nitrogen, the larger is the amount of bases left as carbonate in the 

 ash. 



VI. NITEOGENOUS SUBSTANCES. 



(i) Albuminoids. This word is used in different senses by dif- 

 ferent writers ; by some it is employed as synonymous with proteid, 

 by others as the name of a small class of nitrogenous substances, dif- 

 fering from the true albumins (e.g., gelatin and mucin). Using the 

 term proteid as the generic name for the large group which may be 

 called " Albuminous substances," it becomes difficult to define the 

 exact meaning of the word. They are of highly complex constitution, 

 generally non-crystallisable, and contain carbon, hydrogen, oxygen, 

 nitrogen and sulphur. They occur in all living matter, being essen- 

 tial constituents of protoplasm. 



The ultimate composition of proteids varies between the following 

 limits (Hoppe-Seyler) : 



In vegetable tissues, certain crystallisable proteids have been de- 

 tected, 2 especially in the aleurone grains (e.g., of castor-oil seeds). 



Many schemes for their classification have been drawn up. The 

 following is one often adopted and includes both animal and vegetable 

 proteids 3 : 



I. Protamines, e.g., salmine, sturine, clupeine, cyprinine. 

 II. Histories, e.g., those of the thymus and of blood corpuscles. 



III. Albumins, e.g., egg albumin, serum albumin, plant al- 



bumins. 



IV. Globulins, e.g., serum globulin, fibrinogen, fibrin, edestin, 



crystalline vegetable globulins. 



V. Glutelins, e.g., legumin, conglutin, amandin ; soluble in di- 

 lute alkali. 



iLawes and Gilbert, Phil. Trans., 192 (1900), 207. 



2 Hartig, 1885 ; Vines. 



3 Vide Plimmer, Chemical Constitution of the Proteins, 1908. 



