ALBUMINOIDS. 213 



cells. One of its functions may be to combine with, and render 

 harmless, the vegetable acids formed in the plant. It is found 

 chiefly in the leaves. 



Magnesium is fairly uniformly distributed; little is known of 

 its functions. 



Iron is usually present only in small proportion, but it is 

 quite indispensable. Plants deprived of iron are unable to 

 produce chlorophyll. 



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

 to be essential to the vital processes. It cannot fulfil the 

 functions of potassium. 



The metals mentioned above, in addition to the special 

 functions in connection with assimilation and other processes, 

 act also an important part as carriers of nitric acid ; when the 

 nitrogen is elaborated into proteids, the metals unite with 

 organic acids, which yield carbonates on ignition .* It was 

 found that the richer a plant was in nitrogen the larger was the 

 amount of bases left as carbonate in the ash. 



VI. NITROGENOUS SUBSTANCES. 



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

 different writers ; by some it is employed as synonymous with 

 proteid, by others as the name of a small class of nitrogenous 

 substances, differing from the true albumens (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- 

 cry stallizable, and contain carbon, hydogen, oxygen, nitrogen, 

 and sulphur. They occur in all living matter, being essential 

 constituents of protoplasm. 



The ultimate composition of proteids varies between the foK 

 owing limits (Hoppe-Seyler) : 



Carbon. Hydrogen. Nitrogen. Sulphur. Oxygen. 



From 51-5 6-9 15-2 0-3 20-9 



to 54-5 7-3 17-0 2-0 23-5 



In vegetable tissues certain crystallizable proteids have been 



detected,! especially in the aleurone grains (e.g., of castor-oil 



seeds). 



* Lawes and Gilbert, Phil. Trans. 192 (1900), 207. t Hartig. 1885 ; Vines. 



