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A TEXT-BOOK OF BOTANY. 



a classification of Proteins based chiefly on their solubility under 

 different conditions. There are some eighteen different classes 

 recognized, and these are brought under three principal groups: 

 (i) Simple Proteins, as albumins, globulins, glutelins, prolamins, 

 etc. (2) Conjugated Proteins, or complex substances, as nucleo- 



FiG. III. Phyto-globulins: A, cell of tuber of white potato (Solanum tuberosum) 

 showing protein cyrstals (k), starch grains (st), nucleus (n); B, aleurone grains of the 

 seed of the castor-oil plant {Ricinus communis); C. aleurone grains of fruit of fennel (Fcenic- 

 ulum vulgare) containing large calcium oxalate crystals (Ca) which are strongly polarising, 

 as shown in the isolated grains; D, aleurone grains of Brazil-nut {Bertholletia excelsa); 

 g, globoids; k, protein crystals. 



proteins, which are compounds containing both nucleic acid and 

 protein. (3) Derived Proteins, or compounds resulting from the 

 action of enzymes or acids upon proteins. 



I. Most of the investigations up imtil now have been con- 

 ducted on the Globulins, which are distinguished by being insoluble 

 in w^ater but soluble in saline solutions. A number of them readily 

 crystallize, and these can be generally obtained by diluting their 



