SECTION VIII. 



PROTEINS. 



The term protein is applied to a large variety of bodies occur- 

 ring in the animal and vegetable kingdoms, which occupy a 

 pre-eminent position in the economy of life, owing to their 

 being the chief constituents of protoplasm. 



In the plant, proteins may occur either as solid bodies or 

 in solution in the cell sap. They may be found in all living 

 members ; in roots, stems, leaves, sieve tubes, laticiferous 

 tissue, etc. Reserve proteins commonly are found in the 

 solid state, especially in seeds and in vegetative organs of pro- 

 pagation. 



These protein bodies may be either quite amorphous or 

 crystalline ; sometimes the grains are partly amorphous and 

 partly crystalline, as in the well-known aleurone grains of the 

 seed of Ricinus (castor oil). 



Protein crystals may be cubical, as in the potato, falciform 

 as in the carpellary walls of Gratiola officinalis, and other 

 shapes ; they may occur quite free within the cell, as in the 

 potato, or embedded in other bodies. These embedded crystals 

 may be found in nuclei, e.g. in the leaves of Melampyriim 

 drvense and in the ovary wall of Campanula tvachelium ; in 

 chloroplasts , e.g. Hedera and Canna ; and in amorphous pro- 

 tein, e.g. in the seeds of Ricinus and Bertholletia. 



These last, generally known as aleurone grains, are often 

 somewhat complicated ; the grain is surrounded by a protein 

 membrane, which is less readily soluble than the remaining 

 amorphous protein of the matrix. Embedded in the matrix 

 is the crystalloid, and also a globoid consisting of a double 

 phosphate of calcium and magnesium. The crystalloids vary 

 in shape ; commonly they are hexagonal and stain brown 

 with iodine and are readily soluble in dilute alkali. Also they 

 291 19 * 



