688 SCIENCE PROGRESS 



proteins of leaves and roots we are ignorant of the state in 

 which they are present in the cell sap and the cell protoplasm, 

 and indeed in most cases the proteins of the fully grown plant 

 have received but little attention. The best-known vegetable 

 proteins are those which occur stored up in the seeds as 

 " reserve" food for the growing organism, and they show many 

 analogies with the corresponding animal proteins. As a rule 

 they resemble in their chemical and physical behaviour the 

 animal globulins, i.e. they are insoluble in water, but soluble in 

 salt solutions and precipitated by dilution or by removal of the 

 salts by dialysis. They crystallise very often inside the cell 

 itself and they were the first crystallised proteins known (Hartig, 

 1853). Protein crystals are found especially in the endosperm of 

 oily seeds, in the so-called " aleurone " grains (especially of the 

 Para nut, castor-oil seed, etc.). From their solutions they can 

 again be obtained in a crystalline form by relatively easy means. 

 The methods introduced by Hofmeister for the crystallisation of 

 tgg albumin have not yet been applied to vegetable proteins. 

 It is a remarkable fact that whilst the vegetable globulins 

 crystallise most easily, the animal globulins have not yet been 

 obtained in crystalline form. 



Most of the vegetable proteins, however, like those of the 

 animal body, are characterised by their colloidal nature. Still, 

 numerous transition stages between the colloid and crystalloid 

 condition are also found ; thus many vegetable proteins are 

 soluble in alcohol, have lost their property of being coagulated 

 by heat, and have a diminished viscosity. The vegetable 

 proteins examined so far are all laevo-rotatory. 



Constitution of the Protein Molecule. Protein Hydrolysis. — 

 Elementary analysis and the study of the physical properties 

 and chemical reactions of the animal and vegetable proteins 

 have led to their classification into more or less well-defined 

 groups. These are subdivided according to purel}' external 

 characteristics, such as origin, solubility, coagulability, etc. On 

 the basis of such properties we distinguish to-day some forty to 

 fifty natural proteins, and this number will probably increase as 

 the methods of differentiation are improved. A better know- 

 ledge of the probable constitution of the protein has, however, 

 resulted from the attempts to study systematically their cleavage 

 products. The means which were made use of to obtain this 

 end were the action of enzymes, of acids and alkalies in var3^ing 



