100 HOW CROPS GROW. 



in plants in the alkaline juices of the cambium. The 

 "vegetable caseins," viz., legumin and gluten-casein, as 

 they occur in the alkaline juices or extracts of plants, 

 are probably bodies of this class, and when precipitated by 

 acids unite to the latter, forming compounds with an 

 acid reaction. Casein of milk has been by some consid- 

 ered to be an alkali-proteid, but is probably distinct. 



Proteoses and Peptones. These terms designate 

 bodies that result from the chemical alteration of albu- 

 minoids, under the influence of "ferments" which exist 

 in plants, but which have been most fully studied as they 

 occur in the digestive apparatus of animals. 



The albuminoids, as found in plants, are mostly insol- 

 uble in the vegetable juices, and those which are soluble 

 (probably because of the presence of salts, acids or alka- 

 lies) are mostly incapable of freely penetrating the cell- 

 membranes which inclose them, and cannot circulate in the 

 vegetable juices, and likewise, when they become the food 

 of animals, cannot leave the alimentary canal so as to be- 

 come incorporated with the blood until they have been 

 chemically changed. During the processes of animal 

 digestion the albuminoids of whatever kind undergo solu- 

 tion and conversion into bodies which are freely soluble 

 in water, and rapidly penetrate the moist membranes of 

 the intestines, and thus enter into the circulation. These 

 bodies have been prepared for purposes of study by a 

 partly artificial digestion, carried on in glass vessels with 

 help of the digestive ferments obtained from the stomach 

 (pepsin) or pancreas (trypsin) of animals.* 



It appears from Kiihne and Chittenden's investigations 

 that a series of soluble and diffusible products are formed 

 from each albuminoid with progressive diminution of 

 carbon and increase of oxygen, and, in some cases, of 

 nitrogen. The first-formed products are termed pro- 



* Reference may bo had to riiittondcn's Studies in Physiological 

 ChemlBtrjr, Connecticut Acad., Vols. 11 and III, 1887 and 1889. 



