50 PROTEINS 



residue forming so-called " oil-cake," which is extensively used 

 for the feeding of cattle. Various vegetable oils are, moreover, 

 employed in the manufacture of soap. 



In the plant the fats are formed from carbohydrates and, since 

 they contain a much smaller amount of oxygen than the latter, 

 the volume of carbon dioxide evolved during their formation is 

 considerably greater than that of the oxygen taken in. Con- 

 versely, when fats undergo change into carbohydrates during 

 the germination of a seed, a large quantity of oxygen is absorbed 

 in proportion to the carbon dioxide evolved. In correspondence 

 with their low specific gravity, fats are a frequent form of non- 

 nitrogenous food-reserve in seeds depending on wind-dispersal. 



The carbohydrates or fats found in the various storage-organs 

 are always accompanied by nitrogenous food-reserves, the 

 most important and widespread of which are the proteins. We 

 have already seen that very complex combinations of proteins 

 are organised to form the living protoplasm, but simpler proteins 

 often occur as non-living constituents of the ordinary vegetative 

 cells, and are especially abundant in the diverse storage- organs. 

 In the former case they may either be dissolved in the cell-sap 

 or appear as crystal-like bodies, termed crystalloids, which may 

 even be lodged in the plastids or nuclei. Succulent storage- 

 organs, such as tubers, often likewise contain dissolved proteins, 

 or these may take the form of crystalloids, as in the outer layers 

 of a Potato ; but not uncommonly a considerable part of the 

 nitrogenous matter in these cases is a mixture of simpler com- 

 pounds known as amides (e.g. asparagin in the Potato and 

 glutamin in the Beetroot). 



In seeds proteins generally occur as small grains which are 

 well seen in the cotyledons of a Pea or the endosperm of the 

 Castor Oil. If a section of the former be treated with iodine, 

 the minute protein granules take on a brown colouration, in 

 sharp contrast to the blue or blackish starch-grains with which 

 they are intermingled. On warming a section in a few drops of 

 Millon's reagent (Appendix II), the whole assumes a brick-red 

 colour which microscopic examination shows to be due to the 

 proteins. Heating with concentrated nitric acid gives a yellow 

 colouration which, on addition of ammonia, changes to orange 

 xanthoprotein reaction). 



