ORGANIC SUBSTANCES AS PLANT FOOD 225 



This protease, however, in contradistinction to invertase, is not 

 excreted by the yeast into the surrounding medium but acts only 

 within the cells. It has been called, therefore, endotryptase. 

 Since plant proteases belong to the trypsin type, as a result of 

 autolysis, there is secured a mixture of amino acids. In their 

 quantitative composition, they correspond to the mixture which is 

 obtained through the hydrolysis of seeds by mineral acids. There 

 are found in plants also proteolytic enzymes of the nature of pepsin 

 (peptases). They are especially easily detected in insectivorous 

 plants, where they serve in the digestion of captured insects. 



The physiological significance of the splitting of proteins during 

 the germination of seeds consists in the fact that these proteins 

 first of all must be transformed into a conveniently movable form, 

 since they must be transported from the reserve tissues, such as the 

 endosperm and the cotyledons, into the growing parts of the devel- 

 oping embryo. Moreover, reserve proteins are only slightly 

 soluble or entirely insoluble, and, even in a dissolved form, they are 

 unable to pass through the cell membranes, owing to their colloidal 

 nature. Another no less important result of the breaking down of 

 proteins into amino acids is their loss of specific characters. Every 

 protein being characterized by a combination of amino acids specific 

 to it, it is not possible therefore to change one protein directly into 

 another. In order to do this, it is necessary first to decompose it 

 into its simplest integral parts and only then may they be com- 

 bined into a new protein molecule. This is attained by a complete 

 destruction of the protein molecule into the end products of its 

 hydrolysis. 



It is especially convenient to observe the disintegration of pro- 

 teins in seeds when germination takes place in the dark, to prevent 

 the processes of photosynthesis. It has been shown that over two- 

 thirds of the total proteins are of the form of simple compounds. 

 This, in spite of the fact that germination, even in the dark, is con- 

 nected with the growth of the seedlings, thus producing a consider- 

 able increase in the number of living cells and, consequently, an 

 increase in the total mass of living protoplasm. In an experiment 

 with vetch seeds germinating in the dark, Prianishnikov observed 

 that after 10 days of germination 54 per cent of the protein sub- 

 stances found in the grain remained undisintegrated ; after 20 days, 

 40 per cent; after 30 days, 33 per cent; and after 40 days, when the 

 young plants were already beginning to die from exhaug 



