DIGESTION 257 



the cells of the endosperm, detaching them from each other 

 and giving a curious mealy character to the grain. Its 

 presence was first suspected in the Date-palm, where large 

 reserves of cellulose are found in the hard cell-walls of the 

 endosperm. The embryo dissolves these walls and absorbs 

 their products, the work being effected by an epithelium 

 which covers the part of the cotyledon which remains in 

 the seed during the early processes of germination. This 

 epithelium is composed of elongated cells arranged in a 

 manner resembling that characteristic of those which form 

 the secreting layer of the scutellum. It has recently been 

 shown that cytase is formed in the embryo, probably in this 

 layer, and passes thence into the endosperm. The amount 

 of it that can be detected is very small, however, and the 

 process of the ' decomposition of the cellulose is very slow 

 and gradual. Cytase exists in considerable quantity in 

 some of the higher fungi and in certain bacteria. 



Pectase has recently been found to be very widespread 

 in plants. Its function is not very clear, but it may assist 

 cytase in the swelling up of the cell-wall which is antecedent 

 to solution. It is recognised by its power of forming vege- 

 table jelly from the pectic substances of the cell- wall. This 

 jelly appears to be a compound of pectic acid and calcium. 



The enzymes which digest proteins are frequently on 

 that account spoken of as proteoclastic enzymes. There are 

 three main classes of them known at present. The first, the 

 peptases, represented by the pepsin of the stomach of the 

 higher animals, converts albumins, globulins, and certain 

 insoluble proteins into peptones, several intermediate bodies, 

 known as proteoses or albumoses, being formed during the 

 process. The members of the second group, the tryptases, 

 which may be represented by the trypsin of the pancreas, 

 carry the digestion further and split up certain peptones into 

 amino- and amido-acids, of which the chief that have been 

 observed are leucin, tyrosin, and asparagin. Those of the 

 third class, the ereptases, decompose peptone with the 

 formation of the same amino- and amido-acids. 



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