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 proteims 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 
Gea! dines the ereptases, decompose peptone with the 
formation of the same amino- and amido-acids. 
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