252 



VEGETABLE PHYSIOLOGY 



or nodule of secreting cells, which must be regarded as a 

 rudimentary gland, though it is not provided with any 

 definite outlet or duct. 



In the seed of the cereal grasses there is a special organ 

 separating the embryo from the endosperm. This structure, 

 which is a modification of part of the cotyledon, is known 

 as the scutellum (fig. 119 ) ; its function is to effect the 

 absorption of the nutritive material of the endosperm, and 

 supply it to the growing embryo. This scutellum is covered 

 on its outer face, which is in contact with the endosperm, 

 by a layer of cylindrical cells, whose long axis is at right 



FIG. 119. SECTION OF OAT-GRAIN. 

 9, plumule; r, radicle; *, scutellum. 



FIG. 120. SECTION OF PORTION OF 

 SCUTELLUM OF BARLEY, SHOWING 

 THE SECRETING EPITHELIUM. 



angles to the surface (fig. 120). These cells are very 

 granular in appearance, and form a very marked secreting 

 tissue, producing two enzymes, which are subsequently 

 discharged into the endosperm to effect the digestion which 

 must precede absorption. The aleurone layer of the same 

 grain (fig. 121), which has already been described, is also 

 a secreting layer, resembling the outer layer of the scutellum 

 in several respects. 



The tentacles of the leaves of Drosera, to which allusion 

 has already been made, are very definitely secreting struc- 

 tures ; in addition to preparing an enzyme they produce 

 a weak acid, both of which are present in the glairy material 

 that they pour out over the captured insect. These tentacles 



