136 



THE WHEAT PLANT 



ultimately appears in the completely formed embryo of the grain as a 

 cylindrical space surrounding the root (Fig. 10), the ground tissue of the 

 lower part of the embryo, within which the root has developed, now 

 constituting the coleorhiza or root-sheath. 



(vi.) Integuments and Pericarp. During the growth of the ovule and 

 seed the two integuments undergo considerable change. The outer 

 coat, like its inner companion, at first consists of two cell-layers (c, d, 

 Fig. 109). 



About the time when the fertilised ovum commences its division 

 it begins to degenerate ; its cells lose their turgidity, the cytoplasm 

 disappears, and the elongated nuclei become thinner. Later, the 



x 



a 



FIG. 1 08. A, Longitudinal section of young embryo ( x 175) ; c, coleoptile ; /, first 

 foliage leaf ; e, epiblast ; r, stem apex ; x, initial cells of the periblem of the root ; 

 a, schizogenous cavity ; B, longitudinal section of the root and root-cap region of 

 the embryo ; slightly later stage than A ( x 140). 



cell-contents disappear entirely, the delicate walls shrivel, and are finally 

 disorganised altogether. 



The inner integument, however, remains, and retains its vitality until 

 the grain has reached the maximum size. The cells of its two layers 

 up to this stage of development remain quite distinct and increase in 

 length with the growth of the seed, the long axes of the cells of the separate 

 layers crossing each other at an acute angle. 



A short time before the milk-ripe stage (p. 140) the cell- contents of 

 the outer layer gradually disappear, a semi-permeable cuticle is developed 

 on the outer surface of the layer and the cells collapse (3, c, Fig. 109). The 

 cell-cavities of the inner layer are recognisable for some time longer, and 

 within them, in yellow and red-grained wheats, is formed a yellow or 

 reddish-brown oily or resinous substance, which is partially absorbed by 



