THE FLOWER 



135 



and a short overhanging protuberance arises at the end of the scutellum 

 blade (v, Figs. 10, 107) ; this structure, which is sometimes termed the 

 " cotyledonary " sheath, I regard as the ligule of the scutellum. 



The epithelial layer of the scutellum does not begin to assume its 

 final form until the extension of the scutellum sheath is completed. 



Differentiation of the primary root commences when the embryo is 

 about -5 mm. long. 



Before the embryo is i mm. long the plerome, periblem, and root-cap 



a 



345 6 7 



FIG. 107. Longitudinal section of embryos in different stages of development ( x 50). 

 a, Upper portion of the scutellum (blade of the cotyledon) ; x, intercalary zone of 

 scutellum (sheath of the cotyledon) ; v, " ventral " scale (ligule of the cotyledon) ; 

 c, coleoptile ; /, first foliage leaf ; e, epiblast. 



Length of embryos, i, -25 mm. ; 2, -25 mm. ; 3 and 4, about -5 mm. ; 5, -9 mm. ; 

 6, i-i mm. ; 7, 2 mm. 



are clearly marked out and can be traced back to two or three initial cells, 

 situated in the centre of the undifferentiated mass of tissue which forms 

 the lower half of the embryo (Fig. 108). At this stage the coleoptile and 

 the rudiments of the first foliage leaf are visible round the stem apex. 



Through division of the meristem at the root-tip the length of the root 

 increases and the tissue of the periblem and plerome is augmented at a 

 different rate from that of the root-cap and the surrounding ground tissue, 

 with the result that the adjacent cell walls in this region are torn asunder 

 and a schizogenous cavity is formed round the tip of the root (Fig. 108). 



By further growth in length of the root the cavity is extended and 



