THE EMBRYO 



2*9 



formed into the single cotyledon, m gives rise to the stem tip, n to 

 the root tip, o to the periblem and a part of the root cap, h to the 

 uppermost layer of the root cap, and s to the suspensor composed of 

 three to six superposed cells. 



The embryo of the Gramineae is so different from that of most 

 monocotyledons that it merits separate attention. In Poa annua 

 (Soueges, 19246), which is the most thoroughly investigated species, 

 the first division of the zygote is transverse and results in the forma- 

 tion of two cells ca and cb (Fig. 164 A). The next division is trans- 

 verse in the basal cell and vertical in the terminal cell (Fig. 164B,C). 



B 



D 



E 



G 



H 



Fig. 163. Development of embryo in Sagittaria sagittifolia. (After Soueges, 

 1931.) 



Of the four cells thus formed, the two at the lower end undergo a 

 further longitudinal division, resulting in the quadrant stage (Fig. 

 164D); and each of the quadrant cells in turn becomes partitioned 

 by a more or less transverse wall to give rise to the two tiers I and 

 V (Fig. 1Q4E). At the same time the upper cell ci divides trans- 

 versely to form n and n' , followed by a vertical division in m and n 

 and a transverse division in n' . As further development proceeds, 

 the tiers I and V give rise to the scutellum and part of the coleoptile ; 

 m gives rise to the remaining part of the coleoptile cl-cl', the stem 

 tip pv, and the periblem and plerome of the root tip; n to the root 

 cap, coleorrhiza, and epiblast eU a ; and oand p, daughter cells of n',to 

 the hypoblast and the suspensor (Fig. 164F-7). 



The most conspicuous part of the grass embryo is the scutellum. 



60 This is a small scale-like structure lying opposite to the scutellum (see p. 429) . 



