THE EMBRYO 



281 



merit the tier I gives rise to the cotyledons, V to the lower part of the 

 hypocotyl, and m to the upper part of the hypocotyl. The cells 

 originating from ci form the suspensor except the last cell h, which 

 functions as the hypophysis and contributes to the root tip. 



Beta vulgaris (Artschwager and Starrett, 1933), which is also a 

 member of the Chenopodiaceae, is similar to Chenopodium except 

 for a few minor details. Following the four-celled stage (Fig. 154 A), 

 each cell of the proembryo divides again. The cell I always divides 

 by a vertical wall (Fig. 154B-D) ; V may divide either transversely 

 (Fig. 154C) or vertically (Fig. 154B,D), the latter being the more 

 frequent condition; m may also divide either transversely (Fig. 

 154H-E) or vertically (Fig. 154F); and ci divides transversely (Fig. 



Fig. 154. Development of embryo in Beta vulgaris. (After Artschwager and 

 Starrett, 1983.) 



1545 ,C). Thus, depending on the direction of the partition walls 

 in V and m the eight-celled embryo may comprise five, six (Fig. 

 154D), or seven tiers (Fig. 154#). During the next division, which 

 gives rise to the 16 -celled stage, the walls are so arranged as to 

 produce about eight tiers of cells (Fig. 154F). The authors empha- 

 size that no strict law governs the sequence of cell divisions, al- 

 though "a certain balance seems to be maintained so that the final 

 product is remarkably uniform." 



The four-celled proembryo of Myosotis hispida (Soueges, 1923a) 

 also originates in the same way as that of Chenopodium and Beta, 

 but the cell I divides by two oblique walls so as to form a wedge- 

 shaped epiphysis initial e as in Geum, already described under the 

 Asterad type (Fig. 155 A-C). The epiphysis gives rise to the stem 

 tip, while the other cells produced by the tier I produce the coty- 

 ledons. The tiers V and m form the hypocotyl; and ci gives rise 



