270 EMBRYOGENESIS IN PLANTS 



consists of a spherical, and in PhaJaenopsis of a club-shaped or ellipsoi- 

 dal, mass of meristematic cells, showing no evidence of organ or tissue 

 differentiation, and enclosed in, or surrounded by, embryonal tubes. 



THE GRAMINEAE 



The grasses exemplify the most complex embryonic developments 

 in plants. The embryo, with the scutellum, coleoptile, epiblast and 

 coleorhiza as special developments, and the homologies of these 

 members, have been the subject of investigations and discussions 

 extending over many years, beginning with Malpighi in 1687 {see 

 Bennett (1944), Artschwager and McGuire (1949), and Reeder (1953), 

 for indications of the literature). 



Coulter and Land (1915) held that the scutellum is a lateral organ, 

 the equivalent of a foliage leaf. Worsdell (1916) thought of it as a 

 terminal laminate cotyledon, the coleoptile being its ligule. Sargant 

 and Arber (1915) favoured the view that the coleoptile is the equivalent 

 of a pair of fused stipules. These are but some of the views to which 

 consideration of the grass embryo has given rise. 



The embryogeny of Po«, Fig. 71, may be indicated as a fairly regular 

 and representative type for the Gramineae. In other genera the initial 

 cellular pattern may be considerably less regular. At the two-celled 

 stage, the terminal cell, as in maize, may be small, lens-shaped and 

 semilateral in position (Randolph, 1936). It may divide vertically or 

 obliquely, the latter development leading to the formation of an apical 

 cell of temporary duration, Fig. 72. The subsequent divisions are 

 irregular. The embryo becomes club-shaped; it is narrow at the sus- 

 pensor end and shows a gradient of decreasing cell size into the terminal 

 growing region. 



In a comparative study of embryo development in Zea mays. Arena 

 saliva and Triticum vulgar e, Avery (1930) has concluded (i) that the 



Fig. 71. Grass Embryos 

 Poa annua. A, First division of the zygote. B, C, Later stages in the develop- 

 ment of the segmentation pattern. D. The differentiation of the organs is about 

 to begin. E, An older embryo. The lettering is that of Soueges. The distal, or 

 apical, segment of the zygote is ca, and the basal segment is cb. As the embryo 



develops 

 ca ^- q -^ 1,1^ 



cb -^ m + ci -^ m + n + }0- -^ m + n + o + p 



In E, cl is the coleoptile, eb, the epiblast, and pr the region of the shoot apex. 



(from Maheshwari ; after Soueges). 



F-O, Honlenm safinim. F, Fertilised egg. G-K, Development of segmentation 

 pattern. L, M, Beginning of organ formation; sc, early stages in the formation of 

 the scutellum; .?, nascent shoot apex; r, nascent coleoptile; .w, suspensor. N, O, 

 Older embryos showing the aforementioned parts and also the first leaf (/), the 

 scutellar vascular trace (ct), the root (;), the second or seminal root (,?/), and the 

 coleorhiza (F-K, >: 375; L, M, x 165; N, O, x 54; after Merry). F. Zea mays. 

 Differentiation of plumule-radicle axis; lettering as above (x 60, after Randolph). 



