274 EMBRYOGENESIS IN PLANTS 



entirely irregular manner. About the eighth day a rapid differentiation 

 sets in: an epidermis forms over the distal region and extends down the 

 sides towards the suspensor; in the sub-distal region the cells begin to 

 divide actively by walls at right-angles to the proembryo axis. 'This is 

 obviously a preliminary step in the differentiation of the main axis of 

 the more mature embryo, an axis which does not coincide with the 

 axis of the proembryo' (Randolph, 1936). In the now rapidly enlarging 

 embryo a group of more densely protoplasmic cells is present in a distal 

 lateral position on the anterior side. The rapid growth of the most 

 terminal cells results in the formation of the organ later recognised as 

 the scutellum or cotyledon, and the densely protoplasmic lateral group 

 comes to occupy a position progressively further away from the distal 

 extremity. This lateral meristematic cell mass becomes wedge-shaped, 

 the individual cells grow and divide rapidly, and the region becomes 

 organised as a slightly protruding shoot apex and an endogenous root, 

 this axis being somewhat oblique to the proembryo axis. Fig. 72. 

 About this stage the incipient vascular strand of the posterior-terminal 

 organ, i.e. the scutellum or cotyledon, becomes conjoined with the 

 plumule-radicle axis. Meanwhile the scutellum has enlarged greatly 

 and expanded all round the posterior side of the plumule-radicle axis. 

 These observations suggest that the shoot apex originates distally, but 

 since it develops very slowly as compared with the cotyledon it eventu- 

 ally occupies a lateral position. Like the enlarged foot in lycopod 

 embryos, the greatly enlarged scutellum in the Gramineae is formed 

 adjacent to the endosperm, its strong development being probably 

 related to its proximity to a major source of nutrients. At maturity the 

 plumule-radicle axis is virtually parallel to the scutellum. The suspensor 

 region stops growing soon after the differentiation of the radicle and 

 persists as a vestigial organ. The coleoptile is described as the first 

 ridge of tissue which develops above and later around the shoot 

 meristem. The first plumular leaf arises as a similar ridge on the 

 opposite side of the apical meristem; and eventually three to five 

 additional leaves are formed. 



Akhough Merry (1941) regards the scutellum and coleoptile in 

 Hordeum sativum as 'structures morphologically distinct from the 

 foliage leaves and peculiar to the embryo,' he holds that the scutellum 

 is homologous with the cotyledon in other monocotyledonous embryos 

 —a view well supported by his illustrations. In Paspalum dilatatum, 

 Bennett (1944) also found that the planes of cell division in the young 

 embryo are irregular, confirming the view that it is the growth of the 

 embryo as a whole that is important. Norner (1881) and Soueges (1924), 

 on the other hand, attempted to refer particular embryonic parts to 

 particular cells in the early embryogeny. The mature embryo in 



