PHANEROGAMIA 



457 



cases, however, where the embryo retains in the ripe seed the form 

 of an undifferentiated sphere {e.g. Orobranche, Orchidaceae). 



The number of cotyledons developed is, as a rale, constant and 

 furnishes the most characteristic, although by no means the only method 

 of distinguishing the two divisions of the Angiosperms, which are 

 accordingly termed Monocotyledons and Dicotyledons. 



The embryo shows so much variation, not only in both divisions of the Angio- 

 sperms, but within the different families, that no general scheme of embryonic 

 development can be given. In many Dicotyledons, for example in Oapsella bursa 

 pastoris (Fig. 390), where the development of the embryo is particularly easy to 

 follow, the end of the proembryo farthest removed from the micropyle is converted 

 into a row of cells by the formation of transverse walls. The terminal cell expands 

 into a sphere, and, undergoing division, becomes divided into octants. Each octant 

 cell is further divided by periclinal walls into an 

 outer and an inner cell. The outer cells form the 

 epidermis ; the inner, by continued division, give 

 rise to the fundamental tissue and the vascular 

 bundles. The upper half of the sphere develops into 

 the cotyledons and plumule, the lower half into the 

 hypocotyl and root. The root is derived in part 

 also from the hypophysis, a cell resulting from the 

 transverse division of the next adjoining cell of the 

 suspensor. 



The cotyledons first appear as protuberances 

 from the upper half of the sphere. The plumule 

 does not become differentiated until later. 



In Monocotyledons the single cotyledon is 

 usually developed at the apex of the embryo (Fig. 

 391) ; but in some cases (Dioscoreaceae) it arises 

 laterally, as in the Dicotyledons. 



Adventitious embryos are sometimes pro- 

 duced by both Dicotyledons and Monocotyledons 

 (e.g. Funkia ovata) by the budding of cells of the 

 nucellus in the neighbourhood of the egg -appa- 

 ratus. The fertilised egg, as a rule, does not then 

 continue its development (Fig. 216). In the case 

 of Coelebogyne, adventitious embryos are formed 

 even when no fertilisation of the egg has taken 



place. Seeds in a ripe condition, which contain several such adventitious embryos, 

 afford examples of polyembryoxy. Ovules provided with several embryo- 

 sacs do not exhibit polyembryony, as in that case only one embryo attains full 

 development. 



During the development of the embryo a parenchymatous tissue, 

 termed the endosperm, is formed within the embryo- sac, usually 

 completely filling its remaining free space ; this arises by a process 

 of multicellular formation preceded by free nuclear division (p. 66), 

 or by repeated cell-division. In some species of plants the endosperm 

 is completely disorganised and supplanted by the growing embryo ; in 

 other cases it occupies a larger or smaller part of the ripe seed. 



Fig. 391. — Young embryo of Alisnia 

 Plantago. C, Cotyledon ; v, grow- 

 ing point. (After Hanstein, 

 magnified.) 



