322 EMBRYOGENESIS IN PLANTS 



Cellular Potentiality. As an embryo develops, its cells and parts, 

 or regions, become differentiated in characteristic ways. On this 

 subject many views have been expressed. Driesch (1908) thought of the 

 different protoplasmic potentialities of the zygote as being very early 

 distributed among the cells of the embryo, cells in certain positions 

 inheriting the particular properties or qualities of specific regions of 

 the ovum: this is the conception of the Cellular Mosaic. But other 

 workers, such as Dalcq (1938, 1941), consider that all meristematic 

 cells may retain their totipotency for a long time and that their 

 specialisation in a particular direction is due to the action of hormones 

 emanating from special centres of growth activity. A majority of 

 botanists would no doubt subscribe to the latter rather than the former 

 view. 



NUTRITION, GROWTH AND ORGANISATION 



A developing zygote is a specific dynamic system, operating in a 

 particular environment. Under normal conditions for the species, this 

 environment — prothallus or embryo sac and nucellus — provides the 

 nutrients used by the embryo during its growth. While the specific 

 constitution and organisation of the zygote will determine, or limit, the 

 kind (or kinds) of pattern that may be developed from it, the particular 

 pattern actually developed may be more or less directly due to factors 

 in the environment, e.g. physiological gradients, the balance of nutrients 

 supplied, and so on. In general, in the matter of its nutrition, the young 

 embryo behaves like a shoot in miniature. 



In a number of lycopods, Selaginellas and ferns, the early embryo- 

 geny is characterised by conspicuous carbohydrate metabolism and by 

 a delay in the organisation of the shoot apex and the axial vascular 

 system. The considerable phase of suspensor development in gymno- 

 sperms may perhaps be ascribed to somewhat comparable nutritional 

 relationships; and instances of extensive suspensor or parenchyma 

 development, with delayed organisation of the shoot apex, are also 

 known in the embryology of flowering plants. These observations 

 suggest that in the early embryogeny the supply of nitrogen-containing 

 metabolites, or of substances essential to protein synthesis and the 

 maintenance of meristematic activity, may be limiting. Overbeek, 

 Conklin and Blakeslee (1941, 1942) and Overbeek (1942) have shown 

 that whereas mature embryos of Datura stramonium are completely 

 self-sufficient in respect of growth factors, and, on simple culture 

 media containing minerals and sugar can grow directly into seedlings, 

 very small embryos must have growth factors added to the medium if 

 growth is to continue. 



