FACTORS IN EMBRYOGENESIS 13 



the environment of the egg, the nutrition of the young embryo, the 

 constitution and effect of the adjacent tissue, and the stimuli and 

 mechanisms which may be involved in the processes of morphogenesis 

 and organisation. Nowhere in the botanical literature has an equivalent 

 body of information been gathered together — indeed the most recent 

 books on plant embryology make virtually no reference to these 

 important and fundamental topics — but some relevant data are avail- 

 able and these will be dealt with in their proper place. 



When one contemplates the many distinctive developments that 

 ensue when fertilisation of the ovum has taken place, or when a spore 

 begins to germinate, one cannot fail to realise that, for any adequate 

 understanding of embryogenesis, the existing morphological data must 

 be vastly supplemented by physiological observations. The morpho- 

 logical data merely illustrate the results of embryogenic processes: 

 they tell us little about the underlying causes. But, meanwhile, they 

 constitute the greater part of our information and they are, in fact, the 

 foundation on which the new knowledge must be built. 



POLARITY 



In thallophytes, bryophytes, pteridophytes and seed plants, the 

 earliest perceptible embryogenic development is the estabhshment of 

 polarity. As Bowar (1922) has shown, a characteristic, filamentous, 

 axial or spindle-like development is found in the embryogeny of all 

 classes of plants. The formation of this 'primitive spindle' appears to 

 be a fundamental feature in the embryogeny of plants and affords 

 evidence of the early establishment of polarity in the zygote. There is 

 clearly need for a searching inquiry into the phenomenon of polarity; 

 thus far we have little exact information on the factors which determine 

 it. Here it should be noted that a filamentous development, with early 

 estabhshment of polarity, is to be observed in spore germination just 

 as in the developing zygote (Figs. 1a, b, e, f; 2c; 8a, b, c, d; 9; 

 10a, b). 



The position of the first partition wall in the enlarging zygote has 

 assumed great importance in the literature of embryogeny, for, by the 

 time this wall is laid down, the polarity of the young embryo has been 

 irrevocably determined. Whether polarity is established in the growing 

 but still undivided zygote, and the first dividing wall is consequentially 

 laid down at right angles to what will become the long axis, or whether 

 the position of the first wall is the actual determinant of polarity, cannot 

 yet be decided on the evidence, but several indications favour the former 

 view. In Psilotum, Tmesipteris, Lycopodium, Selaginella, Equisetum, 

 some ferns, and in bryophytes, the first division of the zygote is typically 

 at right-angles to the neck of the archegonium ; but in leptosporangiate 



