Chapter XVI 

 GENERAL CONCLUSIONS 



GENERAL FEATURES OF EMBRYOS 



SPECIES from the major systematic groups — Algae, Bryophyta, 

 Pteridophyta and Spermatophyta — show marked differences in 

 their size, form, structure and reproduction, i.e. they exempUfy different 

 levels of organisation. And while there is general agreement that these 

 levels of organisation afford evidence of the progressive and continuous 

 evolution of plants, more or less wide 'phyletic gaps' separate not only 

 the major systematic groups but even groups of closer affinity. Having 

 now before us the data of a general survey of plant embryology, we 

 may therefore inquire if there are any organisational features which 

 are common to all embryos. In other words, does the relative simplicity 

 of embryos enable us to perceive essential similarities, and possibly 

 relationships, between organisms which become obscured during their 

 subsequent development to the adult state under the impact of genetical 

 and environmental factors? Such homologies of organisation as 

 embryos may show are, of course, of the greatest interest from the 

 causal point of view, quite apart from their value in taxonomy. 



In the zygotic development of all plants, from algae to angiosperms, 

 the following phenomena are of general occurrence : 



(i) In the mature or newly fertilised ovum, the distribution of 

 metabolites is, or quickly becomes, markedly heterogeneous; and with 

 or without an attendant elongation of the zygote, an accumulation of 

 different metabolites takes place in two diametrically opposite regions, 

 the polarity of the new organism being thereby established. These 

 observations have also a general application to the germination of spores. 



(ii) Where the ovum is enclosed, the physiological activity of the 

 surrounding tissue is probably important in determining its polarity. 

 In the free-floating zygotes of algae, factors in the environment may 

 induce the reactions which lead to the establishment of polarity. 



(iii) In the polarised zygote, the apical or distal pole becomes the 

 principle locus of protein synthesis, growth and morphogenesis; 

 whereas the basal or proximal pole is characterised by the accumulation 

 of osmotically active substances, its cells becoming vacuolated and 

 distended. 



(iv) The first division of the zygote is typically by a wall at right 

 angles to the axis, cell division being probably stimulated by the increase 



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