Chapter 11 

 FACTORS IN EMBRYOGENESIS 



THE ZYGOTE OR SPORE AND ITS ENVIRONMENT 



IN studying embryogenesis in any plant, our initial concern is with 

 the growth of a single cell, a zygote or a spore, in its particular 

 environment. In some, perhaps in all, developing sporophytes we 

 probably ought to begin with the unfertiHsed ovum ; for some of the 

 orderly changes that take place during the ontogenetic development 

 may already have been determined by the cytoplasmic organisation of 

 the ovum. If we examine the ovum, whether in an embryo-sac or 

 archegonium, there is evidence that it has undergone some specialised 

 development, e.g. in the archegonium it becomes rounded off and lies 

 free in the venter, and it is a reasonable assumption that it may be 

 affected differentially by various biochemical and physical factors in its 

 immediate environment. In naked, fertilised or unfertilised ova, such 

 as those of Fucus, there is apparently no specialised cytoplasmic 

 organisation which determines the polarity of the young embryo : the 

 polar axis may be established in any direction and it is not necessarily, 

 or invariably, determined by one particular factor or group of factors 

 (Whitaker, 1940). On the other hand, the extruded but still attached 

 ovum of Laminaria gives rise to an embryo whose axis coincides with 

 that of the oogonium. 



We still know very httle about the organisation either of the unfer- 

 tilised or the fertilised egg in plants. Nevertheless, the post-fertihsation 

 developments, especially in archegoniate and seed plants, are such as 

 to indicate that the organisation of the ovum and the zygote is affected, 

 and its subsequent development determined, by factors in the immediate 

 environment, i.e. by the enveloping gametophyte tissue, as well as by 

 physical factors such as gravity. In its most fundamental aspect, the 

 organisation of the ovum and the zygote is due to genetical factors; 

 for in so far as the genes determine the chemical constitution of the 

 reaction system, they also determine the kind of cytoplasmic frame- 

 work or organisation that is possible. 



The position of the ovum in the gametophyte tissue may be 

 important. In some pteridophytes the archegonium typically points 

 downwards, in others, laterally, and in yet others, upwards. These 

 several orientations affect the subsequent embryogenic development. 

 It is probably by the action of biochemical factors, however, that the 



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