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(or 40) chroinosonics arise. Again there are parallel variants in the 

 different stocks. Thus each stock of apomicts is a complex and 

 self-sustaining system of variation (and adaptation) outside the 

 ordinary sexual system, but nevertheless owing its character to an 

 imperfect suppression of meiosis, whose imperfection inherently 

 provides the means of perfecting itself (Fig. 68). 



The breakdown of sexual reproduction comes about in many 

 other ways. One further example will illustrate certain useful 

 principles. In the flowering plants, as a rule, every embryonic seed 

 or ovule contains only a single embryo-sac mother-cell. This cell 

 gives by meiosis four haploid cells or spores. Only one of these 

 spores can, as a rule, develop into an embryo-sac and, as we have 

 seen, the spores may compete for this opportunity, thereby giving 

 the Rermer effect. The embryo-sac in its turn contains a number 

 of cells, only one of which can, as a rule, develop into an egg capable 

 of fertilization. The choice of the egg cell therefore depends on 

 a series of choices during the development of the ovule (Figs. 50 

 and 51). 



Many and entertaining volumes have been written in a language 

 of their own to describe variations and exceptions in this pre- 

 destined course of differentiation, even in the circumstances of 

 normal sexual reproduction. We have already had cause to relate 

 the choice of spore in embryo-sac formation to protein and nucleic 

 acid gradients, and the variations and exceptions no doubt also 

 depend on the variable concentrations and distributions of proteins 

 and nucleic acid available in the ovule. But from our present point 

 of view what is important is that the issue is not always exclusively 

 decided and several cells are not uncommonly available for fertili- 

 zation within one or more embryo-sacs; and at the same time 

 some of these, and others outside the embryo-sacs, whether derived 

 from meiosis or not, arc often available for development without 

 fertilization. Between these potential eggs and potential embryos 

 there must be competition for development; competition whose 

 outcome will depend on the two interacting factors of genotype 

 and of position within the ovule. The Renner effect is thus but 

 a special case of a more general phenomenon. 



The consequences of tliis free enterprise in development are found 

 in many apomictic plants. Perhaps they appear best in Poa pratensis. 



268 



