82 EMBRYOGENESIS IN PLANTS 



bryophytes suggests that the process can be envisaged as a sequence of 

 biochemical phases of increasing complexity, the later phases being 

 characterised either by a greater variety of metabolites, or more highly 

 elaborated ones, or both. The embryo is dependent on the parent 

 gametophyte for all the substances — minerals, carbohydrates, nitrogen- 

 containing and possibly growth-regulating substances — necessary for 

 its growth. The fertihsed ovum as an active centre of metabolism may 

 itself be a source of growth-regulating substances. These and other 

 substances may diffuse basipetally and induce effects in the gameto- 

 phyte. Now, if the substances supplied to the embryo from the gameto- 

 phyte remained unchanged both quantitatively and qualitatively, it 

 would be reasonable to expect that the embryo would continue its 

 vegetative growth indefinitely and give rise to an elongated cylinder, or 

 axial structure. If the substances increased in amount, an obconical 

 axis would result. But, in fact, after a brief embryonic or vegetative 

 phase, the growth and segmentation of the distal apex cease, and, by 

 a succession of what appear to be closely regulated developments, the 

 sporogenous capsule becomes differentiated. Accordingly, as a working 

 hypothesis, it is now suggested that the sequence of distinctive histolo- 

 gical developments is determined by a sequence of changes in the 

 underlying reaction system. For any bryophyte species, the fidelity with 

 which these developments take place is a very remarkable thing. On 

 the hypothesis proposed above, how, in fact, can we account for the 

 regulation of the underlying biochemical processes ? 



In terms of contemporary genetical theory, the explanation might 

 take the form that different genes, or sets of genes, are successively 

 evoked during the ontogenetic development of the sporophyte. At any 

 particular stage, as a result of the biochemical action of particular 

 genes (or gene groups) on the cytoplasm of the embryonic cells, 

 characteristic growth and histological developments will ensue. In 

 relation to the new situation thus produced, the action of other genes 

 will be evoked; and so on, each phase preparing the way for the 

 succeeding one, the whole development being characterised by an 

 orderly and distinctive sequence of physiological events; or, in other 

 words, by the orderly sequence of metabolic activities in a gene- 

 determined reaction system. These become manifest in the successive 

 morphological and histological developments which characterise the 

 ontogeny of the species. While this kind of general statement may 

 approximate to the truth, it tells us little of the processes that are 

 involved; for our ultimate aim in the study of embryogenesis is to 

 explain, in physiological terms and in as much detail as possible, how 

 the orderly morphological and histological developments are brought 

 about. The basic assumption that is made here is that genes are the 



