EMBRYOGENESIS IN THE BRYOPHYTA 83 



primary factors determining the inception and the specific character 

 of the biochemical phases that underhe the morphological development. 

 At any particular stage, the state of the organismal reaction system 

 regulates the entry of particular genes, or their products, into the system. 

 The state of the system at any moment is determined or regulated, by 

 its antecedent state, by the contemporary supply of reacting substances 

 to it, and by environmental conditions. The parent gametophyte, as 

 the sole source of nutrients for the young bryophyte embryo, is 

 evidently very important. (In this connection the experimental investi- 

 gation of gametophyte-sporophyte relationships, using modern surgical, 

 tissue culture and biochemical techniques, may be indicated as affording 

 real scope for advancing our knowledge of embryogenesis.) Important 

 as gametophytic and environmental factors may be, however, the 

 embryo, from a very early stage, is in many respects a self-determining 

 entity. The cumulative effect of the points indicated above is to show 

 that, for any species, the regularity and specific character of the 

 embryonic development are to be attributed to a whole nexus of 

 physical and biological factors and relationships, each of which requires 

 individual investigation. 



The importance of genetical factors is also seen in the early transition 

 from the embryonic to the sporogenous phase and in the limited growth 

 and differentiation of the sporophyte as a whole. 



Phylogeny. In considering the problems of phylogeny, the Ricci- 

 aceae are of special interest because their sporophytes are structurally 

 the simplest in all the bryophytes. A basic question is whether this 

 simplicity is of a primitive or derivative character. Bower (1908, 1935) 

 and Campbell (1940) favoured the former view, and saw in the small 

 and relatively undifferentiated sporogonium of Riccia, which is 

 apparently almost completely dependent on the gametophyte for its 

 nutrition during development, an early stage in the upgrade develop- 

 ment of an antithetic or intercalated sporophytic generation. Goebel 

 (1930), Evans (1939) and Bold (1948), on the other hand, see in the 

 Ricciaceae the culmination of a reduction series which can be traced 

 throughout the Marchantiales. Evans has pointed out that it is 

 erroneous to describe the simpler sporophytes in the Jungermanniales 

 and Marchantiales as being devoid of chlorophyll and therefore 

 entirely dependent on the gametophyte for organic nutrients, as well 

 as for inorganic salts and water. Bold (1938, 1948) has emphasised that 

 chloroplasts have long been known in the sporophytes of some genera 

 and he has shown that some chlorophyll is present even in the small 

 and feebly differentiated sporogonia of Riccia, Ricciocarpus and other 

 genera. As these sporophytes are not entirely dependent, or 'parasitic,' 

 on the gametophytes which bear them, he considers that the evidence 



