324 EMBRYOGENESIS IN PLANTS 



find practically every gradation between species with a well developed 

 suspensor and those in which it is inconspicuous or virtually absent. 

 Sooner or later the angiosperms may be expected to yield evidence as 

 to the kinds of genetical change that determine differences in the early 

 embryogeny; for some of these early differences prepare the way for 

 the very considerable differences between the adult organisms. 



EMBRYOGENESIS AND PHYLOGENY 



In the post-Darwinian or so-called Phyletic Period, botanists were 

 confident that it would be possible to devise a satisfactory, and even 

 fairly complete, phylogenetic classification. It was held that algae, 

 bryophytes, pteridophytes, gymnosperms and angiosperms constituted 

 a continuous evolutionary sequence, i.e. one long-sustained line of 

 descent, protrayed as a genealogical tree consisting of a major stem 

 and lateral branches. Later, however, it was realised that closely 

 comparable morphological and anatomical features might be due to 

 parallel or convergent evolution, and that there might have been evolu- 

 tion along several comparable but quite distinct phyletic lines. Indeed, 

 it is now apparent that many major trends of special interest to 

 biologists, e.g. from isogamy to oogamy in algae and fungi, or from 

 actinomorphy to zygomorphy in floral structure, have taken place 

 independently in unrelated phyletic lines. 



In the Phyletic Period, it was thought that bryophytes and pterido- 

 phytes, both amphibious archegoniate plants, formed part of a con- 

 tinuous line of descent from a filamentous green algal ancestor, the 

 pteridophytes being more progressive offshoots from a bryophyte 

 ancestry. To-day it is agreed that both subdivisions had probably 

 algal ancestors, but it is no longer held, at least with conviction, that 

 pteridophytes were derived from bryophytes : and even if they were, 

 the gap between the two seems unbridgeable on existing criteria of 

 comparison. Some authors have tried to get over this difficulty by 

 saying that the bryophytes have remained at a level of organisation 

 through which the pteridophytes must have passed, the conception of 

 a lineal relationship being thus retained without the necessity of 

 specifying linking organisms. It was also formerly held that the 

 pteridophytes are a coherent group, members of the several classes 

 being vascular cryptogams with the same general reproductive organisa- 

 tion and life history. In contemporary taxonomy the concept of 

 Pteridophyta has been replaced by four major phyletic lines or sub- 

 divisions, i.e. Psilopsida, Sphenopsida, Lycopsida and Filicopsida (or 

 Pteropsida),^ each stemming from some green algal stock, or from a 

 leafless and rootless archetype of vascular plants which was perhaps 



^ For a review of the designation of the vascular plants see Just (1945). 



