326 EMBRYOGENESIS IN PLANTS 



the land. The brown algae, and in particular the Fucales and Lamina- 

 riales, afford examples of organisms which are not closely restricted 

 in their size development: they have attained to a considerable level 

 of somatic organisation and in their embryogeny are not unlike plants 

 considerably higher in the evolutionary scale. 



In their sporophyte and gametophyte generations the bryophytes 

 also show genetically-limited growth development. In the development 

 of a moss zygote, the cell divisions are such as to define an apical cell 

 and this cell grows and divides as if a vegetative axis was about to be 

 formed. After a few divisions, however, apical growth virtually 

 ceases, the further developments resulting in the organisation and 

 maturation of the capsule. This is the general pattern of bryophyte 

 development. An interesting exception is provided by Anthoceros. 

 In this genus {see Chapter V), although the vegetative growth of the 

 distal region of the sporophyte soon stops, the sporophyte continues 

 to elongate by intercalary growth just above the haustorial base. 

 Comparisons have been made between the sporophytes of Anthoceros 

 and of some vascular plants, and it has been suggested that this genus 

 affords common ground between bryophytes and pteridophytes. It is 

 very doubtful if this view can be upheld. The kind of bryophyte which 

 could have been a forerunner of the Pteridophyta, if we assume the 

 real existence of such a relationship, would have been one with a 

 thalloid gametophyte and a robust embryogeny in which the apical 

 growth persisted for some considerable time, i.e. before the sporogenous 

 phase supervened. No bryophyte possessing this organisation is 

 known; indeed, such an organism would have been a nascent pteri- 

 dophyte, probably resembling Rhynia major. It may thus be suggested 

 that during the Early Palaeozoic period there was a group of advanced 

 and actively evolving green algae which possessed a nascent arche- 

 goniate organisation — this common organisation being subsequently 

 retained, though modified in various ways, in all plants above the algal 

 level. The Bryophyta would include those organisms in which there 

 was a constitutional restriction of apical growth in the sporophytic 

 development, while the vascular plants would include all those in which 

 there was no such restriction. From the foregoing considerations it 

 will be seen that the embryological evidence is not contrary to the 

 conception of a common ancestry for archegoniate plants. 



We may now consider how embryological study affects the old 

 conception of Pteridophyta. While the embryos in the several con- 

 stituent classes have some general features in common, they also show 

 characteristic differences. The embryogenies of Lycopodium and 

 Selaginella are closely comparable and so also are those of euspor- 

 angiate and leptosporangiate ferns. But, on the embryological evidence, 



