42 EMBRYOGENESIS IN PLANTS 



somatic state. There are, however, exceptions, e.g. the development of 

 a thin flat expanse of tissue in Ulva, various developments shown by the 

 Chaetophorales {see below), the highly complex construction in the 

 Siphonales, and the characteristic axis with whorled branching in the 

 Charales, Fig. 9. 



Chaetophorales. According to Fritsch (1945), it is to this order of 

 green algae that we should look for evidence of those developments 

 which may have a bearing on the origin of land plants. Morphological 

 features of special interest in this connection shown by members of the 

 group are the polarised filamentous development of the spore or zygote 

 product, growth by an apical cell, and a characteristic mode of cell 

 division leading to an incipient parenchymatisation, or tissue formation. 

 FritschieUa (Fig. 9) shows some of these developments (Iyengar, 1932). 

 Drapamaldia and Draparnaldiopsis aff'ord evidence of the first steps in 

 cellular specialisation in a simple filamentous organism, in that alternate 

 cells remain short and give rise to the lateral branches. Fritsch has 

 suggested that the higher members of the ancestral Chaetophorales 

 were the plants which successfully colonised the land, giving rise to 

 the Bryophyta and the several classes of vascular plants. However, 

 apart from the general points indicated above, the embryogeny of the 

 Chaetophorales yields no specific clues to that of the Embryophyta. 



Among the Chaetophorales, attention is often directed to Coleo- 

 chaete because of its oogamous reproduction and the elaborations which 

 characterise the post-fertilisation phase. These developments, however, 

 like carpospore formation in the red algae, add nothing to our know- 

 ledge of embryonic processes. 



Volvocales and Chlorococcales. Strictly speaking, these groups have 

 no embryogeny equivalent to that of brown algae, archegoniate and 

 seed plants. Yet the development of the larger colonial forms from the 

 single reproductive cell affords some points of interest that are not 

 without relevance. When colonial species of the Volvocales begin to 

 grow from a germ cell, there is a characteristic phase of repeated cell 

 division. Each of the Chlamydomonas-like cells divides longitudinally 

 with the result that a saucer-like multi-cellular body is formed; on 

 further division this assumes the form of a hollow sphere. This sphere 

 now undergoes a curious 'inversion,' i.e. it is turned outside-in, a 

 process which has the effect of bringing the flagella to the outside of the 

 new colony. Some of these developments are not unlike the early 

 stages in the development of an echinoderm from a fertilised egg. In 

 both instances, biochemical, physical and other factors are at work. In 

 echinoderms, the production of a regular segmentation pattern is 

 recognised as a typical embryonic development ; it may be that we shall 

 not be entirely wide of the mark if we regard the Volvocales as also 



