56 EMBRYOGENESIS IN PLANTS 



parallelism between the apical meristems of the Fiicales and the ferns 

 (and also some other pteridophytes, e.g. Selaginella). The dichotomous 

 branching of the thallus in Fucus, Pelvetia and AscophyUum is strongly 

 reminiscent of the dichotomy of the shoot or rhizome seen in ferns and 

 species of Selaginella. But in AscophyUum there is also monopodial 

 branching, and this affords a close parallel with the monopodial 

 branching in leptosporangiate ferns such as Matteiiccia, Onoclea, 

 Dryopteris and Athyhum. In these ferns the lateral branches normally 

 arise later in the development from quiescent bud rudiments, or 

 detached meristems, these having initially constituted part of the apical 

 meristem. (Wardlaw, 1943, etc.). Now, in AscophyUum, Seirococcus 

 and Halidrys, the rudiments of the lateral branches also have their 

 inception in the apical meristem. A longitudinal section parallel to the 

 strap surface of the thallus in AscophyUum or Seirococcus, or through 

 the apex of Halidrys, shows the characteristic apical meristem. Among 

 the meristematic cells are some with conspicuously dense protoplasmic 

 contents. As growth is more rapid in the intervening cells, these dense 

 but somewhat inactive meristematic cells come to occupy depressions in 

 the thallus and are left behind as the apex grows on — a process in many 

 respects closely comparable with the isolation of detached meristems 

 in ferns. Subsequently these meristematic cells may become active and 

 give rise to a lateral branch or branches. In the brown seaweeds, as in 

 the ferns, these lateral apices, and the branches to which they give rise, 

 do not occur at random: they occupy definite and regular positions 

 and hence the adult plant is characterised by a harmonious and dis- 

 tinctive configuration. Halidrys, for example, has alternating branches, 

 while Sargassum has regularly disposed lateral branch systems of some 

 complexity. 



The embryogeny and subsequent development of the Fucales are 

 thus in many respects closely parallel to, and commensurate with, 

 those of vascular plants; in particular, the ferns. The problem of 

 homology of organisation is thus raised in a particularly acute and 

 interesting form, in that it relates to organisms which can have only 

 the most remote taxonomic relationship, if, indeed, it can be held that 

 there is any affinity at all. 



Anomalous Developments. Various anomalous developments have 

 been observed. In some fertilised eggs the nucleus divides first and the 

 rhizoid rudiment develops subsequently; in others, the zygote divides 

 repeatedly but remains spherical, with no evidence of rhizoid formation 

 even in cultures a month old (Farmer and Williams, 1898). The reasons 

 for these developments are not known. As we have seen, this type of 

 development is normal in Himanthalia. Some of the Fucales e.g. 

 Halidrys, are characterised by a delay in the production of rhizoids 



