46 EMBRYOGENESIS IN PLANTS 



Various examples of early development in the brown algae are 

 illustrated in Figs. Ic, d; 2b; 3d; 6e; 10; 11. In these, as in many 

 others which might equally have been selected, polarity is apparently 

 determined as soon as the germination of the settled swarmer or zygote 

 begins. The growing filament soon shows a distinction of apex and 

 base, the latter usually developing as a colourless rhizoidal structure of 

 limited growth, while the former constitutes the actively growing and 

 photosynthetic region of the plant. In some species, growth results in 

 the formation of an entirely procumbent soma, and in several, perhaps 

 many, brown algae, a cushion or a procumbent system is first formed 

 from which one or more erect filaments grow out, e.g. Cladostephus, 

 Fig. IOe. In this connection Fritsch (1939) has pointed to the im- 

 portance of the heterotrichous soma, comprising procumbent and 

 erect filaments, which is characteristic of all filamentous and larger 

 algae. Thus far, the factors determining the two kinds of development 

 remain almost completely unexplored. So, too, are those which 

 determine whether growth will become localised in a distal apical cell 

 or in some intercalary position. Bocher (1951) has discussed some of 

 the relevant problems from the morphological point of view. 



Among the filamentous brown algae, those species in which a 

 conspicuous and physiologically dominant apical cell gives rise to new 

 parts by a regular system of segmentation are of special interest. Fig. 

 10, c, e. As in vascular plants, this aspect of organisation is evident at 

 an early stage in the embryogeny. In these algae there is evidence of a 

 regulated process of development, as in Chara among the green algae 

 (Fig. 9h-k), the subapical cells giving rise to lateral branches in a 

 characteristic and orderly manner. 



In a limited fashion, the brown algae as a class, though growing in 

 an aquatic environment, show many of the organisational features 

 found in land plants, a point which was duly emphasised by Church 

 (1919). These several points gain in importance when we survey the 

 embryonic developments in the large parenchymatous algae, i.e. the 

 Laminariales and Fucales. 



In Laminaria, Figs. Ic; 11a-c, the ellipsoidal ovum is extruded 

 from the oogonium but remains attached to it. After fertilisation, the 

 zygote elongates, divides by a transverse wall and thereafter forms a 

 multicellular filament, usually lying in the axis of the oogonium. This 

 raises the question, as yet unanswered, as to whether the polarity of the 

 germling is already estabhshed in the egg, i.e. under the influence of 

 gradients or other factors in the oogonial environment, or whether 

 polarity is essentially a post-fertilisation development. The embryo of 

 Laminaria, Costaria, and related organisms, at first filamentous, 

 enlarges into a cylindrical, club-shaped, or flattened body and undergoes 



