44 EMBRYOGENESIS IN PLANTS 



exemplifying a kind of embryogeny, though it is of a somewhat unusual 

 kind for plants. Similar observations apply to the reproduction of 

 organisms such as Scenedesmus or Pediastrum. Initially the several 

 products of cell division may be distributed at random within the con- 

 taining wall, but soon, in relation to the forces at work in the system, 

 the cells become arranged in a characteristic symmetrical pattern. This 

 kind of development in the algae reaches its highest expression in the 

 water-net, Hydrodictyon {see Fritsch, 1935). When the haploid poly- 

 hedral resting spore germinates, a phase of abundant cell division ensues, 

 and a very large number of separate cells, actually zoospores, are 

 formed within an enlarged envelope or vesicle. These zoospores are 

 initially distributed at random within the vesicle but after some time 

 they become arranged in a regular pattern; in fact, they form a minute 

 hollow cylinder with a net-like construction. On further growth this 

 structure becomes a new Hydrodictyon plant. 



BROWN ALGAE 



In general, the zoospore or swarmer on settling down grows into a 

 simple filament which may be procumbent or erect; or it may at first 

 be procumbent and branched, soon giving rise to one or more erect 

 filaments. Thus, either from the outset, or at an early stage, there is 

 evidence of a polarised development. Fig. 10. This is also true of the 

 developing zygote. While some brown algae are simple or branched 

 filamentous structures, many show a greater degree of elaboration. 

 Thus we recognise uni-axial, multi-axial and true parenchymatous 

 types, some of the last reaching a comparatively high level of differentia- 

 tion for aquatic organisms. The larger and more elaborate forms also 

 begin as simple filaments, their ontogeny being characterised by a 

 regular sequence of growth phases and by a distinctive segmentation 

 pattern. Thus far, these phases have received comparatively little 

 attention from the standpoint of morphogenesis. The inception of the 

 large apical cell in the Sphacelariales and its regular growth and seg- 

 mentation suggest problems which, in some respects at least, are 

 comparable with those of the apical meristem in the pteridophytes. 

 Again, in the corticated and multi-axial types, the parts and members 

 are formed in an orderly and characteristic manner. These con- 

 figurations, however, do not result from the segmentation of a coherent 

 meristematic tissue, as in Fucus or a higher plant, but from the co- 

 ordinated growth of more or less separate filaments, the development of 

 which is apparently regulated in various ways by their mutual contiguity. 

 This is an aspect of morphogenesis of which we have very little know- 

 ledge. It is important in the fungi and lichens just as it is in the complex 

 filamentous algae. 



