52 EMBRYOGENESIS IN PLANTS 



above, a redistribution of materials takes place within the zygote: a 

 superficial pellicle is soon formed, while later a pale or colourless 

 rhizoid grows out at a particular locus, affording evidence that the 

 materials of the zygote have now a heterogeneous and polarised 

 distribution. The estabhshment of polarity is, in fact, accompanied if 

 not caused by a characteristic and persistent distribution of the sub- 

 stances essential to growth and morphogenesis. Whether there is also 

 some attendant arrangement or orientation of the 'fine structure' of the 

 protoplasm, i.e. of the long-chain protein molecules, is not known. 

 Since the rhizoid grows out from the least illuminated side, and also 

 from the side which is exposed to the highest concentration of diffusate 

 from other zygotes, its downward growth towards the substrate follows 

 naturally. In different natural circumstances, i.e. in the several habitats 

 and environments in which the species of the Fucales grow, some 

 diversity in the early ontogenetic development is to be expected, and 

 this may be accentuated by genetical differences. As a fact, different 

 segmentation patterns are found in different genera and species. Thus, 

 in Sargassum UnifoUum, Fig. 11k, l, the rhizoidal cell is initially small 

 and inconspicuous, while, at an early stage, a median longitudinal wall 

 divides both the upper and lower regions of the pear-shaped zygote. 



When the zygote of Fucus divides, the first transverse wall separates 

 the rhizoid from the spherical region, Fig. 2b. As the rhizoid continues 

 to elongate, a second cross-wall is laid down; simultaneously a trans- 

 verse wall also divides the spherical region into an upper (or distal) 

 and a lower (or proximal) segment. Fig. 3d, the former being normally 

 somewhat larger than the latter. The embryo now consists of a row of 

 four cells. A longitudinal wall next divides the upper segment, and a 

 second longitudinal wall at right-angles soon follows, the distal region 

 of the embryo now consisting of four quadrants, Figs Id, 11f. At this 

 stage the segmentation pattern closely resembles that of certain 

 archegoniate and seed plant embryos. A phase of rapid growth now 

 follows. Further transverse and some longitudinal divisions take place 

 in the rhizoidal portion while the distal segment divides by a transverse 

 wall, so that two quadrant tiers are now present, Periclinal walls are 

 now laid down. Fig. 11 G, the outer or peripheral cells becoming photo- 

 synthetic and histologically distinguishable from these of the central 

 region. Meanwhile the rhizoid has elongated considerably, divided by 

 walls which may be oblique and, in response to tactile stimuli, has 

 become attached to the substratum. The primary rhizoid is later 

 supplemented by secondary ones, a compact holdfast system being 

 formed. All these basal cells, however, have the same physiological 

 character, and differ from those which constitute the body of the young 

 plant above. 



