EMBRYOGENESIS IN THE ALGAE 61 



After the first few transverse divisions of the elongating embryos have 

 taken place, longitudinal divisions follow, these being distinctive for 

 different genera. This is at once evident in a comparison of the 

 segmentation patterns in Ceramium, Polysiphonia, Delesseria and other 

 forms. The close similarity between these embryos and those of the 

 Fucales and archegoniate plants is evident: polarity is determined at 

 an early stage, the distal cell becomes the seat of growth, and the whole 

 embryonic development is characterised by great regularity in the 

 cellular pattern. Further evidence of the existence of important 

 homologies of organisation in the early stages of development is thus 

 afforded by this brief survey of the red algae. 



Like the green and brown algae, many of the red algae show evidence 

 of polarity as soon as spore germination begins. This is particularly 

 conspicuous in the erect type of embryo, seen in the Ceramiales ; it is 

 also seen in the germ-tube type. In the hemispherical and attachment- 

 disc types of germling there is also presumably an early establishment 

 of polarity, though in these organisms polarised or axial development 

 may not be evident for some time; and it is accompanied, or preceded, 

 by quite a different type of segmentation pattern from that seen in 

 filamentous and erect types. Nevertheless, in organisms such as 

 Cystoclonium or Bonnemaisonia it could be argued, on the morpho- 

 logical evidence, that polarity is established as soon as the spore 

 becomes attached to the substratum. If so, then in these organisms 

 there is a departure from the fairly general rule that the first dividing 

 wall is at right-angles to the axis of the embryo. Experiments relating 

 to the effect on polarity of centrifuging have been briefly noted in 

 Chapter II. 



Since the spores are pigmented and photosynthetic bodies, light has 

 an important effect on their development. The food reserves within 

 the spores are apparently relatively small in amount and hence, as 

 Chemin has pointed out, the rapidly growing germs soon show modified 

 and arrested development if kept in the dark. The spores are also 

 readily disorganised by excessive illumination, especially by exposure 

 to ultra-violet radiation. Direct sunlight and temperatures above 35°C 

 also cause disorganisation. 



SUMMARY 



Although at the outset the existence of true embryogenesis in the 

 algae was only tentatively postulated as a working basis, the data which 

 have been set out in this Chapter undoubtedly justify the position which 

 has been adopted: the algae have an embryogeny just as have the 

 higher plants and, indeed, afford many close parallelisms with them. 

 It is true that in some of the algae, especially the green and the simpler 



