WALL STRUCTURE IN THICK CELL WALLS 109 



Assuming that the run of the chains is the same both in the continuous 

 Hght series and the controls (and the run of the striations is, in 

 fact, the same) then the ratio of the intensity of the meridional 

 the equatorial arcs gives a relative measure of the proportion 

 of the chains in the wall which are transverse. In the one case 

 examined — marine Cladophora, both in the Leeds laboratory and 

 at Cullercoats — this ratio is significantly higher in continuous light. 

 Plate V, Fig. 1, gives a forceful illustration of this point. Two 

 other types of observation further substantiate the resulting con- 

 clusion, that under constant light conditions more chains are laid 

 down in the transverse direction than in the longitudinal. Thus, when 

 cells grown under normal conditions are examined between crossed 

 Nicols the (double, upper and lower) walls seen in face view are non- 

 birefringent. In cells from filaments grown for some months in con- 

 tinuous light, however, the walls are strongly birefringent and the 

 birefringence is negative. The non-birefringence in the former case is 

 due to the presence of chains in approximately equal amounts in two 

 directions almost at right angles. In the latter, therefore, the negative 

 birefringence means that more transverse chains are present. Again, 

 the morphology of cells grown in constant light is rather different 

 (sometimes very different) from normal; numerous swellings appear in 

 the otherwise cylindrical filaments, and this can only mean a disturb- 

 ance in wall structure such as we have in mind now. 



Complete confirmation of this point must await further results from 

 cells which have grown only in continuous light, but at the moment the 

 evidence does seem rather convincing. Nothing is known as yet con- 

 cerning the way in which light is associated with this switch in wall 

 structure. It seems rather clear that the periodic change in wall structure 

 must imply a corresponding periodic change in the surface of the cyto- 

 plasm which is in contact with the wall and which is presumably 

 instrumental in defining the orientation in the wall. This is a point we 

 shall return to again. 



Group 2. Halicystis, Hydrodictyon, etc. 



The second group of algae which should come under notice here can 

 in general be typified by Halicystis just as the last group was typified 

 by Valonia. Halicystis resembles Valonia in consisting of very large, 

 solitary cells which do not, however, reach the size of the latter. One 

 might therefore expect a priori that the wall structure would be approxi- 

 mately the same. This, however, is not so; there is at least one very 

 marked difference between this group and the last. Whereas the walls 



