WALL STRUCTURE IN THICK CELL WALLS 107 



fresh-water Cladophora (43(c)) and for Chaetomorpha (43(b)) and for 

 Rhizoclonium(43(c))* where, however, the difficulties are still more 

 acute since these filaments grow by intercalary growth so that the 

 X-ray diagrams are, presumably, those of growing walls. The problems 

 involved in the growth of these cells will be examined later. 



Turning therefore from these difficult and, as yet, completely un- 

 solved, problems, note may be taken of recent attempts to explore the 

 complex of factors involved in the periodic "switch" in wall structure 

 found first in Valonia and now in these filamentous forms. The first 

 point of significance is illustrated in Table V. This is one sample of 

 many similar determinations of striation direction, and it will be clear 

 from inspection that the interstriation angle tends to be constant. The 

 correlation coefficient between the two directions is in fact —0-59. 

 The two directions are therefore not independent. In seeking a possible 

 line of attack on this problem, we were encouraged by the observation 

 reported by Anderson and Moore (44) that cotton hairs grown in con- 

 stant light no longer show the wall lamellation so typical of normal 

 cotton. Attempts have therefore been made to ascertain if here too the 

 change in wall structure (of a much more fundamental nature than that 

 observed in cotton, however) is associated with the change, in nature, 

 from light to dark. All three filamentous forms under discussion here 

 have been grown under constant Hght conditions and carefully compared 

 with controls growing under the normal alternation of light and dark. 

 These algae will not long tolerate exposure to temperatures much above 

 18°C., presumably on account of the gross bacterial infection which 

 then occurs, but the fresh-water species have nevertheless been grown 

 successfully under the two experimental conditions. Marine species are 

 somewhat easier to deal with, and successful cultures have been main- 

 tained both in the laboratory at Leeds and, through the courtesy of 

 Professor Hobson and Dr. Bull, at the Dove Marine Laboratory at 

 CuUercoats. The latter site is remarkably good for this kind of work, 

 for flowing sea water is continually available inside the laboratory. 



The major difficulty in attempting interpretation of these observa- 

 tions lies in the fact that the algae concerned grow very slowly, so that 

 even after some months under experimental conditions the bulk of the 

 walls, in all cells except at the tips of filaments, were present before the 

 experiment began; only a relatively thin innermost layer of wall has 

 been laid down under the experimental conditions. Thus we expect, 

 not a complete change in the X-ray diagram, but merely a modification; 



* The same structure has now been found in Siphonocladus but, strangely enough, 

 not in the Spongomorpha group of Cladophora. 



