200 William J. Hodgetts 
and plane end-walls developed, the order of appearance in the 
autumn being just the reverse 1 . This, as is obvious from the work 
of others, is a fairly general rule, and appears to be correlated with 
the fact, pointed out by Fritsch (15), that the species of Spirogyra 
present in the tropics are mostly broad forms with two or more 
chloroplasts and plane end-walls. Transeau (28) has analysed this 
annual succession of species of Spirogyra in more detail. 
Of course many other factors besides those treated of above 
influence the growth of Algae in a pond, and one which is undoubtedly 
important at times is competition between the various species 
present. In the case of the present pond during May 1919, when at 
least twelve species of Spirogyra developed in the comparatively 
narrow zone of shallow water at the side of the pond, two of these 
(S. protecta, S. longata) were “rather common,” six were “rather 
rare,” one (S. condensata) was “rare,” and the remaining three 
(S. Jurgensii, mirabilis , Weberi) “very rare.” Mutual competition 
between these various species was, in this case, the particular factor 
which determined the extent of development of each. 
VIII. ZYGNEMA AND MOUGEOTIA 
A species of Zygnema (width of filaments 22-24 fi) is a perennial 
constituent of the algal flora of Hawkesley Hall pond, and, as 
mentioned in Section VI, is the dominant filamentous Alga in the 
central province of the pond during the spring, its maximum 
occurring usually at the end of April or in early May, although in 
1921 the maximum fell exceptionally early (see curve in Fig. 5). The 
species was never observed to conjugate, and thus was not identified. 
Like Spirogyra it very rapidly decreased in amount towards the end 
of May, and was generally “rather rare” or “rare” by June. During 
the unfavourable summer-period it is rare and represented by more 
or less isolated and generally very short filaments, the cells of which, 
although comparatively thin-walled, are filled with abundant starch- 
grains, the whole contents of the cell being very dense, with an 
opaque granular appearance 2 . Growth again becomes apparent in 
September, while during the autumn and winter the Alga is often 
“rather common ” (1918, 1920); and the very mild winter 1920-1921 
1 Pevalek explains this by reference to the intensity of the light, but this 
factor alone does not appear to supply a satisfactory explanation. 
2 These short cell-rows with the cells densely packed with food-reserves are 
sometimes termed “cysts” (see West (30), p. 34). 
