224 William J. Hodgetts 
No relation can be demonstrated between the concentration of 
the water and the growth of Nitella. The marked development during 
May—June 1919 coincided with a concentration rising from 18 to 
20 degrees, but the high concentration (27 degrees) of August 1919 
apparently affected this species very little, while the slight maximum 
during April—May 1920 occurred with an extremely low concentra¬ 
tion (cf. Fig. 3). 
With regard to the endophytic Coleochcete Nitellarum it will be 
noted at once on reference to the above table that the growth of 
the Coleochcete followed almost exactly that of the Nitella. This 
means that the amount of the latter plant present in the pond always 
operated as a limiting factor and restrained the growth of the 
endophyte, a condition of affairs just the reverse of that which 
obtained between Aphanochcete and the wide CEdogonium, discussed 
at some length in the last section. With the two last-named species 
it was noted that the epiphyte had a periodicity very different from 
that of its “host,” and this might, or might not, have been the case 
with Coleochcete Nitellarum and Nitella had the latter plant been 
more abundant in the pond. Thus an epiphyte is unable to exhibit, 
so to speak, its true periodicity when its growth is held in check by 
the scarcity of a suitable “host,” and under such conditions its 
occurrence will tend to follow very closely that of its “host,” 
although this does not necessarily mean that the two are influenced 
in the same way by external conditions. 
It may be pointed out that abundant bright sunshine is also 
essential for the development of oospores in Coleochcete Nitellarum. 
This is shown by the fact that in 1920, although development 
commenced in March, no oogonia were observed until the end of 
May (the first, very sunny month); and further by the fact that no 
oogonia were observed in the case of the third maximum (September 
1920—March 1921) which developed during a dull period. 
XVI. THE PROTOCOCCALES 
The principal members of this group which have been at all 
abundant in Hawkesley Hall pond are Dictyosphcerium Ehrenbergia- 
num, Ankistrodesmus falcatus, Scenedesmus denticulatus var. linearis , 
Pediastrum Boryanum and P. tetras. The first-named species was the 
most abundant, and its occurrence is given in Fig. 10, while the 
table below gives the periodicities of the remaining four species. 
