216 
William J. Hodgetts 
The periodicity of T. bombycinum (type), given in Fig. 9, is 
represented by a fairly regular curve. Development commenced 
during September in 1918 and 1919, and the species rapidly reached 
a maximum which lasted throughout the following winter, the 
decline setting in usually in early spring (end of March in 1919, and 
January—February in 1921), while during the warmer months the 
Alga is usually rare 1 . The species flourishes either when the tem¬ 
perature is falling (autumn), or when it remains persistently low or 
rather low; while the rising temperature of spring seems very 
unfavourable. A marked feature of its frequency-curve is the fact 
that during the notoriously cold summer of 1920 the species persisted 
as “rather rare/' whereas it was “very rare” in the hottest months 
of the two preceding summers. 
A marked antagonism appears to exist between the growth of 
T. bombycinum and bright sunshine (cf. data in Fig. 2), the maxima 
of the frequency-curve of this species always occurring when the 
daily mean of bright sunshine during the month is less than three 
hours; although, as with many other species, it is difficult to separate 
the influence of sunshine and high temperature. The adverse influence 
of bright sunshine on the development of several members of the 
Heterokontae was emphasised by Fritsch and Rich(i8), and the 
present observations tend to confirm this in the case of Tribonema. 
It was noted that the species of this genus showed a preference for 
those parts of the margin of the pond which were most shaded by 
the leaves of grasses, and the various marginal Phanerogams. 
There seems almost no relation between the concentration of the 
water and the growth of T . bombycinum , low, moderate, or even 
very high (October—November 1919) concentrations apparently all 
being the same to this species. The very high concentrations towards 
the end of 1919 (see Fig. 3) apparently depressed its frequency-curve 
onty to a slight extent. Some explanation seems necessary to account 
for the early decline of the species in late January and during 
February 1920, at a time when temperature-conditions remained 
favourable (see Fig. 2). February 1920 was somewhat sunnier than 
usual, but hardly sufficiently so to account for the observed decline, 
and a probable adverse factor was the sudden fall in the concentra¬ 
tion of the water during February—March 1920 (Fig. 3), .although 
the evidence is not quite conclusive. T. bombycinum was always 
observed to form abundant aplanospores towards the close of each 
of its periods of abundance (during February—April 1919, March 
1 No indication of two phases, such as were shown by this species in 
Barton’s pool (18), was observed in the present pond. 
