38 
STUDIES OF BRITISH FRESHWATER ALG^. 
rainfall and lack of sunshine during the last four months of 1903. 
Whereas the total rainfall from September to December, 1903 
amounted to 16-4 in. ; that of 1904 for the same period was 7 35 in. ; 
that of 1905, 7*51 in. ; and that of 1906, 11*13 in. The total rainfall 
from September to December, 1907 was 15*3 in., i.e. again consider- 
ably above that in 1904, 1905 and 1906 and very near the amount for 
the same period in 1903. On both occasions, on which the characteristic 
vernal /S/9irop'?/?*a-phase was suppressed, we thus have an exceptionally 
heavy rainfall during the last four months of the preceding year. More- 
over, the rather heavier rainfall at the end of 1906 (viz. 11-13 in. for 
the last four months), as compared with that in 1904 or 1905 also had an 
effect on the ensuing vernal phase, which commenced considerably 
later than usual in 1907 (also noticeable as regards the commencement 
of zygospore-formation 1). Although the evidence therefore seems to 
point to the very marked effect of an abnormally high rainfall in the 
previous autumn on the vernal 8’/>^5•0(9'2/?*a-phase, the direct cause is 
difficult to understand. Unfortunately we have no data as to the 
character of the algal flora in the pond during the latter half of 1903, 
while only two samples were collected in the autumn of 1907 ; these 
samples however, tend to show that the autumnal iS^^iVoyym-phase, 
though developed quite normally in October, had completely terminated 
in November, so that the effect of the heavy rainfall in October seems 
to have made itself felt almost at once. 
There is every reason to suppose that the vernal phase is due to the 
simultaneous germination of the zygospores formed in the preceding 
spring, 2 under the stimulus of a. certain group of factors, and that from 
the end of one vernal phase to the commencement of the next the 
majority (if not all) of these zygospores pass through a peiiod of rest. 
The effect of an excessive autumnal rainfall may either make itself felt 
as a factor affecting the zygospores during their period of dormancy, or 
more probably as a factor having some direct bearing on the germina- 
tion of the zygospores. As regards the former possibility we are 
probably right in assuming that hand in hand with the heavy 
autumnal rainfall went dull days and general lack of sunshine, and it 
is possible that the consequent low intensity of the light operated in 
some way unfavourably on the resting zygospores. This, however, 
seems far less likely than the second possibility, which we will now 
consider. It may be suggested that one of the stimuli necessary for 
germination of the zygospoi-es might, be exposure for some time to a 
certain degree of concentration of the water, and that excessive 
rainfall, owing to its diluting effect, prevents the realisation of the 
necessary degree of concentration at the right time. In 1904 and 
1908 the requisite concentration would then not have been attained 
until other conditions (light-intensity, temperature of the water, etc.) 
were unfavourable for the germination of the zygospores, so that the 
vernal phase was completely passed over, while in 1907 the necessary 
^cf. also Fritsch and Rich, pp. 429 and 436. 
“cf. Fritsch and Rich, p. 425. 
