42 
STUDIES OF BRITISH FRESHWATER ALGiE. 
and 1908, are not exhibited by the Cladophora. In fact, were it not 
for the abundant epiphytic growth at certain times of the year, the 
annual cycle of Cladophora would probably be still more uniform. 
(iii .) — The Diatoinacece of the pond considered as a ivhole. 
Diatoms play such an important part in the flora of the Fish Pond that 
it will be worth our while to consider them in some detail. We may 
distinguish between the free forms and the epiphytic forms, both of 
which are important elements in the flora, though dominant at different 
times in the annual cycle. The free forms are chiefly represented by 
Melosira variant, Fragilaria virescens, Cocconema lanceolatum, and 
Hy'iiedra %dna^ but numerous others (e.g. Navicula spec., Cocconema 
prostratum, C. cymbiforme, etc.) occur in smaller numbers. These free 
Diatoms all show a well-defined maximum during the winter-months, 
whilst during the summer-months they are rare and occasionally 
completely absent. Their abundance during the winter is probably 
to be explained as due to the low temperature of the water, and the 
fact that the latter does not become very strongly heated in summer 
is no doubt responsible for these Diatoms not completely disappearing 
during the warmer portion of the year. Nevertheless, if we compare 
a summer with a winter sample the difference is very striking 
(cf. Plate) ; in the latter chains of Melosira and Fragilaria with 
numerous unicellular Diatoms intermingled fill the field of view under 
the microscope, while in the former free Diatoms require to be sought 
for. It is noticeable that of the two filamentous forms Fragilaria 
generally commences to increase a little later than Melosira, while 
the latter begins to decrease before the former ( cf. the table, p. 43). 
Fragilaria, too, is on the whole the more abundant form of the two 
during the summer months, all this tending to indicate that it is not 
as susceptible to the temperature of the w'ater as is Melosira. The 
species of the genus Cocconema (the third most important genus of 
the free Diatoms) on the other hand have a still more limited maxi- 
mum than the other two genera, and have altogether decreased 
materially in importance since 1905. — It is of course not altogether 
out of the question that the very marked decrease in number of free 
Diatoms with the advent of the spring may stand in relation to the 
sudden increase of Spirogyra, followed by Cladophora, but, since the 
majority of Diatoms are known to favour cold water, the rise of 
temperature in the spring seems the more plausible explanation. 
Of the epiphytic Diatoms Synedra radians behaves much in the 
same way as the free forms just discussed, generally showing a 
maximum during the cold months of the year, although this form 
exhibits occasional marked irregularities, which we are at present 
not in a position to explain: Thus for some unknown reason Synedra 
radians was very abundant during June and July of 1905, and common 
in May of 1908. There is possibly, however, some not yet under- 
stood relation between the species under discussion and the other 
epiphytes which inhabit the surface of the Cladophora, for normally 
