Periodicity of Freshwater Algce in Nature 203 
species remained "rather rare” the whole time. Competition with 
other filamentous Conjugates (particularly Spirogyra spp.), which tend 
to grow mixed with Mougeotia in the shallow-water marginal region 
of the pond, is undoubtedly a potent factor in determining the 
extent of development of M. scalaris at certain times of the year. 
Narrow filaments of Mougeotia were also generally present in the 
pond, but mostly only rare or isolated. Of these narrower forms 
M. viridis became “rather common” at the sides of the pond, and 
produced a fair number of zygospores, during September and 
October 1920, but declined during the following winter, and was not 
again observed. This slight autumnal development of M. viridis is 
interesting since, in this country, it is more particularly an early 
spring Alga. West (30) has pointed out that in the British Isles 
M, viridis is almost invariably the first species of the genus to 
fructify in spring—in the Midlands and Southern counties in March 
with a water-temperature of 5°-7° C. (l.c. p. 33). In the present 
pond it developed when the temperature was falling and the monthly 
mean (air-temperature) was between 13*5 and 10-5° C., and when 
the concentration was falling from 18 to 12 degrees, and the sunshine 
poor. The evidence is incomplete but it seems as though abundant 
bright sunshine, so essential for zygospore-production in the case of 
M. scalaris, can to a considerable extent be dispensed with by 
M. viridis ; in which case an interesting parallel can be drawn 
between Mougeotia and Spirogyra, since the narrower species of the 
latter genus (e.g. S. inflata ) require in a lesser degree the stimulus 
of bright sunshine in order to conjugate than do the wider species 
(cf. Section VII). 
IX. THE DESMIDIACE7E 
Desmids are present in Hawkesley Hall pond all the year round, 
and are represented by a rather large number of species (see 
Section V). The following have been most abundant, and are those 
whose periodicities have been most thoroughly worked out: Hyalo- 
theca dissiliens, Sphcerozosma granulatum, Closterium acerosum 1 , Cl. 
striolatum, Cl. nionilifcrum,Cl. Kutzingii, Cl. lineaium 1 , Pleurotcenium 
Ehrenbergianum, Cosmarium Botrytis 1 , C. Regnellii, Staurastrum 
brevispinum 1 , S. Dickiei 1 , S. furcigerum, Xanthidium antilopceum, 
Arthrodesmus convergens. Many of these tend to be scattered 
throughout the pond, although showing a preference for the deeper 
water, amongst the filaments of CEdogonium, Zygnema, etc., but a 
1 These have produced zygospores. 
5—10 
