34 DR. б. $. WEST ON THE ALGÆ OF 
and subalpine lakes and tarns of the British Islands, in which species of 
Mougeotia and. Zygnema abound, the maximum vegetative abundance usually 
occurs in the late summer and early autumn as the temperature is gradually 
declining ж, In these habitats spores are only rarely produced, the winter 
season being passed in the form of ** cysts " f. 
Only two species of the numerous Desmids of these weedy shores were 
observed with zygospores, namely, Cosmarium contractum var. ellipsoideum 
in March, and Cylindrocystis diplospora in April. T do not think this is at 
all remarkable, notwithstanding the abundance of Desmids in the littoral 
region of the Yan Yean, and it is entirely in accordance with numerous 
observations I have made in the British Islands. The occurrence of zygo- 
spores in the Desmidiaceæ appears to be very spasmodic, and the conditions 
which result in their production are apparently both local and special. 
I have collected and examined, during an extended study of the group, large 
numbers of the zygospores of Desmids, and have kept a careful record of 
all the species which have come under my notice in a fruitiug condition. 
Only two points are manifest on a study of these records: first, that zygo- 
spores are not produced in the colder period of the year ; and secondly, that 
they are more frequently formed in small pools, ponds, and ditches than in 
large bodies of water such as lakes. This is doubtless owing to the greater 
ease with which the necessary conditions may be attained in the smaller 
bodies of water. In no single instance have I known of the formation of 
Desmid-zygospores owing to impending drying-up of the pools, because, as 
a rule, Desmids only occur in permanent pools which are scarcely liable 
to be dried up. Many Desmids produce zygospores in spring, with a rising 
temperature, and many (perhaps the majority) in the late summer just 
after the maximum temperature. Zygospore-formation can be observed in 
stagnant pools and ditches in which there is practically no aération of the 
water; it can also take place at the margins of lakes or in ditches with 
slowly flowing water, in which the aération of the water is fairly good; and 
it also occurs on dripping rocks in which the conditions of aération are at 
their maximum. Sometimes a species produces zygospores in abundance 
* In the littoral Alga-flora of the alpine lakes of the Pike’s Peak region, Colorado, species 
of Zygnema and Mougeotia reach their maximum abundance in September, when the tempe- 
rature is falling. Cf. Shantz, in Trans. Amer. Micr. Soc, xxvii. (March 1907) pp. 92 & 93. 
+ С. S. West, Treatise Brit. Freshw. Alg. (1904) pp. 115 & 121. The survival through 
the winter of many species of the Zygnemaceæ (but especially species of Mougeotia) by 
means of these resting “cysts” is much commoner than is imagined, and it occurs in 
lowland areas almost to the same extent as it does in the upland districts. I have found it 
to occur commonly in the English Midlands in ditches and Spkagnum-pools which I have 
had under continuous monthly observation for two years. Similar “ cysts" occur frequently 
in Ulothrix and in Rhizoclonium. Each “cyst” consists of one or several vegetative cells 
which have entered into a period of quiescence or inactivity, after having developed rather 
thicker cell-walls. Sometimes a gelatinous envelope is also evident, 
