Reproduction of British Freshwater Algae in Nature. 425 
and autumn. Between the vernal and the autumnal phase, however, there 
is always a very marked decrease in quantity, amounting in very many 
cases to complete disappearance (in July and August, cf. S. affihis , 5 . 
jugalis , .S. rivntaris). There is some evidence also to show that between 
the autumnal phase and the ensuing vernal phase some species of Spirogyra 
again practically disappear, being absent in midwinter (cf. S. jugalis (Dillw.), 
Kiitz., 5 . Weber i, Kiitz., 5 . rivntaris, Rabh., in 1905). Since zygospores 
(except in a few cases) are not formed during the autumnal phase, the 
reappearance of the Alga in the following spring must take place at the 
expense of the zygospores formed in the preceding vernal phase. The 
Alga never appears in autumn in such quantity as to warrant the sup- 
position that all the zygospores formed in the previous spring have ger- 
minated ; in all probability the conditions which lead to the autumnal 
appearance of these Spirogyras are only sufficient to stimulate a small 
number of zygospores to germination., and the main mass of them remains 
dormant till the spring. 1 We shall return to this subject below. 
Before passing on to further considerations we may briefly discuss 
some of the data in the literature bearing on this subject. Comere(loc. cit., 
p. 405) records the occurrence of Zygnemaceae in the waters examined 
by him as follows : In the ‘ eaux stagnantes ’ they are found in his second 
vernal, aestival, and autumnal periods (i. e. from the middle of April to the 
first frosts), while in the ‘ milieux passagers ’ they are present only in the 
first and second vernab periods (i. e. from the end of February to the end 
of June). 2 While we have; been unable to recognize any marked differences 
in the occurrence of Spirogyra in permanent waters and in those which dry 
up in summer, Comere’s observations agree with ours in the prevalence 
of This genus in the spring. The waters examined by Comere do not 
apparently show any marked decrease of Spirogyra in midsummer, while 
its absence in winter is much more marked. Petit 3 in his valuable treatise 
on the Spirogyras of the neighbourhood of Paris mentions March — July as 
the period of occurrence of most of the species described, although many 
of them have a much more limited period; .S'. orthospira (Naeg.), Kiitz., 
S.bellis( Hass.), Cleve, and .S', orbicularis (Hass.), Kiitz., are the only species 
found by him also in the autumn. Petit’s observations are particularly 
valuable, as they are the results of investigations extending over many years. 
According to Klebs (loc. cit., p. 229), species of Spirogyra are found ‘ zu 
1 It is not quite impossible that the disappearance of Spirogyra in summer and midwinter may- 
be only apparent, and that a certain number of filaments may sink to the bottom and remain there 
in a dormant condition. In view of the fact that our knowledge of algal reproduction is so scanty, 
it is just worth while keeping this in mind, although there are no data to support it. 
2 The pieces of water with which we are concerned in the present paper belong only to Comere’s 
( eaux stagnantes ’ and ‘ milieux passagers.’ 
3 Paul Petit, Spirogyra des environs de Paris. Paris, 1880, 39 pp. Unfortunately, Petit does 
not definitely state whether the months mentioned by him after each species refer to period of 
occurrence or period of reproduction, but it seems probable that they refer to the former. 
