Reproduction of British Freshwater Algae in Nature . 431 
the occasional reproduction of Spirogyra at other times of the year is 
incomprehensible. With regard to the former point we cannot deny the 
possibility of some of the Spirogyras being biennial as far as reproduction 
is concerned ; one or two years’ further observations will show whether there 
is any truth in this. But from all we know about algal reproduction it 
seems very unlikely, and it is far more probable that reproduction in 
Spirogyra depends on the presence of certain combinations of factors, 
which probably differ for each individual species. The most important 
changes that take place in spring are on the whole the same as those which 
have been enumerated on p. 427 ; it does not seem likely, however, that there 
is any marked annual concentration or dilution of the water during the 
early part of the vernal phase (March and April), although this factor will 
in many years come into play during May and June. All the factors 
concerned, it will be noted, undergo a gradual intensification as the summer 
approaches, and this intensification reaches its maximum somewhere during 
the summer months, after which there is again a gradual diminution. At 
some time in the autumn, therefore, each factor must be present in the same 
degree as in spring, and it might be argued that autumnal reproduction 
should for this reason be as common as vernal reproduction. In the case of 
the autumn, however, each factor is undergoing change in the reverse direc- 
tion (intensity and duration of light decreasing, temperature decreasing, &c.) 
to that which obtains in the spring, and this difference probably accounts for 
the absence of autumnal reproduction — even though combinations of factors, 
similar to those found in spring, must sometimes occur. The direction of 
change of a factor or of a group of factors, is no doubt of great importance, 
and it probably makes all the difference whether, for instance, an Alga after 
being exposed to a low temperature is subjected to a higher one, or vice 
versa. The response in the two cases may be quite different. 
The reproduction of the different species of Spirogyra in the vernal 
phase is thus most probably dependent on the seasonal occurrence of 
certain external conditions, particularly on an increase in intensity of the 
factors liable to change in spring. For each particular species certain 
intensities of the different factors will probably be necessary, and it depends 
on the realization of these intensities as to whether the given species will 
reproduce or not. That these intensities vary for different species, is shown 
by the fact that in the same pond different Spirogyras begin to reproduce at 
different times during the vernal phase, and that the same species reproduces 
at different times in different ponds and in different years (cf. the table). The 
general modus operandi of these factors is illustrated by Klebs’s work. He 
has shown (loc. cit., pp. 233, 237, and 241) that in the case of a number of 
species of Spirogyra zygospore-formation can be induced by placing the 
Alga in water in bright sunlight for a few days. If we substitute a weak 
nutritive solution for the water, it will, unless very dilute, absolutely work 
