Reproduction of British Freshwater Algae in Nature. 427 
(a) The increase in the intensity and duration of the light. 
(b) The increase in the temperature of the water and the consequent 
diminution in the amount of dissolved gases in the water. 
(c) The gradual concentration of the salts dissolved in the water owing 
to the heat, and lack of rainfall in a normal summer. 
(d) ? The increase in the amount of the higher (Phanerogamic) vegetation 
present. 
At the end of a normal summer all these factors undergo modification 
in the reverse direction, and in this way conditions may become favourable 
for the reappearance of Spirogyra. We may point out, however, that 
certain definite combinations of these external factors are probably necessary 
for the reappearance of any given species, and that these combinations will 
vary for each individual species. From observations made last autumn it 
would seem as though dilution of the water back to its ordinary degree of 
concentration were one of the essential factors for an autumnal phase, but 
that if this dilution is delayed too long it may take place at a time when 
the average daily temperature or light-intensity is not high enough to allow 
of the appearance of Spirogyra. Thus in the autumn of last year (1906), 
in which after a very .dry and hot summer rain only set in at a late date, 
Spirogyras were practically wanting in all the ponds examined, whilst in 
1905, in which the rain commenced early and was rather equally dis- 
tributed over the autumn months, many of the Spirogyras showed a very 
well-marked autumnal phase (cf. the table). In the case of one pond 
(Barton’s pond at Harpenden), in which Spirogyras were very common in 
the spring, the water in 1906 only attained its ordinary level after the 
summer shrinkage at the end of October, i. e. at a time when the average 
daily temperature was 10*9° C. (on November 7th the average for the previous 
fortnight was only 6-8° C.) ; there was practically no autumnal Spirogyra. 
The above suggestion that the occurrence and especially the extent of 
the autumnal phase depend on certain combinations of external factors is 
supported by a number of data, which may be briefly referred to ; they are 
mainly derived from Abbot’s Pool, near Bristol, from which we have three 
years’ consecutive observations. N. rivularis was fairly abundant in this pond 
in the autumn of 1 904, but much rarer, though always present in some quantity, 
in the autumn of 1905 and 1906. S. jugalis was quite a common form 
in the autumn of 1905, but absent at this time of the year both in 1904 and 
1906. The special conditions, which led to the non-development of S. jugalis 
in autumn, 1904 and 1906, did not, therefore cause the disappearance of 
N. rivularis , which even flourished in 1904. 
The disappearance of Spirogyras in midwinter between the autumnal 
and the vernal phase, which is not quite certainly established, can only be 
related to temperature and light-conditions. It would be premature to 
discuss the matter further at the present moment. 
