— 103 — 



ditions, and thaï « a sudden change in external conditions checks 

 the végétative growth and tonds to cause the Alga to fruit 

 sexually » (/oc. cit., p. 243). We can only remark with réfé- 

 rence to this that we hâve found no évidence of a sudden change 

 in external conditions being necessary for reproduction, but that 

 a graduai attainment of the requisite factors is quite adéquate 

 (Fritsch and Rich, 1909, pp. 46-49). 



Gopeland (1909) observed a considérable number of species 

 of Spirogyra in nature and in aquaria in the laboratory and 

 concludes that there is « overwhelming évidence in support of 

 the view that the phenomenon of conjugation results not so 

 much from external as from internai conditions » (loc. cit., 

 p. 25). We would regard some of the évidence given in lus 

 paper as direct proof of the contrarv ; thus, the fact that » of 

 300 aquaria in the laboratory at the time when Spi? , ogj/) , a 

 was fruiting most abundantly under natural conditions, there 

 were about 5 per cent which contained conjugating material » 

 (toc. cit., p. 24», surely tends to show that under the différent 

 conditions of the laboratory conjugation was not favoured. 

 We think moreover that raany of the observations recorded in 

 our two previous papers (Fritsch and Rich, 1907 and 1909) are 

 very ditlicult to understand, if we accept Copeland's view. It 

 would for instance be hard to account for a species of Spiro- 

 gyra reproducing in one year and not in another (cf. S. a f finis 

 (Hass.), Petit and 5. Weberi, Ktitz., in Abbot's pool, Fritsch and 

 Rich, 190'.», pp. 46, 47). The occasional occurrence of an auturanal 

 reproductive phase (Fritsch, 1906, pp. 166, 167; Fritsch and 

 Rich, 1907, p. 433, and 190 ( .>, p. 48) speaks quite against Cope- 

 land's view. The conclusions of the latter appear to be based 

 pai'tlv on the observation of simultaneous conjugation of species 

 of Spirogyra in a number of différent pièces of water, which is 

 of course awell-known and strikiDg feature. Wetake it how- 

 ever to indicate that sunshine, which will hâve been much the 

 same in ail cases, is the factor cliiefiy responsible for reproduc- 

 tion in this genus. Observations of the same pièce of water 

 extending over a number ofyears show that the same species does 

 not commence to conjugate at exactly the same time in each 

 year; tlms S. cataeniformis in Barton's pond began the sexual 

 process in April of 1906, in May of 1907 and in June of 1908 

 (cf. also Fritsch and Rich, 1909, p. 47). 



