— 61 — 



appears to be an effect of illumination rather than of tempéra- 

 ture that détermines the period of dominance of Oedogo- 

 nium ' . 



Itwouldappear however that previous rainfall lias some effect 

 on the extent of development of Oedogonium, Thus in 1907, 

 1908 and 1910 the species of this genus attained to a much 

 greater degree of abundance than in 1906 or 1909. The follow- 

 ing table shows the amount of rainfall in inches for six and 

 three months before the commencement of the maximum in each 

 year (i. e. May in 1906, Mardi in 1907, etc.) ® : 



1906. 1907. 1908. 1909. 1910. 



6 months . . 12-79. 15-80. 14-38. 9'64. 12-66. 

 3 months . . 4"56. 5*49. 8'14. 4-66. 6-26. 



Thèse figures speak for themselves except in the case of 1910, 

 where on thé six-months reckoning no defînite conclusion 

 can be drawn. The great development of Oedogonium in May 

 of that year may however hâve something to do with the réduc- 

 tion of compétition (very fewZygnemacese). Although therefore 

 the data are not quite conclusive, it wouid seem as though the 

 extent of development of Oedogonium in a given season 

 depencled somewhat on the degree of concentration of the 

 water, although the time of their maximum abundance in that 

 season dépends on the réalisation of a certain amount of bright 

 sunshine, a resuit which is very similar to that obtained for the 

 Desmids. Before leaving thissubject, attention may be drawn to 

 the behaviourof Oedogonium in the yearl906. The maximum 

 in this year (May and June) was only attained after the period 

 of bright sunshine in April, while in other years the maximum 

 coincides with the first period of bright sunshine. April of 1906 

 had an unusually low rainfall and Mr. Salisbury's data show 

 that the water-level fell appreciably during that month (3) . 



(1) Klebs(1896, p. 266) showedthat in Oedogonium di pi an dm m, Juranyi a 

 change l'rom a lower to a higher température is the chief influence inducing 

 zoospore-formation. This may indicate that the heating; ol the water by the 

 sun is more responsihle for the above phenomena than the light itself. 



(2) Sunshine-data calculated on the same hasis seem to afford no due. in 

 l'art the numher of hours on the six months reckoning is astonishingly 

 uniform. 



(3) The water-level i'ell 3 in. hetween the 9«» and 21 R t of April, 1906 ; it had 

 l'allen another 3 in. by Maj 19 tB , but hegan to rise again in June. 



