— 101 — 



other iwo years, and fchis will hv the month of May Lead to a 

 greater concentration of the water inl906 than in 1907 or 1908. 

 Such conditions Klebs (loc. cit. ) has shown tend to be unfavour- 

 able for sexual reproduction and may well acconnt for the rela- 

 tively scanty reproduction in 1906. In interpreting the above 

 table it raust also be borne in rnind that an honr of sunshine in 

 May or June has a stronger effect than in Mardi or April owing 

 to the greater intensity of the light {cf. the effect from April to 

 May of 1908 with that from Mardi to April of 1006). 



Other examples of the influence of sunlight on tliesexual repro- 

 ductive process in Barton's pond are as follows : 



I. — In the case of Zygnema the period of sexual reproduc- 

 tion is quite short in 1906 and 1908, but much longer in 1907. 

 In the latter year the sunshine after Mardi was considerably 

 below the normal, and it may well be that the amount of sun- 

 shine was toosmall to stimulate a large numberof threads of the 

 Alga simultaneously and that therefore reproduction was spread 

 out over a wide period. 



II. — In 1907 with relatively little bright sunshine during 

 the summer months Mougeotia parvula did not reproduce till 

 August, whereas in 1908 the reproductive process commenced 

 already in May. 



A part from the influence of sunlight however the conditions 

 determining sexual reproduction in the pond are by no means 

 clear; thus, it is difficult tounderstand why sexual reproduction 

 was observed in Closterium Kûtzingii in May 1906, but not 

 at any other time or why Staurastrwm showed conjugation 

 only in July 1908. Another puzzling feature is that Coleochaete 

 formed abundant oospores in 1907 and 1908, but not in 1909, 

 although numerous plants were observed in tins year. 



A consultation of the period ici ty tables on pp. 54 and 59 shows 

 that the Zygnemaceae and Oedogonium (t[) generally com- 

 mence to reproduce sexually as soon as the maximum begins, 

 while the conclusion of sexual reproduction coincides with a 

 great decrease in the quantity of the Alga concerned {cf. also 

 Copeland, 1909, p. 24). But the former statement isnot invaria- 

 blytrue; thus, in 1908 Spirogyra cataeniformis became a 



(1) In what lollows thèse two &ets of forme ar: chietiy taken into considera- 

 tion, since they are the only ones for which adéquate data are available. 



