— 79 — 



The following data indicate a préférence on the part of Eudo- 

 rina (1) and Gloeocystis for relatively concentrated water : 



I. — Both gênera exhibited an exceptional developmcnt in 



Apri] or May of 1906, which is in marked contrast to tlio behav- 



iour of the three other forma. The combined rainfall for Mardi 



and April in the four years is as follows : 



w . . .. 1906 1007 1908 1901) 



March-Apnl ■ ■ ■ • ■ 



2-28 in. 4-14 in. 6 '35 in. 515 in. 



In April 1900 as a matter of fact the water-level was low {cf. 

 footnote 3 on p. 61), and the water will hâve been far more 

 concentrated than in the other three years. This may be respon- 

 sible for the great development of Eudorina and Gloeocystis. 



II. — Confirmation for tins view is supplied in the first place 

 by thebehaviour of Eudorina'm 1908, when with exceptional 

 raini'all in March and April (the critical period) Eudorina fail- 

 ed to put in an appearance at ail. The relatively scanty 

 development of the genus in the spring-months of 1907 and 

 more particularly of 1909 (2) is quite in accordance with the 

 above rainfall data (3) . 



III. — It may also be noticed that the heavy rainfall of June 

 190(3 led to an almost complète disappearance of the hitlierto 

 abundant Eudorina, and that the still heavier rainfall of Octo- 

 ber 1907 put an end to its development in that year. 



IV. — In 1906 and 1907 after the drying up of the pond had 

 ledto agreatdecrease or disappearance of Gloeocystis, it became 

 commoner again (favoured by suitable températures) after the 

 pond commencée! to fill up once more ; on the other hand in 1909 

 in which the pond did not dry up and in which the summer and 



(1) It israther astonishing that the two closely allied gênera Pandorina and 

 Eudorina should lie influenced differently by outside conditions, hut a mère 

 glance at their frequency-tahles shows that this must he the case. 



(2) It may lie that in 1909 the relatively heavy rainfall of Mardi and April 

 led to the absence of Eudorina till May, while the heavy rain of June, July 

 and August prevented any further development in that year. 



(3)If Eudorina favours concentrated water, its dominance in July and August 

 of 1907 does not appear to l>e explicalde. But it must he borne in mind that 

 even in a rainy and cloudy year like lî'OS, owingto the higher températures and 

 greater powerof the sun in summer a certain concentration of the water will 

 ahvays take place, and this may hâve reached a sufficient degree to admit of 

 En dur mu attaining a maximum in July and August, particularly as it was favour- 

 ed bj unusually low températures (p. 75). 



