98 — 



Arcus from January to April of 1908 corresponds to an excep- 

 tionally cold January (cf. Microsjjora and p. 88). 



Thèse cases can only be fully explained on the assumption 

 that the specially favourable conditions at the time of commence- 

 ment of the maximum gave such an impetus to the Alga that 

 it was able to withstand advei'se conditions for some time in sub- 

 séquent months. One would expect that the converse to tliis 

 theory of impulse would also apply, viz. tliat particularly un- 

 favourable conditions might lead to a poor development under 

 subséquent favourable conditions. For tliis there is also some 

 évidence, e. g. the poorly developed maximum of Oedogonium 

 in 190G may be due to the unfavourable conditions in April (cf. 

 p. (31); the brief maximum of Conferva in 1907 may hâve some- 

 thing to do witli the unfavourable influence of the strong sun- 

 shineof Mardi. 



In concluding tliis gênerai considération of periodicity in Bar- 

 tons's pond, it may be well to compare with otherpieces of water. 

 So little detailed work on periodicity lias been done in tliis coun- 

 try ^) however that such a comparison must of necessity be quite 

 imperfect. In larger (especially deeper) pièces of water concen- 

 tration-différences will take place more slo.wly and small varia- 

 tions of température will tend to be lost in the graduai rise from 

 winter to summer and the fall from summer to winter {cf. even 

 Abbot's pool). The effects of sunshine will alone make them- 

 selves as obvious as in smaller pièces of water. The periodicity 

 of a lake therefore will bu detormined largely by the seasonal 

 factors, sunshine being the most important irregular factor. 

 In such large pièces of water the periodicity is therefore far 

 more uniform, and, since most observations hâve hitherto been 

 made on large water-areas, it lias often led to the assumption 

 that periodic changes are largely due to inhérent tendency (cf. 

 Grilliths, 1912, p. 19), although our own and Comère's observa- 

 tions (Comère, 1906 and 1910) hâve already previously indicated 

 that such a view is only tenable (if at ail, cf. above p. 97) to a 

 much more limited extent. The most important contribution 



(l)Brown (Brown 1908) lias investii:ated algal periodicity in a number of 

 ponds and streams in Indiana. As thèse investigations only extended over one 

 year and no exact meteorological data are given, it is impossible to draw any 

 safe conclusions from the results or to compare them in détail with our own 

 {cf. alBoShantz 1903). 



