- 95 - 



Scveral examples of correlated factors hâve been recognised 

 in studying the périodicité in Barton's pond. Thèse may be sum- 

 marised as t'oliows : 



I. — Forms depending on enrichmenè of water by deeay 

 ofother members of the flora : Euglena*, Navicula. An 

 adverse influence possible in case of Trachelomonas. 



II. — Forms influenced in thelr deoelopment by compét- 

 ition tri/// others : Conferva, Microspora, Oedogonium. 



III. — Forms infiuenced in their development by the 

 présence of a suitable substratum : Eunotia tunaris*, 

 Synedra radians*, and no donbt other epiphytes {cf. Fritsch 

 and Rich, 1909, pp. 43, 44). 



In our previous paper on the periodicity of Abbot's pool we 

 pointed out that our observations tended to indicate that « the 

 doctrine of limiting factors will probably be found to underlie 

 the whole scheme of intrieate changes that are so striking a 

 feature of freshwater algal végétation « (Fritsch and Rich, 1909, 

 p. 53). The principle of limiting factors, first clearly propounded 

 by F. F. Blackman (Blackman 1905) and since established fora 

 considérable number of physiological phenomena, is strikingly 

 manifest in some of the features notecl in the preceding section. 

 It will be well to give a list of the more important instances : 



I. — In 1907 and 1908 the extent of development of Zygne- 

 maceae was limited in April by exceptional dilution of thewater, 

 although other conditions as far as we can see were quite suit- 

 able for them [cf. p. 53). 



II. — In 1906 (and possibly also in 1909) the degree of deve- 

 lopment of the Desmids was limited by the water being more 

 concentrated than usual, other conditions apparently being 

 favourable {cf. p. 57). 



III. — In the case of Oedogonium sunshine is the limiting 

 factor, i. e. no maximum is attained until the necessary amount 

 of sunshine lias been realised {cf. p. 59). In other cases(e. g. 1900 

 and 1909)unusual concentration of the water may be a limiting 

 factor {cf. especially, p. 61). 



IV. — In the case of Oedogonium, and more especially of 

 Microspora and Conferva, a limiting factor is probably consti- 

 tuted by the amount of substratum available {cf. pp. 62, 65 

 and 69). 



