— 96 — 



V. — In Conferva and Ophiocytium exceptional sunsliine 

 may act as the limiting factor (cf. pp. 68-71). 



VI. — In the case of Chaetophora and the Protococcales the 

 concentration of the water appears to be the limiting factor 

 determining the degree of abundance, since the requisite tempe- 

 rature-conditions are always realised at some period of theyear 

 (cf. pp. 72-77) 



VU. — In Anabaena the limiting factor appears to be the 

 amount of sunlight {cf. p. 81). 



VIII. — in the case of Euglena the amount of organic sub- 

 stance dissolved in the water seems to be the limiting factor 

 (cf. p. 84). The same may possiblj apply to Navicula lanceo- 

 lata andiV eœilis (p. 91). 



IX. — The représentation of Eunotia Arcus appears to be 

 limited by température (cf. p. 88). 



X — Lastly, in the case of the epiphytic Diatoms the limiting 

 factor is the présence of a suitable host in the necessary quanti ty. 



Whilst in most of the cases above enumerated there is only 

 one limiting factor in nature, the others being apparently 

 alwajs realised, the case of Oedogonhun is particularly inte- 

 resting because we hâve obtained some évidence of the existence 

 of two limiting factors, one of them the amount of sunsliine and 

 the other the degree of concentration of the water. In the case 

 of Conferva and Microspora also, where the main limiting 

 factors are amount of sunsliine and température respectively, 

 compétition on the substratum is no doubt a second limiting fact- 

 or. With the exception of the cases mentioned under IV and X 

 the eftect of the limiting factor is probably always in the direct- 

 ion of hampering végétative growth (incl. zoospore-production 

 and végétative propagation), i. e. the growth and conséquent rate 

 of végétative increase remains at a minimum until the necessary 

 factor lias been realised. In the two exceptional cases however 

 there is not necessarily a direct hampering of growth, but rather 

 an absence of the necessary substratum on which tins growth 

 could take place. 



It should be noted that in the case of forms like Con ferra. 

 Microspora and the stérile species of Mougeotia the réalisa- 

 tion of a maximum merely dépends on the growth and propaga- 

 tion of preexisting filaments, whereas in forms like Spirogyra, 

 Zygnema and Anabaena, which disappear more orless compte- 



