— 94 — 



tracts of water, are rather masked by the irregular factors in so 

 small a pièce of water as Barton's pond (or Abbot's pool), but 

 are responsible for the gênerai trend of the annual cycle consi- 

 dered on p. 45 It is the irregular factors (abnormal drought or 

 rainfall, abnormal sunshine, abnormal températures), however 

 that help to cast most light on the conditions determining the ■ 

 existence of the diverse members of the algal flora,and they hâve 

 been used freely in the preceding section. The resuit of thèse 

 observations may be summarised as follows : 



I. _ Rainfall (i. e. degree of concentration of water) : 



(a) Favoured by relatively dilute water : Desmids*, 

 Oedogonium, Chaetophora* , Pandorina*, Sphaero- 

 cystis*, Inefflgiata{l), Peridinium tàbulatwm. 



(b) Favoured by relatively concentrated water :Zygne- 

 maceae*, Eudorina*, Gloeocystis*. 



II. — Sunshine : 



(a) Forms favourably affected by sunshine : Zygne- 

 maceae, Desmids, Oedogonium*, Euglena (,?), Protococ- 

 cales. 



(b) Forms adversely affected by sunshine : Confer- 

 vales*, Anabaena*. 



III. — Température : 



(a) Favoured by low températures (probably niainly 

 affecting amount of dissolved gases) : Microspora*, 

 Eunotia Arcus* : , E. lunaris, Synedra radians, Navi- 

 cula (?). 



(b) Favoured by relatively low températures : Zygne- 

 maceae, Oedogonium, Chaetophora. 



(c) Favoured by higher températures : Desmids, Proto- 

 coccales* (especially Ineftïgiata), Anabaena*, Peridi- 

 nium tabulatum*. 



It will be noticed thatseveral of the forms concernedare enum- 

 erated under two or even three of thèse headings, and it is pos- 

 sible that tins should be done in every case. There is however 

 usually a determining condition on which the degree of abun- 

 dance or the réalisation of a maximum dépends, and tins lias 

 been indicated in the preceding list by placing an asterisk after 

 the form concerned. 



