Algae. 325 



the pond and of the annual cycle in the flora (Winter, Early Spring, 

 Spring, Early Summer, Summer, Autumn, Earh' Winter). At the 

 end of the paper the results of the investigations are briefly sum- 

 marized as follows: 



The algal flora of Barton's Pond is dominated by a successive 

 association (formation?) of Microspora, Eiinotia Arcus, Conferva, 

 Zyg)ieniaceae , Oedogoniuni, Protococcales, Englena, Anahaena, spe- 

 cies of Tyachelojnotias, and epiphytic Diatoms. The flora is very 

 rieh and shows a very pronounced periodicit3% related somewhat to 

 that of the Phanerogamic flora. 



Four main phases are distinguishable in the annual cycle, viz.: 



a. Winter phase with an abundance of Microspora and Diatotns. 



b. Spring phase with dominant Zygnemaceae and Oedogonium , 

 and numerous Protococcales as subsidiär}^ forms. 



c. Summer phase with dominant Eiiglena and Anahaena, Ineffi- 

 giata and Desniids being important subsidiary forms. 



d. Autumn phase, chiefly characterized by renewed develop- 

 ment of many of the filamentous forms and by the presence ot 

 species of Trachelornov.as and Oscillarieae. 



The flora differs fundamentalh'' from that of Abbot's pool in 

 the absence of Cladophora and Melosira, and in the abundant develop- 

 ment of Confervales. These differences are probabl}^ characteristic 

 of two distinct algal associations (or formations?). 



The general trend of periodicity is the same in Barton's pond 

 and Abbot's pool, there being in both cases a Winterphase cha- 

 racterised by a hard}^ filamentous form and Diatonis, a spring 

 phase in which Zygnemaceae play an important part and a mixed 

 autumn phase with Oscillarieae. The principal difference lies in the 

 summerphase and is due to the normal drying-up of Barton's 

 pond during that period. 



A profound relation between the frequency of the algal flora 

 and the meteorological conditions has been established. 



The species of Spirogyra present in Barton's pond show no 

 autumnphase, which is regarded as being due to the usual drying 

 up or extreme concentration of the water of the pond in the sum- 

 mer-months. The presence of S. varians, in 1907 alone, is shown 

 to be related to special meteorological conditions. Evidence is brought 

 forward to show that the exact time of dominance of Zygnemaceae 

 in the spring depends on the water attaining to a certain degree 

 of eoncentration. 



The Desmids in Barton's pond attain their chief development 

 after the Zygnemaceous phase is over. It is shown that the abun- 

 dance of development of the Desmids in a given year probably 

 depends on the degree of dilution of the water, sunshine apparently 

 being the most important factor determining their time of appea- 

 rance in the pond. 



The time of maximum development of the species of Oedogo- 

 nium in the pond is determined by the first month Avith plenty of 

 bright sunshine. Dilute water is more favourable to their develop- 

 ment than concentrated water. 



The time of dominance of Microspora during the winter is de- 

 termined b}^ the lowest winter- temperatures, although competition 

 with Oedogonium and Conferva also comes into play. It has not 

 been possible altogether to explain the frequency-table of J/ä"ros/)ora. 



The development of the Confervales in the pond is adversely 

 afifected by much bright sunshine. Conferva is also influenced by 



