758 TRANSACTIONS OF SECTION K. 



5. Some In^jressions of South African Vegetation, 

 By Professor R. H. Yapp, M.A. 



6. Algal Ecology and Biology. By Miss Florence Rich. 



Periodical observations are being made, in conjunction with Dr. F. E. Fritscb, 

 on the algal vegetation of ponds, in the hope that by this means light may be 

 thrown on problems of algal ecology and biology that have hitherto received but 

 scant attention. From certain ponds monthly samples have been taken, from 

 others fortnightly ones. These exhibit marked periodical variations. 



Great differences have also been observed in the algnl vegetation of neigh- 

 bouring ponds. Certain algse appear to be commonly associated with one another; 

 it may be possible after a time to classify waters according to the types they con- 

 tain. There may also be a connection between the algae and the aquatic 

 Phanerogams present. Determinations of the external conditions to which 

 any given piece of water is subjected, such as average temperature, average 

 illumination, variations in shading and exposure, nature of drainage received, 

 and so on, must ultimately lead to recognition of the dominant factors to which 

 different kinds of algal growth are due. The combination of conditions in- 

 fluencing the reproduction of one particular species in different ponds will, it is 

 hoped, gradually be made out, and then the probable factors affecting reproduction 

 may be recognised, though the results will still want testing by experimental 

 methods in the laboratory. 



7. The Algal Vegetation of Ponds. By Norman Walker. 



The ponds examined are situated above the Bramhope railway tunnel, near 

 Leeds, and occupy excavations in clay which were made sixty-seven years ago. 

 An abundant flowering-plant vegetation which has established itself encourages 

 mud formation and renders the ponds favourable to a rich algal vegetation. The 

 following are the available sites for attached algae : — 



Site 1. — Winter shoots of CEjianthe fstulosa. These form a pale green zone, 

 from 2 to 3 yards wide, extending from the edge of the pond to a depth of 

 about 9 inches. In winter this site is brightly illuminated and the water is 

 thoroughly aerated by surface movements, caused by wind. During the course of 

 the year a succession of algal associations occurs here. The factors controlling 

 this distribution are, probably, changes in temperature with accompanying varia- 

 tion in dissolved gas content, and interference of light by the Plankton, chiefly 

 Peridinium tahulatum. 



September to May. — Tribonema bombycinum (Ag.) Derb. and Sol., dominates, 

 with abundant Ulothrix subtilis var. variabilis, Kutz. 



June. — Tribonema and Ulothrix disappear and Zygnema spp. (sterile) is esta- 

 blished, which in turn is followed by Tolyj)othri.v lanata (Desv.) Wartm. 



July and August. — (With increasing temperature of water, accompanied by 

 rapid decomposition of vegetable xa&Xier) Anabcena oscillarioides, Bory, followed 

 by Oscillatoria anguina, Bory, with Phormidium molle (Kutz.) Gom. These are 

 displaced by Oscillatoria amphibia Ag., and finally, in small warm pools, where 

 decay is very rapid, Beyyiatoa alba and Cladothri.v dichotoma are found upon the 

 remains of the foregoing plants. 



Site 2. — Shoots of Potamogeton natans and Sparyaniumramosum occ\xxv\r\g'\n 

 the deeper water (1 to 3 feet). In winter the old decaying shoots of these plants 

 afford a considerable surface at various depths and degrees of illumination for the 

 attachment of numerous epiphytic forms. The vertical distribution of these algse 

 is in some cases striking, and seems to be affected by surface commotion caused by 

 wind and by differences in the illumination. 



Winter. — Species of OEdogonium and Bulbochsete dominate. 



Spring. — Sfiroqyra Weberi, Kutz, and other filamentous conjugatae. 



