— 57 — 



attempted the culture of €onjugatae, that they are remarkablv 

 sensitive to changes in the concentration of the water, and just 

 asthis factor appears to influence the early or late appearance 

 of the Zygnemaceae, it seems to hâve its effect upon the abun- 

 dance of tlio Desmid-flora in such a pond as Barton's. Inas- 

 much as the total amonnt of sunshine was much thesamedurino- 

 the period preceding the main development of Desmids in tlio 

 différent years and the temperature-data show no important 

 variation, \ve are no donbt right in assuming that in the month 

 of June a larger volume of water (involving a lesser degree cl" 

 concentration) was présent in the pond in 1907 and 1908 tlian 

 in the ôther two years; unfortunately exact measurements of 

 the amount of water in the pond at différent times are lack- 

 ing (1) . It appears fherefore that the greater the concentration 

 of the water the less do the Desmids prosper. It may be noticed 

 that tins conclusion is in complète accord with one put forward 

 be Messrs. W. and G. S. West i West & West, 1912, p. 429) on 

 the basis of data derived from other considérations, althongh 

 Brown (1908) lias obtained somewhat contradictory results with 

 ClosteriumEhrenbergii. Since such concentration of the water 

 does not affect the actual appearance of Desmids in the pond, 

 but only their abundance, we must conclude that it opérâtes in 

 the direction of hampering growth and végétative reproduction 

 in some way and thus brings about the paucity of individuals < 2) . 

 In tins connection it may be noticed that dividing individuals of 

 Cosmarium were seen much more frequently in 1907 and 

 1908 than in the other two years (cf. table on p. 56). It will be 

 noticed that the (two) species ai Staurastrum lare apparently 

 much more sensitive to the conditions just considered than those 

 of the other two gênera. 



It may be well to draw attention to the différent behaviour 

 exhibited by the Zygnemaceae and Desmidiaceae. Iïoth dépend 

 on the concentration of the water, but whereas the Zygnema- 

 ceae put in an appearance at a time when the concentration of 

 the water bas reached a certain point (the time of their appear- 

 ance being therefore influenced by the amount of previous rain- 



(1) The data furnished by Mr. Salishury however lend some support to this 

 conclusion. The water-level in June of the différent years is described as l'ollows: 

 1906, very low ; 1907, ratherhigh ; 1908, high : 1909, rather high. 



(2) Cf. however Brown, 1908, p. 234. 



