200 



Mr. W. West and Dr. G. S. West. [Oct. 30, 



In the first place, it has been recently pointed out* that the association of 

 the rich Desmid-areas with the older strata is (in the British Islands) most 

 probably due in part to the antiquity and consequent hardness of the rocks. 

 The mountainous regions which have resulted from those changes in the 

 earth's surface which have produced folding and contortion, and from the 

 resistance of these old, compressed rocks to subaerial denudation, are not 

 only directly responsible for the rainfall owing to their geographical position, 

 but are themselves most suitable for the formation of peat-bogs. In these 

 areas Desmids flourish, and therefore, so far as the British Islands are 

 concerned, the richness of the Desmid-flora bears a distinct relationship 

 to the antiquity of the geological formations of any area under consideration. 

 It seems probable that the determining factor is a chemical one. It is 

 certainly something more than mere suitability of habitat, otherwise how is 

 the relative poorness of the extensive bogs of the more recent formations to 

 be explained ?f 



It is very probable that the chemical composition of the water plays an 

 important part in determining this distribution. That the chemical factors 

 are quite apart from the occurrence of brown, peaty water, is evident from 

 the poorness of the Desmid-flora of so many peat-bogs, and also from the 

 fact that the best and richest Desmid-floras only occur in clear water with 

 no obvious peaty characters. Wesenberg-Lund is incorrect when he states 

 that Desmids chiefly thrive in " brown water rich in humic acids."+ Some of 

 them certainly do, and often profusely, but these are generally the common, 

 ubiquitous species which have almost a world-wide distribution. The great 

 majority, including most of the western British types, prefer clear water 

 with little peat. An excess of the brown peaty material is distinctly 

 unfavourable. The plankton-Desmids also occur much more abundantly in 

 the clear lakes than in the brown peaty ones. 



Although it appears so probable that chemical factors determine the 

 distribution of many Desmids, no definite information on this point has yet 

 been obtained. The drainage water which has percolated through the old 

 formations (rocks and soil) may possibly contain minute quantities of 

 something in solution which greatly favours the development of certain types 

 of Desmids, and may be directly responsible for their restricted distribution^ 



* G. S. West, in ' Linn. Soc. Bot. Journ.,' 1909, vol. 39, p. 10. 



t How is it, also, that the peaty bogs and ditches of the fens of the east of England, such 

 as are left of them, are poorer in the Desniidiacese than any other part of the British Islands 1 

 J Wesenberg-Lund, loc. cit., 1908, pp. 280 and 281. 



§ There would be nothing remarkable in this, as diatoms thrive and build up their 

 siliceous cell-walls in water containing silica in such minute quantities that ordinary 

 chemical analysis reveals no trace of it. 



