Tlie Plankton . Algae of the Palisades Interstate Park 113 



does not follow definite rules ; they may constitute an important 

 portion of either the Baltic or the Caledonian associations or they 

 may be lacking. On the other hand the Baltic community has a 

 relatively large proportion of Myxophyceae, while the Caledonian 

 contains a small proportion of these algae. In view of the fore- 

 going it seems unadvisable to attempt a subdivision of the Baltic 

 association, but more logical to regard it as constituting a formation 

 of somewhat varying qualities in which Myxophyceae are always 

 present. 



Relation of the Plankton Formations to Geological Con- 

 ditions. In their first article calling attention to the Caledonian 

 formation W. & G. S. West do not ofifer an explanation of the 

 phenomenon. Two years later ('05) they suggest that the occur- 

 rence of numerous desmids in the plankton is due to the correlation 

 between desmid distribution and geological formation, a correlation 

 which Wi'ttrock had previously pointed out. They state that the 

 waters draining into lakes rich in desmids are peaty and slightly 

 acid ; this may produce a favorable medium for the growth of 

 desmids but does not account for the richness in species. They 

 then go on to show that while lakes in Carboniferous areas may 

 receive peaty waters they are relatively poor in desmids, and that 

 lakes in Secondary or Tertiary geological formations are even poorer 

 in desmids. From this they conclude that the diversity of species 

 in the desmid flora is directly correlated with the geological antiquity 

 of the region, while abundance of individuals is due to the absence 

 of lime and the presence of humic acids. They have also investi- 

 gated the phytoplankton of other regions (W. & G. S. West, '06, 

 '09a, '09b; and G. S. West, '07, '09), and in every case in the dis- 

 cussion of the flora call attention to the correlation between richness 

 in desmids and old geological formations. In their article sum- 

 marizing the phytoplankton of Great Britain ('09) they take up the 

 distribution of British desmids in general, and show that eastern 

 England is poor in species but that the western part contains areas 

 very rich in species. They hold that two factors are essential for 

 an abundant desmid flora, — a geological formation older than the 

 Carboniferous, and an abundant rainfall. If either of these factors is 

 lacking a region will not contain numerous desmids. This is shown 

 in a striking manner by their map of Great Britain, which gives the 

 distribution of rainfall, geological areas, and abundance of desmids. 

 Turning to the phytoplankton they state that lakes lying in rich 

 desmid areas are usually rich in plankton desmids, the same factors 

 that favor a general development of desmids favoring the occurrence 

 of plankton desmids. 



This hypothesis of geological correlation has not been allowed to 

 pass unchallenged, and other explanations have been offered to 

 account for the predominance of desmids in the plankton of Great 

 Britain. In a comparison of Scotch and Danish lakes. Wesenberg- 

 Lund ('05a, '05b) thinks that the peaty character and richness in 

 humic acids are the factors favoring the growth of desmids. In a 



