80 Proceedivgs oi the Royal Irish Academy. 



dilution from different catches. They have not been treated to give as many 

 different organisms as possible in any catch in the field of view at once. 

 Neither have they been made to indicate different species. They serve to 

 show the appearance of the plankton at the different dates named, as seen in 

 a low-power view. What determines, then, the character of the Lough Neagli 

 plankton ? We may give the following known factors : — 



1. Physical environmental factors — 



(a) Latitude — climate. 



(6) Depth of water, affecting temperature, relation to bottom, 

 light, &e. 



(c) Geological nature of rocks drained by lake-water. 



(d) Area of surface. 



(e) Purity of water— chemical condition of water. 



2. Biological environmental factors — 



(a) Character of vegetation on banks and bottom. 

 (J) Interaction of animals and plants of the plankton, 

 (c) Interaction of animals and plants present in the water other 

 than plankton. 



Now, we have dealt with several features which give a characteristic 

 plankton-ethos to Lough Neagh plankton. Briefly enumerated, they may be 

 taken as — 



1. Large quantity of plankton present always. 



2. Great development of the blue-green algae. 



3. Huge quantities of Gomphosphaeria, Oseillatoria, Tabellaria. 



4. Presence of arctic and central Eui'opean organisms together. 



5. Desmid flora and prodigious quantities of certain species of 



Staurastrum at special times. 



6. Great abundance of My sis relicta. 



7. The relative abundance of all the different organisms that make 



up the Lough Neagh plankton. 



It seems to us that this study of the plankton-ethos has been rather 

 neglected. In only a few cases have attempts been made to correlate the 

 presence of peculiarities with other features in the physical or biological 

 environments. 



It is owing to the complex interaction of so many conilieting factors that 

 the plankton of two lakes in the same country and at the same latitude may 

 be entirely different. 



Very many of the fresh-water plankton organisms are cosmopolitan. 

 Notwithstanding this, the plankton-ethos of any lake is quite marked. 



