20 G. T. HARRIS ON THE COLLECTION AND 



those of the bogs. A feature that strikes one in connection with 

 them is that they are often occupied almost exclusively by one, 

 or at least a very few, species. One moorland pool I examined 

 on Dartmoor was so filled with Staurastrum hrachiatum that 

 from every part of it the net was crowded with specimens, and 

 another small pool was quite green with the rod-like threads of 

 Hyaloiheca dissiliens, and practically nothing else occurred there. 

 Yet another pool was the exclusive preserve of Xanthidium 

 variahile. 



It is obvious that by adopting this method of collecting, bottles 

 might be altogether dispensed with, the residue of the filtering 

 being saturated with the preservative, drained and wrapped in 

 waterproof paper. This would be a very convenient method for 

 anyone wishing to cover a considerable district during a holiday 

 without being encumbered with storing appliances. 



One drawback is associated with the material collected in the 

 manner just described ; it is so concentrated, and is such a 

 heterogeneous mass, that when it comes to mounting the desmids 

 contained in it the difficulty of isolating them from the debris 

 of sphagnum, the aquatic larvae, rotifers, entomostraca and a 

 multitude of denizens that constitute a bog fauna, the mounter 

 may be excused if he loses patience with such material. Unfor- 

 tunately I have no remedy to oiler, in spite of the many endeavours 

 to devise some method which should, even partially, separate the 

 desmids from their surroundings. The best method I can suggest, 

 and the one I myself use, is to take a small portion of the material 

 in a pipette, place it in a shallow tray and well dilute it with water ; 

 then with a very fine pipette the desmids can be picked out with 

 a minimum of bog debris. They are transferred to a watch-glass 

 containing clear water, and if necessary to a second and a third, 

 obtaining clean desmids eventually by a process of elimination- 

 Of course the gelatinous secretion of desmids would in any case 

 make perfectly clean mounts very difficult to achieve. 



Mounting desmids is admittedly both a tedious and an uncer- 

 tain process. I have used practically all the formulae I could 

 find for mounting media, and however divergent the formulae^ 

 in one particular they all agreed, the chloroplasts sooner or later 

 were distorted in shape. I quite soon formed the opinion (hetero- 



