1908.] Tlie British Freshwater Phytoplanhton. 169 



constituent of the plankton, and sterile filaments of slender species of 

 Spirogyra, Zygnema, and Mougeotia occur in abundance. The more slender 

 species of Mougeotia are generally the most abundant of these filamentous 

 Algae, and in some instances they exhibit a coiling of the filaments such as is 

 known to occur in certain of the plankton-forms of Melosira. (Consult fig. 1.) 



Fig. 1. — A Coiled Filament of one of the Sterile Species of Mougeotia from the Scottish 



Plankton, x 300. 



The Protococeoideae are not very abundant in the deeper Scottish lochs 

 most of them having a decided preference for shallower and warmer water. 

 The most frequent are Botryococcus Braunii, Sphcerocystis Schroeteri, Glceocystis 

 gigcis, and Ankistrodesraus falcatus. 



Diatoms are conspicuous, largely due to the occurrence in quantity of 

 a few species. They occur throughout the spring and summer in large 

 numbers in the deeper lochs, as the temperature of the water never becomes 

 very high. Many of them are adventitious constituents washed into the 

 plankton from the bogs and shores, but some have become established as true 

 plankton-species. There are about 18 well-established species in the 

 plankton, but only the three species of RMzosolenia are of exclusive limnetic 

 habit, all the remainder occurring in other situations. 



The Myxophycese (Blue-green Algae) are very poorly represented in the 

 deeper lochs, and are by no means abundant in the smaller, shallower lakes. 

 The phenomenon of " water-bloom " is very rarely met with, and discoloration 

 of the water, even of the smaller lakes, does not occur either very often or 

 with any great regularity as the result of the accumulation of large 

 quantities of Blue-green Algae. 



The only periodic collections which have so far been reported upon were 

 those made in Loch Ness and examined by Bachmann,* and our own 

 * Bachmann, loc. cit., 1907, pp. 85 — 88. 



