442 Ecology 



Botryococcus Braunii (which the present author regards as a member of 

 the Heterokontae ; consult p. 406) is a ubiquitous plankton-form, although by 

 no means confined to the plankton. It is much more abundant in some 

 lakes than in others and at times forms a yellow-green scum on the surface 

 of the water. This scum may subsequently become of a yellowish-red or 

 brick-red colour, owing to the development of a pigment, and the entire lake 

 may be coloured by it. 



The DesmidiaceaB occur in abundance as plankton-organisms mostly in 

 those lakes which receive a drainage- water derived from geological formations 

 older than the Carboniferous (W. & G. S. W., '05 ; '09 B ; G. S. W., '09 B). 

 The western British lakes are particularly remarkable for the richness of 

 desmids in the plankton. Some of the Scandinavian lakes are also rich 

 in this respect and the Yan Yean Reservoir, Victoria, is a notable Australian 

 example. As a rule, the surrounding drainage-basins of such lakes contain 

 a rich desmid-flora, but in most cases the desmids of the plankton differ very 

 greatly from the desmid-community of the surrounding area. The common 

 desmids of the bogs are only found in the limnetic region of the lakes as 

 casual or adventitious constituents, and the great majority of individuals 

 brought by the rains into this limnetic region, with its new conditions of 

 life, find it impossible to maintain their further existence, and rapidly perish. 

 On the other hand, the true plankton-desmids may be put into three cate- 

 gories : those which are exclusively confined to the plankton, those which are 

 exclusively plankton-varieties of species which frequently occur in other 

 situations, and those which are more abundant in the plankton than else- 

 where (W. & G. S. W., '09 B). Many of them show floating devices, such as 

 increased length of spines and processes, copious development of mucilage, 

 etc. The desmid-phase is at its maximum at the end of the warm period 

 and during the autumnal decline in temperature. In the British Islands 

 they are most abundant in September or, more rarely, in August, although 

 in some lakes certain species attain their maximum in June. It would 

 appear that the maximum vegetative activity of the group as a whole is 

 just after the highest temperature has been reached. In some lakes certain 

 species are perennial, but these also have their maximum at the same season 

 as the others (W. & G. S. W., 12). 



There are a few other Green Algae which are sometimes of importance 

 in the phytoplankton. A form of Tribonema bombycinum occurs in Lough 

 Neagh in fairly large quantities and also in the Danish lakes. All these 

 lakes are shallow and very different in character from the deep rocky lakes 

 of mountainous areas, in which Tribonema does not occur in the plankton. 

 On the other hand, in these lakes (and also in many of the shallow lakes) 

 the plankton contains various species of Zygnema, Spirogyra and Mougeotia, 

 principally in the late spring and summer. They are usually the slender 



