u 



BREAKING OF THE MERES." 119 



any of its modifications in it. We may take it, then, that 

 Oscillatoi'ia agardhii is a normal plant, able by means of its 

 chlorophyll to fix the carbon dioxide contained in the water, just 

 as any other alga does ; while the Totteridge specimen, though 

 probably of the same species, has taken on a saprophytic habit, 

 has lost its chlorophyll, and is able only to derive its nourishment 

 from the organic matter in a state of decay present in its environ- 

 ment. This seems probable, because, while the organism in the 

 large body of comparatively clean water in '' The Welsh Harp " 

 reservoir has plenty of light and very little decaying matter at 

 its disposal, the smaller horse-pond at Totteridge is much more 

 shaded by houses, and no doubt, from the use to which it is put, 

 contains a large amount of partially decomposed organic matter, 

 thus favouring the tendency towards a saprophytic habit which 

 some of these lowly organised plants possess. Another detail 

 that seems to point in the same direction is the fact that the 

 green specimens lived much longer and seemed to flourish better 

 in a small aquarium than did the yellow ones, these latter not 

 being supplied, of course, with the impure water which would 

 contain the necessary food. When, however, this suggestion was 

 made to the authority already referred to, he replied that he 

 should not expect the plant ever to become saprophytic. It is 

 most likely that neither of these plants is very common ; I have 

 never seen either before. Mr. Hammond, who kindly drew for 

 me " The Welsh Harp " species, had obviously met with it, and 

 had a drawing in his portfolio which exactly agreed with mine. 



Of course, in some form they must always be present in the 

 places in which they occasionally appear so abundantly ; but the 

 causes which enable them to multiply in this manner seem to be 

 unknown. It cannot be a seasonal increase alone, such as we have 

 in flowering plants, which at the proper time develop and then die 

 away. In that case the "breaking of the meres" would be an 

 annual occurrence, or nearly so, with more tendency to regularity 

 than it seems to have. Clearly there must be some simultaneous 



JouRN. Q. M. C, Series II. No. 67. 9 



