JAMES BURTON OX HYDRODICTYON RETICCLATU.V. 591 



that non-sexual reproduction occurs when the water is clear 

 and there is abundance of chemical food material present with 

 appropriate temperature and light in fact with favourable 

 vegetative conditions while under less favourable conditions, 

 and with the presence of organic matter, decaying plants and so 

 on, in the water, there is a tendency for sexual reproduction to 

 take place. And he states that either condition may be readily 

 brought about at will, with plants grown under observation. 



Perhaps not the least interesting fact about Hydrodictyon is 

 the manner of its occasional appearances. After being plentiful 

 on one occasion it will totally disappear, and for perhaps several 

 years nothing will be seen of it. Then again, owing to no 

 particular cause which is understood, it has another outbreak, 

 and the water from which it has been absent for long is again 

 filled with it. These outbreaks are known in some parts as the 

 " breaking of the meres," and by other similar terms. I knew 

 that in times past Hydrodictyon frequently appeared in the 

 lake in Kew Gardens and for many years more than thirty, 

 I believe looked out for it in vain. During all this time I only 

 found one very small and unsatisfactory specimen. Then last 

 autumn a tremendous outbreak occurred, the water was so full 

 of it that at the lee end of the lake the Hydrodictyon was massed 

 together to such an extent that it was impossible to get good 

 examples. Two boats were on the water, with men gathering 

 it in with rakes and piling it in heaps on the shore. In rather 

 less than four weeks I again visited Kew, and though diligent 

 search at every part of the lake was made, not a single specimen 

 could be found. Prof. West in speaking of this phenomenon 

 in regard to various other algae says ' ; they usually consist of 

 species that are normally present in the waters." But that can 

 hardly be said in this case ; normally it is impossible to find an 

 example of Hydrodictyon in the lake at Kew. 



Personally I cannot suggest any better explanation of the 

 cause of the phenomenon than I gave once before. Speaking 

 of a similar outbreak of another alga it was said : "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 flower- 

 ing plants, which at the proper time develop and then die 



