TROPICAL EPIPHYTES. I 95 



plant (StcphaiiodiscKs, RichicricUa) \ arrangement of the indi- 

 viduals of a colony in a more or less stellate manner {AstcrioiicUa, 

 some TahcUarias) ; assumption of a delicate acicular shape 

 (Syncdra) ; formation of fat in the cell (many Diatoms and Cya- 

 nophyceae). and so on. 



In spite of these adaptations, however, most of the non-motile 

 organisms of the plankton sink to the bottom of the containing 

 vessel in the space of a few minutes after they have been col- 

 lected. How is it that this does not happen in nature? It has 

 been suggested that the continuous currents in the water, due to 

 the wind and other causes, help to buoy up the organisms of the 

 plankton ; but it is of course also possible that in collecting such 

 delicate forms they are damaged in some way or other so as to 

 deprive them of that power of floating which makes them so well 

 suited to their natural habitat. An interesting point connected 

 with the development of the diverse floating mechanisms is that in 

 some plants they have been found to be far more strongly devel- 

 oped in the summer than in the winter forms ; this is, imdoubtedly, 

 in some way connected with a lower specific gravity of the water 

 in summer, although the exact relation is not yet quite evident. 



If the plankton of any piece of water is examined from week 

 to week or month to month, we find not only astonishing varia- 

 tions in the quantity of organisms present, but also very marked 

 dift"erences in the specific constitution of the pelagic life. The 

 quantity of the plankton is generally very much less in the winter 

 than in the summer months, and the organisms composing it are 

 quite different in the two seasons. Thus in the Thames there 

 are four well marked annual phases, each characterized by its 

 own peculiar plankton. This periodicity exhibited by the pelagic 

 life stands in close relation to the external seasonal changes ; 

 some of the forms prefer cold, others warm water, and conse- 

 quently they flourish in those seasons which are most to their 

 liking. Some plants are particularly sensitive and consequently 

 only put in appearance for a very short space of time each year. 

 During their period of absence from the plankton these organ- 

 isms persist as resting spores in the mud at the bottom of the 

 piece of water ; when favorable conditions return the spores ger- 



