202 Dr. F. E. Fritsch. On the Subaerial and [Jan. 2, 



formations, except as regards the two fundamental groups of marine and fresh- 

 water Algae, is by no means so marked a one as in the case of the terrestrial 

 higher plants. It is often, only by a study of extreme cases, that we can draw 

 any definite limits between one kind of algal vegetation and another, for 

 generally numerous intermediate types are to be met with. Algae, and 

 probably water-plants generally, are much more plastic than terrestrial plants, 

 and many forms of aquatics are no doubt capable of adapting themselves to a 

 variety of modifications in their environment. But there are also amongst 

 the Algae certain forms which are much more susceptible than others to such 

 variations in their environment, and which are therefore of more limited 

 distribution, and these are the forms which will characterise certain prevailing 

 conditions in the surrounding medium. They are the character-plants, which 

 will help us to determine our aquatic formations, and it is requisite to study 

 them primarily, and to determine the exact conditions which influence their 

 presence or absence. Other species will then be found to be almost constantly 

 associated with these character-plants, and will make up the subordinate 

 members of the formation. Whereas amongst terrestrial plants each formation 

 probably always has one or more character-plants, which occur in this formation 

 only, and are absolutely distinctive of it, aquatic formations will often lack so 

 emphatic a character ; they will probably in many cases be distinguished by 

 the way in which the different members of the formation grow together, 

 rather than by any individual species. It even seems quite possible to imagine 

 two algal formations of almost identical specific composition, but differing 

 from one another biologically or physiologically ; as far as I am aware, such 

 formations are not known to occur amongst terrestrial plants. 



The preceding remarks are intended to indicate the scope for research in the 

 field of aquatic ecology, a subject on which practically nothing is known at 

 the present day, but the importance of which it is well to recognise at the 

 outset of considerations like those with which we are dealing. We are 

 almost entirely ignorant of the true factors which lead to the development of 

 certain algal forms in one piece of water and of others in a second. Certain 

 preliminary researches on this subject, which have been undertaken in the 

 past years on the British fresh-water algal flora, have given a number of 

 interesting results ; some of these have been dealt with at another place, where 

 the subject of algal formations, briefly touched on above, is considered in 

 greater detail.* My time in Ceylon was, of course, far too limited to admit of 

 collecting more than a few meagre observations on possible tropical fresh- 

 water formations. Moreover it is quite impossible to fully characterise an 



* See Fritsch, " Problems in Aquatic Biology, with Special Reference to the Study of 

 Algal Periodicity," ' New Phytologist,' vol. 5, 1906, p. 149 et seq. 



