F. E. Frit sell. 
150 
As in the case of terrestrial vegetation we can consider 
freshwater algal growth from several points of view. We may in 
the first place merely compile a list of the species occurring in any 
piece of water, or in all the waters of a given area, i.e., determine 
its flora; such floras are of considerable phyto-geographical and 
systematic value, but in their present condition go little further. 
There is a remarkable lack of precision in the large majority of such 
floras, evident not only in the complete absence of adequate 
descriptions of the different habitats, but also in the lack of data 
regarding the relative abundance of the different forms. A true 
flora must be able to call up a picture of the vegetation of the 
district dealt with ; it must point out to us what are the important 
and characteristic forms and must correlate them with the habitat. 
Then only is it of any ecological value; if more precision in these 
directions had been employed in the past years we should start the 
ecological study of freshwater algal vegetation better equipped than 
is the case. 
In the second place we may study the aspect of the aquatic 
flora, i.e., arrange the species composing it, in their relative order 
of abundance as dominant and subordinate forms. This can only 
be done satisfactorily by examining the vegetation in question at 
frequent intervals in the course of a complete year (or better still 
of a number of years), although hand-in-hand with such complete 
observations casual examinations of other pieces of water are 
bound to yield good results. If a sufficiently extensive investigation 
of this kind is carried out it will be found that different pieces of 
water (often far apart from one another) contain the same dominant 
and to a considerable extent the same subordinate forms and we 
shall be able to classify these different waters according to their 
algal vegetation. A basis of this kind once established we shall in 
many cases be able to determine to which type a certain piece of 
water belongs by a single examination of its algal flora. Periodical 
observation is, however, always a desirable feature of this line of 
research and the necessary tardiness of this method of observation 
is compensated for by the fact that it yields numerous other 
important results on the biology of aquatic growth. It is a familiar 
fact that in the case of the Algae a form may be very abundant one 
month and almost absent in the next (cf. the table on p. 164 ); even 
in the case of species which persist throughout the year, it is 
generally found that they are more abundantly represented at one 
time than at another. These changes in the relative abundance of 
