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E E. Fritsch. 
which often holds good between Cladophora (or Rhizoclonium ) and 
the epiphytic Diatoms ( Cocconeis , Synedra, etc.) which it bears. 
The latter increase to a certain maximum abundance, when they 
almost completely cover the surface of the green Alga and probably 
begin to impede seriously the access of the amount of light 
necessary for assimilation ; this leads to a great decrease in the 
amount of the Cladophora , followed by dying off of some of the 
epiphytes and colonisation of a new substratum by the rest. When 
the period of least abundance of the epiphytes approaches, the 
green Alga begins to increase in amount and slowly attains its 
maximum abundance, but renewed appearance of the epiphytes 
involves a gradual decrease of the Cladophora to a minimum again. 
In such cases the periodicity of relative abundance exhibited by the 
Cladophora depends intimately on the periodicity of its epiphytes. 
Irregular correlated factors depend on the co-operation of irregular 
external factors; in this way the effect of an irregular factor may 
be multiplied and may involve complicated irregularities, difficult to 
understand unless the starting factor is known. If periodical 
observation is to be of avail in biological investigations every care 
must be taken to secure adequate data regarding all possible 
factors. In addition to the external factors the inherent tendency of 
the plant has to be taken into account; but nothing can be said on 
that point till our knowledge of the influence of external conditions 
is more advanced. 
In determining the conditions for the development of the flora 
of any piece of water we still require a knowledge of other factors ; 
we may call them the determining factors. Such are for instance 
the geological formation in which the piece of water occurs, and its 
altitude, the chemical composition of the water, the maximum depth 
and relative inclination of the bottom, the relative exposure to sun, 
shade, wind, etc., the nature of the fauna, the relative air-content, 
the nature of the banks and bottom, the nature of in- and outflow 
and possible human influence ( e.g ., sewage, etc.) A knowledge of 
these factors is also necessary for a proper interpretation of 
periodicity, but they are of primary importance for ecological 
purposes. 
In collecting periodical data I have during the last years 
forwarded to those Botanists who have had the kindness to assist 
me schedules with questions, one of which has been returned with 
each sample of Algae. The questions are as follows :— 
(a) What Phanerogamic aquatics are present, in what amount, 
