Periodicity of Freshwater Algce in Nature 159 
still less from the rainfall data alone. In the case of the present 
pond the question at once arises, how is it that during certain months 
the concentration may be increasing when the water-level is rising? 
It has already been mentioned in Section II that during the autumn 
the very considerable quantity of floating leaves of Potamogeton 
natans are in process of decay, and there seems no doubt that the 
products of this decajf, which must be considerable and which of 
course are returned to the water, are responsible for the marked 
increase in dissolved matter noted during October—-November 1920, 
and September—November 1918 and 1919. Moreover, as will be 
referred to later in Section V, during the autumn months (September 
—November) very considerable masses cf (Edogonium, growing 
between the leaves of the Potamogeton natans, gradually decay, and 
the resulting products must also have contributed to the increase in 
dissolved matter noted during the autumn months. 
Later on (in Section V) reference will be made to a great maximum 
of Zygnema, culminating in May (and occurring in all four years), 
but a quick decline setting in during June, at the end of which 
month this Alga is generally rare. Now between May 8th and May 
29th, 1920, the water-level fell 2-5 cm., while the concentration at 
the same time increased from 9 to 18 degrees—an amount which it 
seems impossible to account for by the mere evaporation of the 
water; and from May 29th to June 17th of the same year the water- 
level rose 1-3 cm., but notwithstanding this dilution the concentration 
increased still further to 20 degrees. There is some evidence, therefore, 
that the products of the decay of Zygnema are partly responsible 
for this increase in dissolved matter observed during May and June, 
1920. During these months in 1919 the water-level was falling, so 
that it is not possible to say how far the rising curve of dissolved 
substances was the result of decay of Algae, or of mere evaporation. 
It will be convenient here to refer to some periodic determinations 
of the freezing-point of pond-water made by Xranseau( 28 ) during 
1913-1915. He states that his results “indicate that the highest 
concentrations coincide with the periods of greatest rainfall and 
higher water-levels, and the periods of low concentration are coin¬ 
cident with low water-levels and drought ” ( l.c . p. 131 ). He considers 
that these results are due to the rains bringing in soluble salts from 
the upper layers of the soil, and by the silt, clay and suspensoids, also 
brought in by the rains, slowly settling to the bottom of the pond and 
canying “nearly all the soluble salts with them.” He limits this 
statement to “pools, ponds and streams fed by surface run-off,” 
