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EDGAR NELSON TRANSEAU 
are here also in harmony with the idea that the length of the vegetative 
cycle is a function of the specific surface. 
Reproduction.— It is evident from the foregoing that in the Zyg- 
nemales and probably the Oedogoniales that the length of the vegeta- 
tive period is the important period for study and experimentation, 
while the period of sexual reproduction is the supplement to this 
period. In some of the other orders of algae this period of accumu- 
lation seems to be less important and reproduction may be induced 
at all times by changes in the environment. Zoospore production 
is induced in nature by a sudden rise of temperature, and by changes 
in light. The effect of nutrients and other chemical substances on 
reproduction cannot be determined from field observations. But 
when the collections from waters of sand and shale regions are com- 
pared with those of the prairie there can be no doubt that the number 
of fruiting species is very small. The length of the time required for 
the formation of the gametes, the union of the gametes and the for- 
mation of the spore is undoubtedly shortened by a rise in temperature, 
and lengthened by a lowering of the temperature. 
In these more specialized groups, then, the environmental factors 
operate in general to accelerate, or retard or even inhibit the process 
of maturing. 
One factor, however, deserves especial attention because of the 
fact that it has been emphasized so many times in connection with 
the reproduction of algae in nature: the concentration of the water. 
This matter of concentration has been appealed to both by those who 
think of it in terms of osmotic pressure, and those who think of it as 
increasing the mineral nutrients. The same idea is sometimes ex- 
pressed in terms of the water levels. In all these cases the assumption 
has been made that the lowering of the water levels in a pond or pool 
results in the concentration of the pond solution. Three years ago 
I pointed out that, in general, algae fruit more abundantly during 
high-water periods than during low-water periods. This statement 
can be made on a still surer basis to-day than at that time. Now 
some of Klebs's experiments have shown that the fruiting of algae 
may be accelerated by increasing the concentration. Perhaps it is 
these results coupled with the experience of seeing waters in aquaria 
become concentrated through evaporation that has led to belief in 
the correlation between concentration and fruiting. 
Since my figures ran contrary to this idea that algae fruit when 
