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EDGAR NELSON TRANSEAU 
the concentration experiments, they are. But it is doubtful whether 
they need bear any relation to these experiments, for the concentration 
used is not ordinarily attained or even approached in nature. 
The Method of Reproduction. — -I have already spoken of the seem- 
ingly slight changes in conditions which initiate the production of 
zoospores. In nature they are for the most part produced during 
the earlier part of the vegetative period. I wish to speak more 
particularly regarding the reproduction in the Zygnemales. As is 
well known conjugation may occur between cells of the same filament 
(lateral conjugation), or between cells of different filaments (scalari- 
form conjugation), or they may produce aplanospores without con- 
jugation. This last method I have found to be much more common 
than has been hitherto supposed. The question arises, are these 
different methods of reproduction controlled by environmental factors 
or do they indicate different hereditary strains of the species? Field 
observations point to the conclusion that they are hereditary qualities 
rather than effects of the medium in which they grow. The evidence 
may be summarized as follows: 
I. All three modes of reproduction, indicated above, may be 
found in adjacent cells of the same filaments; (2) the three forms of 
reproduction may occur simultaneously in different species making 
up a single small mass of algae; (3) in certain ponds certain species 
have been found annually, producing only aplanospores, in other 
similar ponds producing only zygospores or both ; (4) in several species 
of Zygnemales the conjugation may be almost entirely lateral in one 
mass collected and just as completely scalariform in another. With- 
out a knowledge of the field conditions these last two proofs may seem 
as good evidence for the other side, but knowing the conditions under 
which these differences occur I think no one would question the in- 
terpretation given here. 
Summary. — In conclusion, I wish to summarize the more important 
facts brought together in this study of algal periodicity. 
1. Although algae germinate, develop vegetatively, and produce 
spores throughout the year, they may be conveniently grouped, on 
the basis of their complete life histories, into winter annuals, spring 
annuals, summer annuals, autumn annuals, ephemerals, and perennials. 
2. The contradictory results of experimenters on the production 
of spores in the algae may be due to the neglect of the normal pe- 
riodicity and vegetative age of the algae used in the experiments. 
