THE PERIODICITY OF FRESHWATER ALGAE 
forms, but in general is more abundant during May and June. Some 
local examples of perennials are Rhizoclonium hieroglyphicum (Ag.) 
Kiitz., R. fontanum Kiitz., Cladophora glomerata (L.) Kiitz., C. fracta 
(Dillw.) Kiitz., Pithophora varia Wille, Tolypothrix tenuis Kiitz., 
Hyalotheca dissiliens Breb., Desmidium swartzii Ag., Mougeotia genu- 
flexa (Dillw.) Ag. (in permanent ponds) and Zygnema pectinatiim 
(Vauch) Ag. (in permanent ponds). 
6. The Ephemerals are species having vegetative cycles of a few 
days or at most a few weeks' duration. Generations succeed one 
another rapidly during the periods of favorable conditions. These 
species are mostly plankton and soil algae. Zoospores are the usual 
means of reproduction, and aplanospores and akinetes carry them over 
the unfavorable seasons. Some examples of ephemerals are Botrydium 
Walrothii Kiitz., Ineffigiata neglecta W. & G. S. West, Pediastrum 
Boryanum (Turp.) Meneg., Scenedesmus guadricauda (Turp.) Kiitz. 
and 5. bijuga (Turp.) Wittr. 
The recognition of these classes is of ecological interest because of 
the greater ease of description of the life habits of a particular species. 
In the present connection it is of importance because the finding of 
great irregularity in the behavior of some form or group of forms may 
suggest the causes correlated with the irregularity, or a method of 
experimentation which will lead to the causes. I believe that the 
most prolific source of contradictory results among experimenters 
has been a lack of knowledge of the normal periodicity of the algae in 
the field, and the failure to appreciate the importance of the time of 
germination and the vegetative age of the materials used in the experi- 
ments. I have hoped throughout this study of the behavior of algae 
in the field, that it might furnish new points of departure for experi- 
mentation. 
The other results of these field observations may be conveniently 
presented under four heads: (i) The time and conditions of germina- 
tion, (2) the vegetative cycle, (3) the reproductive period in relation 
to external conditions, and (4) the method of reproduction in relation 
to heredity and external conditions. 
Germination. — The vast majority of zygospores, oospores and 
aplanospores germinate in the spring. But the period during which 
the spores of a particular species germinate is often extended over 
several weeks. This is indicated both by field observations and 
laboratory experiments. Further, there is some germination going on 
