fares Ormapo GON LA CE AE 25 
personal interpretations, must rest upon one’s knowledge of 
all species and their variations. 
The ultimate criterion must be the constancy of such 
characteristics as methods of reproduction, size (within limits), 
shape of cells, opening of oogonia. How much are all these 
altered by ecological conditions of the body of water in 
which the alge are growing? Unfortunately our data here are 
far from complete. When one finds a combination of the same 
characteristics in a given alga year after year in various habitats, 
one can be reasonably sure of the constancy of the species. 
If an alga occurs year after year in the same habitat with a 
known species, differing in one or more morphological character- 
istics but evidently closely related to the type, it should be 
classed as a ‘‘variety.’’ As an example, the robust variety 
(majus) of Oe. grande has been collected with the type in nearly 
all collections from the United States. Such differences can 
hardly be accounted for on the basis of environmental influences. 
The category ‘‘form’’ should be regarded as a temporary 
disposition only. If an alga varies from the description of a 
given species and has been seen only once or twice, it is hardly 
possible to know whether the variation is constant or merely 
an ecological variation. If the form occurs with the type and 
is found on several occasions to maintain its morphological 
variation, it should be raised to varietal rank. <A ‘‘form”’ 
then should be retained only when the data are insufficient to 
pass on the constancy of the variations noted in the description. 
In the identification of members of the three genera— 
Bulbochaete, Oedogonium, and Oedocladium—and in the use of 
the keys to the species certain terminology is necessary to be 
able to differentiate among the species. The preceding pages 
have been written with the hope of elucidating such terminology 
and at the same time giving a brief introduction to the life 
history of species of the genera of the Oedogoniacee. 
Keys to plant genera and species have not always been 
distinguished from synopses. It seems to the author that a 
key is merely an arrangement of characteristics, by which through 
proper manouvering, an ‘‘unknown’’ becomes a ‘‘known.”’ 
A key has sometimes been defined as a scientific puzzle made by 
one expert to disturb another expert. Such keys may be 
interesting contributions if one already knows the field covered 
by them, but it is needless to say that the student, or any 
beginner for that matter, will find scant encouragement in them 

