te 
Seas 
A REVISION OF THE AMERICAN LEMNACEAE. 93 
vinced that from a morphological standpoint Wolffedla is 
more closely related to Lemna than is Wolffa. 
In synonymy I have aimed to cite all original descrip- 
tions of the American species, and in addition to include 
those published names which are found occurring in the 
collections examined. In these herbaria occur many man- 
uscript names which are of interest only in designating 
various forms. These it seems wise to omit. 
Lemnaceae is an order comprising four well defined 
genera and about twenty-eight species, distributed through- 
out the torrid and temperate zones. In our range the four 
genera are represented by about thirteen species and one 
variety. Of these perhaps but two species and the varicty 
are peculiar to it alone and ten are found only in the western 
hemisphere. To some extent the classification must necessar- 
ily be incomplete and perhaps slightly erroneous, due largely 
to the fact that in almost every species there is a strong 
tendency to vary widely according to the environments, 
and further because the flowers and fruit are but partially 
known in some species and wholly unknown in others. 
Again a difficulty is met with in undertaking a classification 
when it is found that a single species may have at least 
three marked stages of growth differing from each other 
so widely as to give rise to different specific names for each. 
In several species I recognize two stages which I designate 
as the active stage, the period when vegetative reproduc- 
tion goes on most vigorously, and the fruiting stage, or 
that period when the fruit is being produced, at which 
time the fronds are always smaller and more or less un- 
symmetrical. In other species three marked stages are 
recognized, the two above mentioned and a third which 
may be considered a resting stage. This, from its impor- 
tance, deserves more than passing mention. Hegelmaier, * 
in his monograph, describes and figures what he terms 
«< winter fronds ”’ in Spirodela polyrrhiza. The name, no 
* Hegelmaier, Friedr. Monogr. Lemnac. 81. May 1868. 
3 
