22 MISSOURI BOTANICAL GARDEN, 
of his original monograph, installing therein many changes 
from his former classification and adding several new 
species and farther territorial distribution. Other iso- 
lated notes and descriptions of species of the order 
found in America have been published by various authors 
at different times. 
It is the object of the present work to bring together 
the results of all these labors, together with my own re- 
searches of the past two years, into a revision that will 
give an accurate record of the Lemnaceae of North 
America, north of Mexico. This revision is based on a 
study of rich collections contained in the herbaria of the 
Missouri Botanical Garden, containing the Engelmann col- 
lection; of Columbia College, containing the Torrey and 
Austin collections; of Harvard; and of the California Acad- 
emy of Science: and furthermore upon a study of abun- 
dant living material of nearly every species, contributed 
by many correspondents or collected by myself. I desire 
to acknowledge as one of my greatest helps, in preparing 
this work, the exhaustive notes and sketches made by Dr. 
George Engelmann in his extensive study of the order 
during his lifetime. I am especially indebted to Miss 
Bertha Henney of Santa Cruz, California, for living 
material collected at that place and in San Francisco, 
and for valuable field-notes which have greatly aided to 
clear up many doubtful points that have hitherto ex- 
isted concerning the western forms. Lastly my sincere 
thanks are due to Dr. Wm. Trelease at whose sug- 
gestion the work was undertaken and to whom, for his 
kind aid in many ways, is largely due any merit it may 
possess. 
In classification I have followed, in a general way, that 
of Hegelmaier’s revision of his monograph, he being the 
recognized authority on the order, deviating from it only 
where a thorough study of a group or species seemed, to 
my mind, to demand a change. For instance, I have 
reversed the positions of Wolfia and Wolfiella, being con- 
2 
