2, MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA UNIVERSITY 
It is, however, not out of place to mention the most valuable monographs and 
revisions of the genera. The most important are without doubt those by Dr. Christian 
Lehmann, of Hamburg, in his “ Monographia Potentillarum” of 1820, in his “ Revisio 
Potentillarum” of 1856, and in Hooker's “ Flora Boreali-Americana.” His “ Revisio,” 
an octavo of 230 pages, six tables of distribution, and 64 fine plates, will always remain 
one of our standard works. These, together with the following, form the basis of the 
present study: Nestler’s Monograph of 1846, that of Seringe in De Candolle’s Pro- 
dromus of 1825, of Torrey and Gray in their Flora, in 1849, of Gray in the Proceedings 
of the American Academy, Vol. 6: 1865; of Watson, in the same journal, Vol. 7: 1878 ; 
of Brewer and Watson in the Botany of California, Vol. 1: 1876, and of Greene in 
Pittonia, Vol. 1: 1887. To these should be added the scattered descriptions and other 
information in the works cited under the different species. 
In the original plan I intended to include only the species native and naturalized 
in the United States and British North America, and these species are nearly all illus- 
trated. As the additional North American species are not very numerous, it was 
thought advisable to include them also. Of the species exclusively Mexican and 
Central American or growing only in Greenland, as well as of a few introduced, which 
can scarcely be said to have established themselves, the descriptions are made somewhat 
shorter and printed in smaller type. None of these are illustrated. 
The plates are reproduced from pen and ink drawings made by Mr. F. Emil, from 
herbarium specimens. ‘The enlargements are from dissections and sketches made by the 
author and finished by the artist. Plate 1 is drawn entirely by me. 
At first it was not intended to include the revisions of Fragaria and Duchesnea, but 
as these two genera are so closely related to / otentilla, the Monograph would seem in- 
complete if they were omitted. The change in the plan was, however, made so late that 
the illustrations could not be included; their preparation would have caused a c¢ onsid- 
erable delay in the publication. 
HISTORY. 
The pre-Linnaean history I shall omit altogether, as the descriptions and the refer- 
ences to the literature of that time are in such a chaotic condition that it would be 
nearly impossible to write a history of any value. I refer any one interested in such a 
history to Pickering’s “Chronological History of Plants.” 
The real history of botany as a science begins with the issue of Linnaeus’ ‘“‘ Systema 
Vegetabilium” (1785), “Genera Plantarum” (1737) and “Species Plantarum” (1753). 
By these publications a foundation was laid upon which a systematic science could be 
