18 MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA UNIVERSITY 
Pickeringii. It is true that H. argyrocoma differs from the three Horkeliae mentioned by 
the number of stamens, which is 20; but there are two species of true Horkeliae with 
numerous pistils that also have 20 stamens, viz., H. purpurascens and H. pinetorwm. 
The characters which have been used to separate Horkelia and Ivesia, viz., the number 
of stamens and pistils and the dilated filaments in Horkelia, fail altogether. The nu- 
merous and more or less imbricated leaflets generally found in the Jvesiae, is a charac- 
ter lacking in I. Baileyi, but found in H. sericata and H. Howellii, which are in other 
respects good Horkelias. I have, therefore, come to the conclusion that it is best to 
unite the two genera. The characters that have been used to separate Jvesia from Po- 
tentilla, viz., the fewer number of pistils in the former, is, of course, of no value. — [vesia 
pygmaea Gray has very often as many as 20 pistils, while such species as Potentilla 
Coloradoensis has often not more than 12 or 15, and Ivesia Lemmonii, which is a good 
Potentilla and should be transferred to that genus, seldom has that many.’ There is, 
however, another character that can serve very well to distinguish Jvesia as well as 
Horkelia from Potentilla. The character was partly used in the original description of 
Horkelia, in which the dilated stamens were not referred to. In Potenti//la the stamens 
are inserted in the bottom of the hypanthium, very near the base of the receptacle, and 
at their bases there is a more or less distinct, annular and somewhat glandular thick- 
ening of the tissues. In Horkelia (with Ivesia included), the insertion of the stamens is 
farther out from the center of the hypanthium, so that there is a large open space be- 
tween the base of the receptacle and the stamens, and there is no trace whatever of a 
thickening (except perhaps in H. sacosa from Lower California). As Horkelia (Ivesia 
included) is a genus characteristic of the drier regions of the North American Pacific 
slope, it is well to keep it separate from Potentilla, even if the distinctive characters are 
not very prominent. 
LIMITATION OF SPECIES. 
It will be seen from the following that my opinion as to the limitation of species 
differs widely from that expressed in our manuals and from that of Dr. Watson in his 
Revision.2 As there is a tendency to acknowledge many and small genera, so is there 
also a tendency among botanists to admit many more species than was customary only 
a few years ago. My conception of a species agrees in the main points so far as Potentilla 
is concerned with that of Lehmann, as expressed in his Revisio of 1856. In Horkelia 
1 P. Newberryi Gray (Ivesia gracilis T. & G.) and P. Arizonica Greene (J. pinnatifida Wats.), are typical Potentillae, and 
were described in the wrong genus. 
2 Proc. Am. Acad. 8 : 549-573. 
