188 LEAFLETS. 
The Genus Tridophyllum. 
The species of the very old genus Potentilla, were distributed 
by Tournefort among his three genera Quinguefolium, Tormen- 
tilla and Pentaphylloides. Linnaeus’ retained Tormentilla and 
relegated the other twenty-five or thirty species to one genus 
Potentilla. Since then many a taxonomist has studied the Lin- 
naean Potentilla with a view of resolving its incoherent elements 
into a number of natural and acceptable genera. Among all the 
segregate genera that have been proposed, not one is better 
entitled to the rank of a genus than Necker’s Z7ridophylium. As 
its name indicates, it is founded upon species of Linnaean 
Potentilla having trifoliolate leaves. But this mark of the 
foliage is not the one which he considered essential. He makes 
the generic rank of the group to rest on the very small ovaries, 
greatly reduced styles, and minute naked achenes. The so-called 
Potentillas that evince these characters have other marks more 
obvious. Their roots are annual, or now and then of biennial 
duration. All other plants that ever were referred to Pozentilla 
are perennial, and very many suffrutescent. The whole aspect 
of this group is such as to enable an experienced botanist to 
recognize a member of it at first glance. This is not true of 
even such genera as Drymocaliis and Horkelia, for these are not 
habitally distinguishable the one from the other, or either one 
from certain plants that are still retained in Potentilla. 
The North American species of TRIDOPHYLLUM are perhaps 
more numerous than the European; but they are every one 
western, and in Necker’s day were yet undiscovered. His type 
of the genus is P. monspeliensis, and he does not seem to have 
realized the fact that P. supina has all the essential marks of 
TRIDOPHYLLUM. 
The genus in a small one, but is far more widely dispersed 
than almost any other segregate of Potentilla. The prompt ma- 
turing of the individual as annuals, and the small smooth seeds 
