188 LEAFLETS. 



The Qenus Tridnphyllum. 



The species of the very old genus Poientilla, were distributed 

 by Tournefort among his three genera Quinquefolium, Tormen- 

 tilla and Peniaphylloides. Linnaeus' retained Tormeniilla and 

 relegated the other twenty-fire 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 Tridofhyllum. 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 bieunial 

 duration. All other plants that ever were referred to Potentilla 

 are perennial, and very many sufErutescent. 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 Drymocallis 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 



