DISTKIBUTION OF RHAMNUS IN AMERICA. 85 



where the mercury seldom in the coldest wiuter indicates the 

 freezing point of water, and snow never falls, up to where 

 zero of Fahrenheit is reached, and snow falls to the depth of 

 ten feet, or even more. Nothing approaching these climatic 

 extremes are to be found within the limits of the East Amer- 

 ican Eihamnus territory. Indeed, so very great is the diver- 

 sity of environment on that narrow strip of the far- western 

 field, that one might with reason expect a greater number of 

 species than have been recognized hitherto. 



Before taking up the subject of the actual dispersion of the 

 species it may be useful to indicate certain natural and easily 

 recognizable sub-divisions of the genus. The name of 

 these^^and their characters are as follows: 



1. Khamnus proper. Leaves thin, deciduous: fruits 

 black: pyrenes thin- walled, indehisceut: seeds with carti- 

 laginous testa, grooved on the back; cotyledons foliaceous, 

 revolute-margined. 



2. Frangula. Leaves firmer: fruits black: pyrenes 

 thick, indehisceut: seeds with thin testa, not grooved; coty- 

 ledons fleshy, plane. 



3. Xanthorhamnus. Leaves coriaceous, evergreen: ber- 

 ries cherry-red: pyrenes crustaceous, promptly dehiscent: 

 seeds with crustaceous testa, deeply grooved or excavated on 

 the back; cotyledons fleshy, strongly curved. 



Rhamrms proper, though strongly developed in the Old 

 World, is very feebly represented in America. In fact, we 

 have but a single species which can with perfect confidence 

 be so referred, namely, 



/ 1. R. LANCEOLATA, Parsh, Fl. i, 166. This, though ap- 

 parently nowhere plentiful, nor continuously distributed, 

 occurs at intervals between western Pennsylvania and eastern 

 Nebraska; thus however illustrating a general law respecting 

 the distribution of the Rhamni of the East, that their great- 

 est range is east and west, whereas, on the opposite side of the 

 continent the species take a meridional line of dispersion, 

 holding to such lines very strictly. 



