BOTANICAL SURVEY— SUGAR GROVE REGION 



287 



The Unifolium Society. This is tiie Unifolium society, composed 

 almost purely of Unifolium canadense, or mixed with the partridge 

 berry (MitcJiella repens) (fig. 22). This association occurs on shaded, 

 flat-topped boulders, which in the course of time become covered with a 

 few inches of almost pure sand (l(M-ived from the decay of the rock 

 beneath. Isolated as they are, plants in such habitats have no access 

 to a supply of permanent ground water, but are dependent on the 





.*>St 



Fig. 22. "Wild Lily of the Valley" (Unifolium) on a Detached Boulder. 



Betula lenta at Left. 



amounts which can be conserved from rainfall in the scanty sub- 

 stratum. Such water is, of course, very meagre, llic more so since the 

 habitat is not adapted physically to retain watei-, and at the same time 

 drains freely on all sides. Wlicii siicli rocks are exposed to tlic dessica- 

 tion of direct sun and wind, none of the seed plants can endure the 

 conditions, Init when protected by dense shade of the forest canopy 

 above them, Unifolium develops thriftily. Tliis ])l;nit is not. however, 

 Jimilcd lo i.solated l)oiil(h'i-s. ])ut occurs all through Ihe jirca in niiiurrous 

 situations, varying from this periodically xeropliytic habitat to tlie 

 extremely nicsopliylic hemlock forest. It may Ihcrefore \u' inrci-rcd 

 that tlie purily of the as.sociation is dui' not to any pi-ffn-cnci' of I'ni- 

 folium for Ihe habitat, but to the inabilitv of other i)hints successl'uUv 



• I • 



to invjidc it. 



