BOTANICAL SURVEY— SUGAR GROVE REGION 



287 



The Vnifolium Soviet ij. This is the rnifoliuin society, composed 

 ahnost purely of UnifoUuin ca)iade)ise, or mixed with the partridge 

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

 fiat-topped bouhkM-s, wincli in tlie course of time become covered with a 

 few inches of ahnost pure sand derived from the decay of the rock 

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

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





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Fiar. 22. "Wild Lily of the Valley" (Unit'olium) on a Detached Boulder. 

 Betula ienta at Left. 



amounts which can be conserved from rainfall in the scanty sul)- 

 stratuuL Such water is, of course, very meagre, the more so since the 

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

 drains freely on all sides. When such rocks are exposed to the dessica- 

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

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

 above them, Unifolium develops thriftily. This plant is not, however, 

 limited to isolated boulders, but occurs all through the area in numerous 

 situations, varying from this periodically xerophytic habitat to the 

 extremely mesophytic hemlock forest. It may therefore be inferred 

 that the purity of the association is due not to any preference of Uni- 

 folium for the habitat, but to the inability of other plants successfully 

 to invade it. 



