187 



from, e. g., succession on trap is seen in the earlier, pioneer stages. 

 Crustose and foliose lichens are never present — a statement which 

 holds true for unconsolidated rocks in general. The first stage 

 in the sand plain series may not inappropriately be termed an 

 edaphic desert. The soil lacks humus and the vegetation is very 

 open, the plants growing scattered about over the otherwise bare, 

 sandy soil (figs. 5,6); and since the distinctive species are either 

 annuals or rhizome perennials (or biennials), there are seasons 

 of the year when to all appearances these tracts are almost des- 



FiG. 6, Sand plains with scattered tufts of Andropogon scoparius; North 

 Haven. Most of the trees are black caks. Photograph after W. E. Britton. 



titute of vegetation. Here and there, however, distributed at 

 irregular intervals over the surface of the plain, are trees (most 

 commonly Quercus velutina) whose presence may doubtless be 

 attributed to the activities of squirrels since, despite the abund- 

 ance of seeds with which the ground underneath the trees is 

 strewn every fall, they exhibit no tendency to spread.* 



* The acorns "fall upon the sand under the trees, probably germinate and dry- 

 up before the radicles can reach a sufficient depth to obtain the necessary water. 

 In some unpublished investigations Prof. J. W. Toumey has found that in hard 

 soil the radicle is not able to work its way into the soil, but on account of the 

 lightness of the acorn it is tumbled about on the sand" (Britton, op. cit., pp. 

 578, 579). 



