PLANT ASSOCIATION OF WESTERN PENNSYLVANIA 



WITH SPECIAL REFERENCE TO PHYSIO- 



GR.APHIC RELATIONSHIP 



J. E. CRIBBS 



Grove City College, Grove City, Pennsylvania 



The development of the vegetation of any given region is 

 subject to a number of important factors. The variation of one 

 or more of these may suffice to initiate very noticable changes 

 in the composition of the vegetation. 



The water content of the soil, for instance, is the most import- 

 ant single factor influencing plant development; and other 

 conditions being equal, will determine whether the vegetation 

 will be of a desert, prairie, open forest, or mesophytic forest 

 type. The increased available supply of soil moisture, up to a 

 certain point, always affords a proportional increase in the rich- 

 ness and abundance of vegetative development. There are 

 always additional factors, however, which in conjunction with 

 this one, give rise to a wide range of habitats. Of these tem- 

 perature, exposure to wind, exposure to sun, and soil composi- 

 tion, are of most significance. 



One of the striking features which occurs is the close relation- 

 ship between these physical factors and the local topographical 

 conditions, so that it is customary to encounter a similar com- 

 bination of factors in like physiographical situations (e.g.: in 

 the ravine, valley, or on the bluff, or flood plain). This general 

 likeness in the combination of factors encountered under similar 

 conditions, produces habitats in which are found characteristic 

 types of vegetation. These vary as the factors vary; but never- 

 theless there is a striking resemblance noticed when the vegeta- 

 tions of these positions are compared. 



Western Pennsylvania may be divided into two general areas; 

 (1) the glacial drift, and (2) the unglaciated clays. The 



THE PLANT WORLD, VOL. 20, NO. 4 

 APRIL, 1917 



