194 THE PLANT LIFE OF MARYLAND 



not a barren tract but one which has recently been released from 

 cultivation. Thus in any but a virgin forest, the historical factor 

 as it may be called must be considered. In other words, it is neces- 

 sary to ascertain how long the area has remained undisturbed. But 

 conversely, the rapidity with which succession proceeds on any given 

 tract depends on the character of the soil; soils markedly deficient 

 in humus acquire but slowly the humus requisite for the support 

 of a mesophytic type of vegetation. One of the objects constantly 

 kept to the front in the progress of this Survey was the establishing 

 of a relation between the various types of soil and the vegetation 

 upon them. Yet a perusal of this chapter will show that the forests 

 have not been grouped in accordance with the thirteen or more types 

 of soil found in the region, for there is satisfactory evidence for 

 believing that on the same tract several types of forest may follow 

 one another, and further that these same forest types may occur on 

 soils as different from one another as clay and sand. In spite of 

 the variety of soils, the water content seems to be the most potent 

 edaphic factor; the differences in the vegetation supported by a light 

 and heavy soil, when both are placed in similar conditions as re- 

 gards drainage, are trifling compared with the differences between 

 upland and lowland vegetation. 



One matter which has rendered the study of this problem difficult 

 is the old enemy of two variable factors. It may be seen from an 

 inspection of the soil maps* that any particular kind of soil is apt 

 to occupy a fixed topographic position. For example, in St. Mary's 

 County the Leonardtown Loam uniformly occupies high ground and 

 is usually drained by a layer of Susquehanna Gravel underlying it, 

 while Sassafras Sandy loam just as regularly occupies low ground, 

 so that we cannot be sure that if Leonardtown Loam occurred at a 

 low level it would not have the same vegetation as the Sassafras 

 Sandy loam. But it has been possible to pick out areas of a num- 

 ber of soils occupying situations which are at any rate similar, 

 even if not identical. A comparison of the vegetation of these areas 

 has not revealed differences of a very striking nature. The really 



*U. S. Department of Agriculture, Bureau of Soils, Reports for 1900 and 

 1901. 



