196 THE PLANT LIFE OF MARYLAND 



supports the most mesophytic vegetation of any of the upland soils, 

 and its merits are attested by the presence of nurseries and by the 

 large areas under cultivation. 



The nature of the forest growth now found on some tracts in this 

 district seems to be due not only to the facl that it represents an 

 early stage in the process of succession, bul to the fact that the tracts 

 have been used for so long a time for the growing of tobacco that the 

 supply of certain soil constituents, as for example, lime, has become 

 exhausted. The importance of the preservation of a balance among 

 the various soil solutes has been shown by Osterhout* who finds that 

 magnesium and potassium salts used separately are poisonous to 

 plants, but when mixed together the poisonous effects disappear. 

 Apart from the practical suggestion that these soils may be improved 

 by application of lime and of green manure, the inference to be 

 drawn from these facts is that we are here dealing with complex 

 conditions, hence the following statements must be made with reserve. 



Susquehanna clays. On this hard and impervious soil the vegeta- 

 tion is scanty and of a xerophytic character, except where a cover- 

 ing of humus has been accumulated. The Black Jack Oak is espe- 

 cially characteristic, with much Scrub Pine, also in smaller num- 

 bers oaks of other species. Chestnut, Hickory and Dogwood are 

 hardly represented. Plate XXXVI. , Fig. 2, gives a fair idea of 

 the clay country near Bowie, Prince George's County, on the line 

 of the Pennsylvania Eailroad. The trees shown are Scrub Pine ami 

 Black Jack Oak. 



Leonardtown Loam. This is known in some localities as "White 

 Oak Land." It undoubtedly supports much of this species, but is 

 characterized just as truly by Post Oak and Spanish Oak and by 

 Scrub Pine and Sweet Gum. Black Oak, Hickory and Dogwood 

 occur commonly; Black Jack Oak is found with Scrub Pine in 

 xerophytic situations, while Loblolly Pine is common locally. 



Collingtoii Sandy Loam. This soil shows the most mesophytic 

 vegetation of the region; White, Black and Bed Oaks, Hickory and 

 on the slopes, Beech and Tulip Tree, form the leading plant cov- 



*Osterhout, W. J. The antagonistic action of magnesium and potassium 

 Botan. Gaz.. vol. xlv, 1908, pp. 117-124. 



