1913.] STEVENSON— FORMATION OF COAL BEDS. 153 



long on dry ground, and that they could have been retained even 

 after the forms had migrated to a swamp. Seward and Hill,^^^ on 

 the other hand, recognized indications in the structure that the con- 

 ditions of growth required development of characteristics associated 

 with the xerophytic habit. 



The readiness with which certain types of plants accommodate 

 themselves to the extreme dampness of swamps or to the aridity of 

 sands has been, long time, subject of investigation. Davis^^- says 

 that swamp plants growing at the water-level are drought-resisting; 

 their leaves are contracted, have dense cuticle and are often coated 

 with waxy or resinous materials. The condition against which they 

 are protected exists in swamps as well as in dry soils. Peat, though 

 holding much water, parts with it reluctantly ; even after the centrif- 

 ugal test, the retained water equals 142 per cent, of the weight of the 

 dried peat, and the material appears to be merely damp. Under 

 similar conditions the retained water in sand is but 2 to 4 per cent. 

 There is physiological dryness in peat; the water is ample but not 

 available. 



Coville^^^ has shown that another agency is important. He 

 recognizes fully the fact of physiological dryness, but he regards 

 another agency as of equal or in some cases .of much greater impor- 

 tance. The blueberry grows luxuriantly in swamps, but equally well 

 in the sandy soil of pine and oak woods on the Coastal plain and in 

 the spruce woods of the White mountain slopes. The factor deter- 

 mining distribution of this plant is acidity, it cannot thrive if the 

 soil be alkaline or neutral. The surface in the pine and oak woods, 

 as in the spruce forests, is covered with a litter of decomposing twigs 

 and leaves, whence organic acids are carried to supply the plant's 

 needs. The rootlets are without the fibrous appendages, which 



"^A. C. Seward and A. W. Hill, "On the Structure and Affinities of a 

 Lepidodendroid Stem," Trans. Roy. Phys. Soc. Edin., Vol. XXXIX., 1900, 

 p. 928. 



^^' C. A. Davis, cited in "Formation of Coal Beds," these Proceedings, 

 Vol. L., 1911, p. 601. 



^°'F. V. Coville, "Experiments in Blueberry Culture," Bur. PI. Ind., Bull. 

 193, 1900; "The Formation of Leafmold," Journ. Wash. Acad. Sci., Vol. III., 

 1913, pp. 87-89. 



