1G6 THE PLANT LIFE OF MARYLAND 



the average is 10 inches, indicating that most of the timber is sec- 

 ond growth. The characteristic "knees"* of the cypress are fairly 

 displayed in this swamp, although one misses the hanging bunches 

 of Spanish moss, Tillandsia, which help to give the weird effect to 

 typical examples of these swamps in the southern states. Other 

 trees, especially Black Gum, usually accompany the Cypress but are 

 present in small proportion. Very little undergrowth occurs, owing 

 partly to the dense shade and partly to the inundated condition of 

 such swamps. 



Succession. — The general principle of succession has already 

 been stated. It is desirable to consider at this point whether the 

 associations already described conform to the principle. Begin- 

 ning with the pine association, it was found by inquiry that cer- 

 tain tracts now covered with pine (or- pines with a sprinkling of 

 oaks) were in a high state of cultivation forty years ago, but that 

 after the Civil War were neglected on account of the difficulty of 

 securing labor. There is abundant reason for believing that in such 

 areas Scrub Pine is the pioneer invader, and the features of this 

 tree which fit it for such a role have been pointed out. But in spite 

 of its hardiness the Scrub Pine is a short-lived tree, so that sooner 

 or later openings appear in the originally pure stand. Besides old 

 age, the woodman's axe, lightning strokes and severe storms aid in 

 making gaps. Meanwhile seeds of hardwoods have been dropped 

 among the pines by the agency of wind, birds and squirrels, the 

 mode varying with the seed in question. Although oaks have a seed 

 which is apparently not easily distributed, the trees generally oc- 

 cur in such numbers that a large number of acorns are produced. 

 Moreover oaks as a rule will grow in a soil not well supplied with 

 humus. Hence in the openings in the pine woods oak seedlings 

 spring up, and owing to their having greater tolerance than have 

 pines, are able to grow, even though slowly, in such a position, while 

 the pines on account of their intolerance are unable under such con- 

 ditions to reseed the spaces left by a tree falling. As these openings 



*See Kearney, T. H. Report on a Botanical Survey of the Dismal Swamp 

 Region. Contrib. U. S. Nat. Herb., vol. v, 1901. Also Coulter, S. M., An 

 ecological comparison of some typical swamp areas. Rep. Missouri Bot. 

 Garden, vol. xv, 1904. 



