1919 ^HI9' '^^ ^^9 Succession 203 



Transeau, Nichols, and others, the center in some cases is as much as 6 

 meters (20 feet) higher than the margin. Our bogs do not usually have 

 a uniform surface, but are characterized by hummocks of Sphagnum often 

 6 dm. (2 feet) or more in height. Some, however, have a rather uniform 

 surface over a good deal of their area. Our bogs are usally surrounded 

 by a marginal ditch which in winter may contain 3-6 dm. (1 to 2 feet) of 

 water while no water at all stands on the bog itself. 



In the development of this depression at Covington into a bog we 

 need suppose only the lapse of time and the introduction of the ordinary 

 bog plants — Labrador tea, swamp laurel and cranberry. The continued 

 growth of Sphagmmi and the accumulation of the products of its decay 

 under the anaerobic conditions will gradually kill out the few bunches 

 of sedge present as well as the hardback except that in the Populus-Poly- 

 trichum zone. Hardback is the most characteristic shrub of the mar- 

 ginal ditch of Puget Sound bogs, but it is not commonly found where the 

 growth of Sphagnum has progressed far enough to form a layer of de- 

 cayed material of any considerable depth beneath it. The occasional 

 aspens in the lower two zones will likewise be killed by the Sphagnum. 

 When the sedge, the hardback and the aspen, now the only three species 

 of spermatophytes occupying the main portion of this depression, disap- 

 pear, we shall have, in the Sphagnum and the products of its decay, ideal 

 conditions for the establishment of bog plants. Since there is a large bog 

 only a mile distant we need suppose only the means of transporting these 

 this short distance. 



The Populus-Polytrichum association at present marks the portion of 

 this depression which in the course of natural development would evidently 

 become the marginal ditch of the bog. It was noted that where the leaves 

 of the aspen had fallen upon the substratum the Sphagnum covered by 

 them was either dead or dying. This was especially evident where a few 

 leaves lay close together forming a continuous covering, but was clearly 

 noticeable even where only a single aspen leaf rested upon the Sphagnum. 

 In one case an abundant growth of the mycelium of a fungus was found 

 upon the dead moss under a single aspen leaf. The leaves of the hard- 

 hack are much smaller and lighter, and it requires a large accumulation of 

 them to cause serious injury to the growth of the moss. Very few places 

 could be found where they had caused any injury at all, while the injurious 

 effect of the aspen leaves was evident wherever this species was abundant. 

 It seems probable that the conditions above described are a considerable 

 .(•Ktor in the formation of a marginal ditch around so many Puget Sound 

 bogs, as Turesson has pointed out. 



The flora of the marginal ditch varies a good deal in different bogs. 

 Hardback is, however, usually abundant, forming a dense thicket. The 



