112 



referred back to this for its origin, indicating that all fonner asso- 

 ciations of this type have sooner or later been destroyed. 



THE^ POIvYTRICHUM ASSOCIATION 



In the Dixon area the hydrophytic series is carried further, and 

 a new association, characterized by Folytrichuui jniiiperimnn, also 

 appears. The Solidago and Sali.v associations are also represented. 

 In all, six depressions show one or more of these associations and 

 illustrate not only the successions between them but their develop- 

 ment as well. For convenience they will be referred to by letters. 

 These depressions are near the Northwestern tracks al)Out four miles 

 west of Dixon. Blowout A is on the north side of the track ; B is 

 near the track on the south side ; C is east of the deposits of a large 

 blowout south of the track; D, B, and F are in this blowout or its 

 southern extension. 



Blowout A is a shallow depression, but with rather moist sand. 

 It is occupied mainly by a dense growth of Solidago graniinifolia, 

 with Bqinscfinii an'cnsc, Carex sp., and Spiraea salicifolia as acces- 

 soiT species. 



Blowout B is smaller in width and length, l)ut deeper and with 

 steeper sides. On the outside there is a ring- of Solidago gvaininifoUa 

 with an abundant growth of Arisfida tuberculosa. The most 

 abundant accessory species is Lespedeca capitata, anrl others of less 

 frecjuenc}' are Andropogou fureatus, Cassia- Clianiaeclirista, and a 

 few sterile g'rasses which could not be identified. Arisfida and Soli- 

 dago are almost equally abundant except at the inner margin, where 

 the former is slightly in excess. This zone extends up the hillside to 

 the typical bunch-grass and is rather sharply delimited from it. In 

 the center of this ring is the PolxtrieJiuni association. The moss 

 grows in dense mats, occupying all the surface in the deepest part 

 of the depression. These mats are very thick and spongy and sink 

 beneath the feet several inches. The clead stems grade ofif beneath 

 into a thick, brown, moist, spong}' laver of a somewhat peaty texture. 

 The mats are sparsely occupied bv solitaiy plants of the several acces- 

 soiy species. Solidago graminifolia is the most abundant of these 

 and extends entirely across, but the ])lants are much snialler than in 

 the association outside. The others are Lycopus americanus, Hyper- 

 icum uiajus, Sali.v pedicellaris, and Aster si\ There are also a few 

 depauperate relic bunches of Panicuin -Argatuui. There is a narrow 

 tension zone between the two associations, in which the mats of 

 moss are less close and the stand of Solidago less pure. The moss is 



