116 



out. The appearance of the SaUx association can not follow in time 

 that of the Solidago association, as has already been explained. Poly- 

 trichuni may colonize not only under the Solidago but also under a 

 more xerophytic type of vegetation as well. Both indicate moist 

 sand: the mosses by retention of moisture, the willows by retention 

 and depth of position. Consequently each develops contemporaneously 

 with the Solidago association, and in the early stages the associations 

 are not differentiated. Later the Solidago association is forced to 

 the outside. 



In both cases the succession is in a xerophytic-hydrophytic di- 

 rection. Nothing has been observed to succeed the Salix association, 

 even in the oldest and deepest blowouts. The mats of Polytrichum, 

 on the other hand, produce a peaty layer over the sand, which be- 

 comes so thick that it retains standing water and admits of the devel- 

 opment of a pond society. These ponds must be held by a water- 

 tight bottom, otherwise their water would soon drain out through 

 the sandy subsoil. As it is, they are conspicuously higher than the 

 general level of the country. The zones surrounding the ponds move 

 outward and upward and permit the continued growth of the pond. 

 This is evidenced not only by the position of young mats of Foly- 

 trichuin, 1>ut also by relic bunches of such typical sand plants as 

 Paniciini virgatuiii, now actually in the standing water. If continued 

 far enough the increase of the pond might ultimately lead to the 

 establishment of other associations, such as pondweeds or water-lilies. 

 Its growth is retarded, however, by the gradual deposition of wind- 

 blown sand, by the accumulation of soil by the aquatic plants, and by 

 loss of water because of increased pressure on the mucky bottom. 

 Most important of these is the deposition of sand, which will mix 

 with the peat and eventually raise the level of the soil somewhat 

 above the water-table. The later stages in the succession are prob- 

 ably similar to the meadows in the Kankakee area, except that the 

 latter represent primary successions on a large scale, instead of sec- 

 ondary successions in a small area. 



SuccKssioN 01' THE Prairie Formation by the Forest 



It was a matter of great interest to the first explorers and settlers 

 in Illinois that so much of the surface was occupied by prairie, and 

 that the forests were confined to certain physiographic divisions, es- 

 pecially the stream valleys. In seeking to account for this natural 

 feature, the earlier generation of scientists, and to some extent even 

 the modem ones as well, were influenced, or even prejudiced, by two 



