1920] TAYLOR— SUCCESSION OF MOSSES 471 



On the oak upland and in most oak and oak-hickory forests of 

 the subclimax type mosses are nearly absent, particularly where 

 decayed logs are not to be found. The same paucity of mosses 

 occurs in the beech-maple climax forests of this region, where 

 competition with other plants or chemical conditions of the soil 

 may be the leading cause. The increase in moss flora along the 

 Des Plaines River at Wheeling seems to be a result of former and 

 present better supply of moisture in soil and atmosphere. 



Rock successions. — The rock successions are poorly repre- 

 sented in the Chicago region. The early pioneer stages of lichens 

 and mosses, however, can be distinctly traced at Lemont, Illinois, 

 near the Des Plaines River, on rocks of Niagara limestone which 

 have recently been exposed, on the sides of an old stone quarry, 

 on a cliff in an open pasture, and in several small ravines. The 

 early crustose lichens are followed by Br yum argenteum and 

 Grimmia apocarpa. Ceratodon purpureus seems to succeed these 

 or even to appear with them on the flat rock surfaces, either on the 

 top of the cliffs or on the bowlders. Many rocks have been exposed 

 during recent excavations in straightening the channel of the 

 stream. These are frequently well covered with crustose lichens, 

 and the first moss to invade the lichen zone is Bryum argenteum, 

 so that in this case at least this species is a pioneer moss. Else- 

 where on rocks it seems often to come in later than Grimmia. At 

 the mouth of the ravines, wherever the rocks are still exposed to 

 xerophytic conditions, the struggle is going on between the mosses 

 and lichens. The pioneer mosses usually smother out the crustose 

 lichens, but in turn may be covered up by small species of the 

 foliose lichen group. The mosses here never become very abundant, 

 nor do they occupy large spaces. On the vertical faces there are 

 numerous small cracks and pits in the rock which offer a better 

 hold for typical crevice species, such as Funaria hygrometrica and 

 Gymnostomutn rupestre. Crevice forms are somewhat more 

 abundant in the cracks of a stone wall at Palos Park where the 

 mortar has disintegrated. At the quarry near Thornton, where 



limestone 



pu 



pureus. Within the limits 



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