4 8o BOTANICAL GAZETTE [june 



cannot cause the absence of mosses. Competition with the abundant 

 herbaceous flora either in the spring or summer is only a secondary 

 cause, if worthy of consideration at all. If competition were a 

 prime factor, we should find somewhere in the floodplain succession, 

 either in the horizontal series from the water back to the upland 

 or in the series from the standpoint of time from the floodplain 

 formed by the younger stream as it begins deposition, up to the 

 old floodplain of the mature river which has nearly reached base 

 level, an association in which mosses take an important part. This 

 has not been observed on any of the floodplains under consideration. 

 It is not, therefore, a case of being crowded out by other plants, 

 but rather an inability to survive the unfavorable dynamic con- 

 ditions along a depositing stream, which are as effective in elimi- 

 nating mosses as was the active erosion of the earlier stages in the 

 stream's development. 



Spring stream succession. — At Otis, Indiana, and New Lenox, 

 Illinois, are numerous springs, the water of which is highly impreg- 

 nated with iron compounds. As the water comes in contact with 

 the oxygen of the air, bog iron ore is produced which builds up 

 mounds about the outlets of the springs until the water can no 

 longer force its way to the top for escape, and finds a lower exit 

 where there is less resistance to be overcome. Very frequently 

 numerous species of plants make up a large part of the foundation 

 structure of the tufa. Taking part in this tufa formation is a 

 coarse moss, Brachythecium rivulare. The chemical substances in 

 the water penetrate the plant tissues which, as they grow old, 

 resist decay and form a porous rocklike mass. In the larger 

 stream forming the outlet of such springs at New Lenox are 

 several species of Amblystegium growing on submerged sticks and 

 stones, but these do not enter into the tufa formation. A few 

 other species, not typically water forms, grow on sticks which 

 emerge from the water. 



A somewhat comparable case of the formation of travertine in 

 the waterfalls of the Arbuckle Mountains in Oklahoma has been 

 described by Emig (4), in which the two mosses Did y mod on topha- 

 ceus and Philonotis calcare are the species involved. Still another 

 species, Cratoncuron filicinnm, has recently been collected by 



