1 9 20] TA YLOR—S UCCESSION OF MOSSES 479 



sequent species, as they are found only in somewhat xerophytic 

 species. 



We have, therefore, a retrogressive succession indicated by the 

 moss flora, which is a mixture of relic or antecedent, typically 

 mesophytic species and the subsequent xerophytic forms. Such 

 retrograde successions are not uncommon wherever surface condi- 

 tions of soil water and exposure to evaporation have undergone 

 rather gradual modification. 



Hydrarch successions 



Under this heading have been included all successions originat- 

 ing in water or very moist habitats, with the exception of the moist 

 rock succession already described. 



Floodplain succession. — This succession was studied at 

 several points along the Des Plaines River, as at River Forest, 

 Riverside, on the east bank at Wheeling, and also along Carroll 

 Creek. The work has been of importance only for its negative 

 value in establishing the fact of almost entire absence of mosses 

 in such associations. Late in the season a few immature plants may 

 sometimes be found, but these seem never to reach maturity if 

 growing on soil, although a few well developed sporophytes may be 

 found on plants growing on logs above the high water level. The 

 true floodplain is subject to inundation during spring rains and 

 during high water at any season. A great quantity of fine alluvial 

 sediment is carried over the land and settles to the bottom with 

 the recession of the water, leaving a crustlike layer of variable 

 thickness over the ground and on any vegetation which may be 

 present. The moisture conditions, except during the flood period, 

 are favorable to spore germination; but the frequent deposit of 

 fine material, particularly at the period when the moss plants 

 would begin the season's growth, seems to be sufficient to destroy 

 the ephemeral protonema which by any chance may begin to 

 develop. The immature plants found later in the season probably 

 come from late germination of spores which have escaped destruc- 

 tion or which have reached the floodplain from the surrounding 

 uplands after the spring inundation. 



Evaporation on a floodplain is not excessive, and the available 

 supply of soil moisture is high, so that these two conditions 



