56 UNIVERSITY OF MISSOURI STUDIES \_^9^ 



of its most characteristic species are verging upon extinction, 

 and in many places the ledges are becoming more barren year 

 by year. Naturally the vegetation is inaccessible to ordinary 

 processes of destruction; it suffers also little from competition, 

 hence is a zone of slight tension, as is shown by the fact that 

 here on the verges of the cliffs the prairie plants have found 

 a final asylum from the encroaching sylva. The destruction of 

 the flora comes about from two causes. The processes of quarry- 

 ing and blasting destroy it outright, and the clearing of the forests 

 at the summits and at the bases of the cliffs causes a gradual dry- 

 ing of the whole cliff affected, and allows the soil to be eroded 

 from the summits and slopes. The cliff vegetation suffers easily 

 from drouth. While little moisture is needed, that little must 

 always be at hand. Thus from the great drouth of 1901 no 

 other shrub suffered as severely as the red-bud (Cercis Cana- 

 densis), it being almost everywhere killed to the root, yet the 

 red-bud is common at the summits of cliffs, where at all times 

 there is excessive drainage. The drying up of springs and 

 streams as the result of deforesting is especially fatal to the cliff 

 vegetation, to which an always humid air is necessary for its 

 best development. 



A cliff may be divided into four subzones. i. The subzone 

 of wet and dripping rocks. 2. The subzone of comparatively 

 deep soil, usually at the foot of the cliff, or at the top of flat 

 ledges. 3. The subzone of naked, often perpendicular rock. 4. 

 The subzone of cliff summits. The first subzone is properly 

 hydrophytic, the second is mesophytic, the others xerophytic in 

 the main. These subzones tend to fuse and certain plants, such 

 as Cystopteris fragilis and Houstonia angustifolia, are found on 

 the cliffs from top to bottom. In no other portion of the region 

 do cryptogams play so important a role as here. Liverworts, 



