12 UNIVERSITY OP MISSOURI STUDIES [154 



6. CONTROLLING FACTORS OF PLANT-ASSOCIATION. 



Since meteorological factors are common to the whole re- 

 gion, the primary divisions of the flora must rest on other 

 grounds. If water be regarded as the dominant factor in plant 

 life, since the rainfall is uniform, its eflfect is dependent solely 

 upon the drainage. Hills and steep cliffs retain the rainfall least, 

 hence are clothed with a vegetation more or less xerophytic, 

 while the valleys and swales receive the surplus waters and re- 

 tain them as streams and ponds, hence support a hydrophytic 

 flora. But drainage conditions do not enable us to come to a 

 full understanding of the flora. The rupestrine vegetation must 

 be treated as a unit, and here the controlling factor is the pres- 

 ence of limestone rock. In the mesophytic forest plain light 

 and shade appear largely decisive. Moreover the region is one 

 of tension between forest and prairie, the forest being, however, 

 normally dominant. 



There is usually a correspondence between definite plant- 

 associations and edaphic conditions. Thus the palustrous vegeta- 

 tion needs not only an abundance of water, it must have a soil 

 rich in organic materials, if it is to thrive. There are to be seen 

 vast marshes, the humus of whose soil has been burned by forest 

 fires, which have supported nothing for years but a thin layer of 

 Marchantia polymorpha and a few dwarf willows and now and 

 then a clump of Polygonum or Bidens. About artificial ponds, 

 which here are in all stages of development both as to soil and 

 vegetation, only a few semimarsh plants, the polygonums, 

 Bidens, Juncus, occur at first. As the soil becomes richer. 

 Heleocharis is frequent, then Alismaceae, Typha and Acorns 

 come in. If flats are sandy and pebbly, the vegetation becomes 

 semixerophytic in spite of an abundant water supply. A hillside 

 covered with a rich soil has an altogether different flora from 



