177 



These changes go on very slowly. There are now areas of 

 black-jack covering several square miles with scarcely a trace 

 of leaf-mold. This is illustrated especially by the black-jack 

 timber south of Havana between Bath and Ealbourne, where, 

 in a belt five miles wide, there is nothing but pure sand without 

 any covering of humus. In the country east of Havana, where 

 the sand is mostly confined to long wooded dunes, and near the 

 Illinois River, the formation of the soil appears to be more rapid. 

 Some of the wooded ridges back from the river have a coating 

 of leaf-mold only half an inch to an inch in thickness, and the 

 cacti still growing in it show that it has been but a few years 

 since its formation. 



With the first traces of leaf-mold such semi-xerophytic 

 plants as AquiU'tjut conudensis, Triosteum aurantiacum, Silene 

 stellata, Anemone virginiana, and Agrimonia mollis begin to ap- 

 pear, together with many other species common to most up- 

 land-wood associations, although many of the sand-loving xero- 

 phytes, such as Callirrhoe triangulata, Rhus aromatica, and Les- 

 pedeza capitata, still persist. As the soil increases in depth 

 more characteristic mesophytes appear, including Vagnera 

 racemosa, Vagnera stellata, Geum canadense, Asclepias exaltata, 

 and Eupatorium ageratoides. Parthenocissus quinquefolia be- 

 comes very abundant, climbing up most of the trees, and trail- 

 ing prostrate on the sand, covering it with a dense mat. The 

 arborescent flora is still unchanged ; the two oaks and the 

 hickory constitute nearly all of the forest, and the only addi- 

 tions are small scattering trees of Cercis canadensis, Morus ru- 

 bra, and Celtis occidentalis. 



None of the black-jack forests observed has as yet passed 

 beyond this semi-mesophytic stage except in a narrow belt 

 along the Illinois River (PI. XXL, Fig. 2), where plants from 

 the neighboring mesophytic and hydrophytic forests may spread 

 more quickly over the sand ridges. In such places the forests 

 of the wooded dunes contain but a small proportion of black- 

 jack oak, its place being taken by bur-oak (Quercus macrocarpa I 

 and white oak {Quercus alba). The leaf-mold is deep, and the 



