175 



soil is still a pure sand, but since it is protected by the trees 

 from the wind and from the light and heat of the sun, it does 

 not dry out so rapidly. For the same reasons the atmospheric 

 humidity is conserved, and the oxidization of organic matter 



TABLE X. BLACK-JACK ASSOCIATION. 



Cracca Virginian a 

 Opuntia huniifusa 

 Aristida tuberculosa 

 Euphorbia corollata 

 Teucrium canadense 

 Rhus aromatica 

 Crotonopsis linearis 

 Helianthemum niajus 

 Monarda punctata 

 Ambrosia psilostachya 

 Helianthus occidentalis 

 (Enothera rhombipetala 

 Leptilon canadense 

 Asclepias tuberosa 

 Sporobolus cryptandrus 

 Oyperus filiculmis 

 Lespedeza capitata 

 Panicum sp. 



Open 



Brush 



b*d c a a 

 a b a a b 

 o o g f f 

 o a a b a 

 o a a b o 

 f o o o o 

 c f c d d 

 o a o a a 

 o o a a a 

 ooooo 

 o o o c b 

 ooooo 

 ooooo 

 ooooo 

 ooooo 

 ooooo 



O 



ooooo 



odgbdobofcfo 



a b c c o o o b b 1) a 1) 

 ede fc feoeedf 

 a a o o o a b a a a o a 

 d o I) a o o c b o o a c 

 g f ooeoogoeao 

 o ooooooooood 

 o oooooaaaooo 

 o o o b o b c a o o o o 

 o o o a o b o o o I) o o 

 o o o o o o o o o o o o 

 o ooaooaooooo 

 o ooooaoooooa 

 o a o o o o o o o o o o 

 o o o a o o o o o o o o 

 o o o o o o o o o a o o 

 o o u o o o o a o o o o 

 oooooooooooa 



Other species: Chrysopsis camporum, Rudbeckia hirta, Bouteloua 

 hirsuta, Euphorbia Geyeri. The trees were nor. included. 



lying on or near the surface of the sand is retarded. The an- 

 nual leaf- fall is of course much larger, and instead of being- 

 blown about by the wind it is held by the dense growths of 

 Rhus and Cracca, and forms a layer over the sand which in- 

 creases annually in thickness. This holds a portion of the 

 rain-water at the surface of the sand, and is eventually con- 



See p. 159. 



