8 



physiography. The most important physiographical feature of the 

 region studied is the soil. Sand has peculiar physical and chemical 

 properties, which greatly influence the other edaphic factors. These 

 factors may now be discussed in detail. 



Temperature variations in sand during day and night and from 

 the surface to the subsoil are great. When the association is open, as 

 it usually is, the insolation is heightened by reflection ; heat and light 

 are thus both intense (Gleason, '10:34). 



The moisture relations of sandy soils are peculiar. Coarse-grained 

 soils hold relatively small amounts of water; but, on the other hand, 

 more of this water is available to plants than in fine-grained soils. 

 Coarse loose soils dry very readily at the surface ; but there is a, com- 

 pensating tendency here, too, as the dry surface-layer forms a mulch 

 which retards further evaporation, so that (in Illinois) there is always 

 considerable moisture in the sand a few inches below the surface. The 

 amount of evaporation therefore depends more upon the degree of 

 openness of the association than upon the kind of soil. It depends 

 upon the temperature of the evaporating surface, the relative hu- 

 midity of the air, and the velocity of the wind (T. Russell, '88). 

 Quantitative evaporation experiments in plant associations have been 

 carried on by means of porous cup vaporimeters- at the Desert Lab- 

 oratory at Tucson, Arizona, by Livingston ('06) and on Long Island 

 by Transeau ('08). Transeau's work shows that in a given locality 

 the evaporation (at ground level) is greatest in open associations. 

 Dr. F. C. Gates, during the summer of 1910, carried on a number 

 of evaporation tests in the different associations in the Havana local- 

 ity, and has kindly permitted the use of his results. The figures in 

 the table below express relative amounts of evaporation. These read- 

 ings extended over most of the month of July. 



Relative Evaporation in Associations near Havana, July, ipio 



Standard evaporimeter, in sandy flat near marginal dune i.ooo 



River beach, on dry sand 933 



Bottomland forest across the river (Sali.r-Acer) 560 



Bottomland forest composed purely of Salix 443 



Bunch-grass, Leptoloma consocies 1.178 



Bunch-grass, Bragrostis consocies i .040 



Blowout, large bare area 1.270 



Black-oak forest 660 



Black-oak forest, beginning of mesophytic stage 550 



Mixed forest, near the center 293 



Mixed forest, near the margin 356 



