260 



these, the most important in itself is probably water. This region is 

 abundantly supplied with precipitation quite uniformly distributed 

 throughout the year. In addition, it lies in the immediate proximity 

 of the water-table level of Lake Michigan, which makes it to a large 

 degree independent of precipitation. The sandy soil is quite favor- 

 able for furnishing the plants with water, which the particles of 

 sand hold as capillary films. The physiological supply is probably 

 about 95 per cent, of the physical supply. 



What seems the second factor in importance is the food material 

 in the soil. Sandy soil is notably deficient in soluble food material. 

 The relatively rapid eremacausis* characteristic of sandy soils, 

 caused by ready admission of atmospheric oxygen, accounts for the 

 destruction of much of what would have been available plant food 

 under other environmental conditions. Furthermore, soluble mate- 

 rials, and even insoluble ones, are gradually leached out of the soil as 

 the rain percolates through it instead of running off as it does in 

 most soils. 



With respect to light, plants of the sandy soils thrive best with a 

 maximum, and this partially explains the lack of density in the vege- 

 tation under trees on the sand. Wind has a marked influence upon 

 the vegetation of the dune regions, although for the most part its 

 action is upon the environment directly and upon the plants only 

 more or less indirectly. Wind increases the evaporation of water 

 from the plants, but many of those which are modified to reduce trans- 

 piration have an abundant supply of water, so, at least to a certain 

 extent, such modification is inherent in the species and is not pro- 

 voked by the direct eft'ect of the environment. 



Influence oe Lake Michigan 



Lake Michigan exercises a leveling influence upon the region in 

 so far as temperature is concerned. The most evident influence is, 

 of course, upon the shore itself, which in places is built out and in 

 others is torn down. This has had a very marked effect upon the 

 beach associations, which will be discussed in the proper place. The 

 fluctuations of the lake within the last sixty years are shown in 

 Plate XLIII. Tidal waves are of rare occurrence (May 12, 1905, 

 and April 29, 1909). They may violently modify the vegetation, but 

 they do not occur sufficiently often nor are they sufficiently powerful 

 to permanently modify it. Such waves are seldom over 1.5 meters 



*The state of affairs in v.hich humus-forming matter is so rapidly oxidized 

 that no humus is formed. 



