10 



Most of the physical properties of sand are discussed in connection 

 with the other edaphic factors. The chemical properties are also im- 

 portant. Organic matter and mineral substances in solution occur in 

 sand in very small proportions, consequently it is very deficient in 

 plant food. The soil is usually slightly acid, and this tends to keep it 

 poor in nitrogen, as the nitrifying bacteria do not thrive in acid soils. 

 The low content of lime keeps out animals with calcareous shells, such 

 as land snails. 



Rain sinks readily into the porous soil, and surface drainage is 

 very poorly developed. No streams at all are found on the sandy 

 islands, and those of the loamy flats are merely sluggish ditches, usu- 

 ally dredged. The map (PI. I) gives one an idea of the compara- 

 tive development of streams in sand and in clay. The ground water 

 issues from the sand all along the river bank. The rapid drainage 

 takes away the soluble content of the soil almost as soon as it is 

 formed, and this continual leaching keeps the soil poor, greatly re- 

 tarding the development of the vegetation. 



On the whole, physical conditions are severe in sand prairie, re- 

 sulting in more or less open associations. 



The Plant Environment 

 local history oe the vegetation 



In the general account of the sand region, it was shown that the 

 uplands on both sides of the river were covered by a growth of xero- 

 phytic prairie, and that this prairie was the first vegetation to invade 

 the sand-bars and islands of the overloaded river. It is probable that 

 a subsequent decrease in the volume of water left many of the shallow- 

 stream valleys mere broad, poorly drained flats. These were soon in- 

 vaded by the swamp and bog associations, now found in the north, 

 which must have been abundant along the bottoms and bayous of the 

 rivers. Certain relic colonies of the bog associations still persist at 

 the head of Matanzas Lake (Gates, 'lib) and along the north shore 

 of Quiver Lake. They are characterized by northern plants, as Bcnila 

 crccta, Peltandra virginica, the fern Onoclea, the lizard's tail, Saiirii- 

 rus ccrmius, and others. The old bottom of the Mackinaw River is 

 filled with peat, and the broad level areas were partly grown over 

 with swamp vegetation. The dry parts of the level areas probably 

 developed a xerophytic prairie growth somewhat less open than the 

 bunch-grass of the loose sand ridges. 



The arid climate which accompanied the retreat of the glaciers was 

 at about this time gradually changed, the rainfall was materially in- 



