292 BOTANICAL GAZETTE [MAY 
Again, the soil furnishes the plant certain organic and inor- 
ganic compounds, and hence its chemical properties should be 
considered. Asa rule, inorganic salts are present in sufficient 
quantities in the soil; indeed in salt marshes the presence of too 
great a quantity of salt in the water excludes trees, except ina 
few instances. Soils are more often deficient in organic com- 
pounds. Organic decay, since it furnishes most of the available 
nitrates, is of great importance. The amount of /umus is so 
essential that it often determines the character of a forest, not 
only in furnishing the nitrogen to the plants, but also in 
ameliorating the physical properties of the soil. Nitrifying 
bacteria are necessary for the conversion of organic matter into 
nitrates, and if these are excluded the processes of organic 
decay discontinue, and consequently a condition like that present 
in sphagnum swamps is developed. Here, generally speaking, 
trees are excluded, except a few characteristic species. 
We must look to the soil factors for an explanation of our 
most characteristic plant societies. There must be a soil. The 
water in the soil must be sufficient. The soil must be aerated. 
The amount of organic and inorganic compounds must not be 
too great or too little, and usually the soil must contain bacteria 
or other fungi for organic decay. If all these conditions are 
present in the right proportions, the soil is capable of supporting 
a luxuriant tree growth. 
ATMOSPHERIC FACTORS. By atmospheric factors are meant 
those which influence the aerial parts of plants. They include 
radiant energy in the form of heat and light, and also the influ- 
ence of wind. Of these light is the most important. The tree 
by virtue of its many planes of plagiotropic branches gives a 
greater surface on which the light may fall than is found in any 
other plant form. Principally because it has worked out this 
successful light relation, it is ecologically the culminating type 
of plant body. Once a dense forest is established, all forms of 
low vegetation, except those species that have special shade 
adaptations, are driven out. Attention has already been called 
to the necessity of heat as a climatic factor. Heat may also be 

