

1901 | GENETIC DEVELOPMENT OF FORESTS 291 
formation there are prairie, beach, dune, heath, swamp, and other 
plant societies ; also the forest itself may be divided into a num- 
ber of different forest societies. This leads naturally to the 
consideration of those factors that make up the plant society 
conditions. 
B. Ecological factors. For convenience this set of factors may 
be divided into edaphic, atmospheric, hydrodynamic, and biotic. 
Epapuic Factors. Schimper3 has regarded the soil and its 
properties so important that he has given the name edaphic for- 
mations to those societies which owe their existence to the 
peculiarities of the soil. More than any other form of vegeta- 
tion trees need a firm anchorage medium, and hence a deep 
soil. Since the roots are the absorbing as well as the holdfast 
organs, the water content of the soil is important. Warming’s 
classification of plant societies‘ into xerophytic, hydrophytic, and 
mesophytic is based upon the amount of water in the soil. 
More plant societies doubtless owe their characteristic physiog- 
nomies to the amount and condition of the water in the soil than 
to any other one factor. Since trees present a greater transpira- 
tion surface than other forms of plants they must occupy those 
positions where there is sufficient water to maintain the trans- 
piration current. This excludes them from those regions where 
the water content of the soil approaches the minimum; a stagnant 
condition of soil water is likewise injurious to trees. Probably 
here the exclusion of air and the presence of humic acids pre- 
vent the healthy growth and activity of the root system. In 
order that the forest condition may be obtained, therefore, the 
soil must be well drained, as well as watered. 
The physical properties of the soil play an important réle, 
for upon them depends the capacity of the soil to hold water. 
The water-holding properties of the different kinds of soil are 
too well known to need treatment here. The heat-absorbing 
and heat-retaining qualities of soils must be taken into account, 
for they often determine the presence or absence of certain 
plants. 
3 OD. cit. 4 OD. cit. 
