

1901 | GENETIC DEVELOPMENT OF FORESTS 293 
an ecological factor. In exposed situations the water and humus 
contents of the soil may be dried out. For example, a forest 
may be cleared and thus exposed to the sun’s rays to such an 
extent that the soil factors will be changed considerably. It is 
obvious that it is difficult to separate the light and heat factors. 
While it is a benefit for a plant to get as much light as possible 
up to a certain point, along with the light the plant may absorb 
too much heat. 
The carbon dioxid content of the air is so constant that it 
plays little or no part as an ecological factor. Those plants 
nearest the ground are slightly favored by the greater amount of 
carbon dioxid in the lowest layers of the atmosphere. The 
effect of winds on trees, however, is often pronounced in exposed 
situations. Besides destroying trees the wind may injure them 
to such an extent that in the struggle with disease and with 
other trees they will be the first to succumb. The wind is of 
great importance also in that it causes excessive transpiration. 
The effect of wind on seed distribution is so pronounced that, 
other things being equal, those plants whose seeds are most 
easily distributed will stand the best chance in a given area. In 
conclusion, therefore, the atmospheric factors of light, heat, and 
wind must be taken into account in trying to explain the presence 
or absence of certain plant societies. 
HypRopyNAMIc FACTORS. The term hydrodynamic is used 
here to designate the action of tides and waves upon strand 
vegetation, and the action of stream and ocean currents in dis- 
tributing seeds. These factors may play an important paft in 
determining the peculiarities of plant societies, hence the neces- 
sity of keeping them in mind. 
Bioric Factors. Two plants cannot occupy the same soil 
at the same time. The struggle for a foothold in the soil may 
take place between species of the same kind or of different 
kinds. Again, the struggle may be between plant societies, as 
the forest and heath, or forest and prairie. The line along which 
two societies meet has been called the tension line. Here it is 
that the struggle is most pronounced. If the other ecological 
