8 Richard H. Boerker 
them. Foresters and botanists, in general, have proceeded on 
the assumption that light and soil moisture are necessary for 
germination. They have also noted that germination is acceler- 
ated in sand as against a heavier soil like loam or clay. Little 
has been done to inquire further into these relations. On the 
whole the effect of habitat factors upon the early development of 
plants has received more attention than their effect upon germina- 
tion. In the following historical summary, light in relation to 
germination and early development of plants will be considered 
first, since probably more work has been done upon that particular 
phase of the problem than any other. 
One of the oldest notions regarding light and its relation to 
plant growth is the one concerning the effect of artificial or 
natural shade upon atmospheric and soil moisture conditions. 
The forest experiment stations of Europe have long since worked 
out this relation in the forest, so that to-day these results are 
more or less well known to all foresters and botanists. Several 
Americans, working on the effect of artificial shading upon the 
growth of tobacco, have brought out results similar to those 
secured in connection with forests. Hasselbring (3) has shown 
that the transpiration of plants grown in the open is nearly 30 
per cent. greater than the transpiration of plants grown under 
cheese-cloth shade. The transpiration per unit of leaf surface 
was nearly twice as great in the sun plants as in the shade plants. 
Stewart (4) records the results of observations made in the 
course of tobacco experiments in Connecticut on the climate and 
soil conditions as affected by tents in producing a certain kind of 
tobacco. He concludes that under the shade of tents the soil 
retains more moisture, there is a greater relative humidity, and 
there is a reduction in wind velocity, all resulting in plants which 
are larger and of more rapid growth as compared to those grown 
without tents. To sum up the effect of shade it might be stated 
tersely: it lowers the air and soil temperatures and breaks the 
action of the wind; these factors increase the humidity of the 
air and this increased humidity results in less evaporation from 
the soil and less transpiration from the plant; the final con- 
sequence is a greater soil moisture content with its correspond- 
ingly good effect upon the growth. of the plant. 
