8 Richard Ii. Boerkcr 



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. 



