322 W. A. CANNON 



posed to the diurnal temperature changes of the air about them. 

 Since the soil temperature exercised a predominating influence in 

 control of the rate of root growth, as appeared repeatedly, no 

 especial attention was paid to the air temperature, except to 

 record it. However, it may be said that most of the experi- 

 ments were conducted in a chamber where the air temperature 

 varied less than 2°C. during any experiment. Usually no at- 

 tempt was made to control the illumination, although at the 

 Coastal Laboratory constant illumination was secured by the 

 use of a 100-watt nitrogen electric lamp placed approximately 

 2 meters from the thermostats. 



In the experiments all of the seedlings were used, and none 

 were discarded because they were "abnormal." Therefore, 

 the maximum range of variation in rate of growth was secured. 

 One of the results obtained was a surprising effect on the vigor 

 of the seedling caused by varying the depth of earth which was 

 used to cover the seeds, suggesting in this fact a prolific cause 

 for failure to survive in nature. To quite do away with this 

 as a possible cause for variation in development seemed rather 

 difficult, although uniformity in planting was attempted. As 

 an instance of the effects of deep planting on the vigor of the 

 seedlings a single experiment mil be cited. Five seeds were 

 covered with 30 mm. of moist sand, and five seeds were barely 

 covered with the sand. Cotton plugs were used to prevent 

 excessive evaporation. Both cultures were treated in an other- 

 wise similar manner. At the end of about two weeks it was 

 found that the roots of the seedlings from the lightly covered 

 seeds were from 83 to 153 mm. long, while the roots of those 

 which developed from the seeds that had been covered deeply 

 measured between 44 and 75 mm. in length, with one that was 

 only 5 mm. long. Although there was thus a great difference 

 in the development of the two groups of seedlings, there was 

 nevertheless marked variation in the rate of growth in each 

 group, and this variation occurred in such a manner as to lead 

 to the conclusion that its cause lay in difference of whatever 

 nature in the plants themselves. 



