LIMITING FACTORS TO GROWTH MEASUREMENTS. 36] 



Atmospheric Pressure and Velocity of Wind. ■ 



Other factors which may influence growth are (1) Pressure 

 of the Atmosphere and (2) Velocity of the Wind. 



Jaccard (7) has shown that a diminution of pressure has the 

 effect of greatly increasing the rate of growth. It is also well 

 known that wind of high velocity checks the growth of trees. 

 This is partly from the direct breaking effect of very high 

 winds and also, as Schimper (18) has pointed out, from the 

 increase of transpiration caused by the wind. Neither of 

 these factors however is of much importance with regard to 

 the present or similar investigations. 



The variations of atmospheric pressure in the course of a 

 series of growth measurements are not likely to be large enough 

 to have any appreciable effect on growth. Under ordinary 

 conditions also the wind will not be strong enough to affect 

 the growth very much except in the way of increasing trans- 

 piration. 



Summary of this Section. 



To sum up, the three effective factors of the environment in 

 cases of undoubted reserve food supply are Light, Temper- 

 ature, and Water supply. Of these, the action of light is 

 doubtful, in many cases absent or negligible, and when pre- 

 sent cannot be conceived of as a limiting factor as that term 

 is used by Blackman (2) and in this paper. The other two 

 factors may either limit growth throughout or become in turn 

 limiting factors according as their intensities vary. 



It must be noted that the true temperature for the purposes 

 of this theory is the temperature of the growing organ itself 

 and not the temperature of the air. Temperature is therefore 

 a complex factor in so far as it may depend not only on the 

 temperature of the air, but also on the intensity of solar radia- 

 tion incident upon the growing organ. 



Water supply is a much more complex factor and depends 

 upon (1) Absorption and (2) Transpiration both cuticular and 

 stomatal. These factors in their turn depend (1) upon (a) 

 Soil Temperature and {b) Water content of soil ; (2) upon (a) 



9(8)06 (34) 



