;U»i SMITH : APPLICATION OF THE THEORY OP 



.it one time. It is of t lit- essence of the theory of limiting 

 ta« tors to suppose that given any physical factor in excess of 

 requirement, it will not, however much it fluctuates, affect 

 the growth which is all the time limited by another factor. 

 Thus, if a temperature fluctuating round, say 15° C.,on a dry 

 day is low enough to limit the growth, then if the same tem- 

 perature prevails on a wet day we do not expect any increase 

 of growth, for the growth being still limited by the low 

 temperature the large amount of water available on the wet 

 day is merely superfluous and does not affect the growth. 

 Similarly if on a day of given humidity and temperature the 

 growth is limited by the supply of moisture, then as long as 

 the humidity remains the same an increase of temperature 

 would not increase the growth, since this would be still limited 

 by the low humidity. The extra temperature would in this 

 case be merely superfluous. 



Thus, though the physical factors of the environment may 

 change sufficiently during 24 hours to bring in first one and 

 then another as limiting factors, yet at any one time {i.e., any 

 sufficiently short period) the growth is not the product of two 

 or more of the physical factors, but is regulated by one alone, 

 the other being in excess of requirements to a greater or 

 smaller degree. It seems probable therefore, since the curves 

 for Vitis Lindeni show in each case the effect of only one 

 factor throughout, in January temperature, in July humidity, 

 that whatever the ultimate explanation of the facts may be, 

 it will be one which can be brought under our theory or under 

 some slight modification of it. 



HI.— CONSIDERATION OF PREVIOUS WORK. 

 The theory of limit in-j factors being on the whole so clearly 

 applicable to the results given above, it remains to consider 



how Ear previous results ran be CO-ordinated by its application. 



Unfortunately it is seldom thai such observations have been 

 taken With sufficient accuracy or under such conditions as to 



famish any clear evidence for or against our point of \i«w 



