LIMITING FACTORS TO GROWTH MEASUREMENTS. 346 



growth permitted by the higher temperature. Such is how- 

 ever not the case, for the temperature rises on January 9th 

 to 27 '3° C. and on January 10th to 26- 1° during the day, while 

 on July 6th it rises only to 25 • 1° C. and on July 7th to 25 ■ 0° C. 

 That a temperature of 27° C. in January on a day of low 

 humidity should result in increased growth, while a lower 

 temperature 25°C. in July on a day of much higher humidity 

 should be followed by a very marked decrease in growth-rate 

 is inexplicable on our theory, nor do I see any suggestion or 

 theory which can account for the facts and help us out of the 

 difficulty. The records of soil temperature at one foot below 

 the surface at the Experiment Station, about a mile away, were 

 obtained and showed a difference of * 5° F. between the two 

 periods at 9.30 a.m. and 3.30 p.m., the temperature being 

 higher by this small amount for the July period. This if it 

 were sufficient to have any visible effect, would act in the 

 direction of increasing the water supply and therefore would 

 make it still more inexplicable that at this period the water 

 supply should be insufficient during the day. The number 

 of rapidly growing roots from this plant was greater 

 in July than in January, so that it is possible that the water 

 supply which was always in excess of requirement for the few 

 roots growing in January, would be found to be inadequate 

 for the larger number of roots growing in July. Another 

 possible loophole of explanation would be furnished if it should 

 be noted that the leaf -area of the plant was greater in July than 

 January, for then the much larger transpiration from the 

 greater leaf-area would presumably allow of a smaller water 

 supply for the growing roots. Though at the time sufficient 

 attention was not directed to this point, yet there is no reason 

 to suppose that a marked leaf-fall took place previous to the 

 January readings, or it would not have escaped attention. 

 Further observations on this point may serve to clear up the 

 difficulty. It remains now to notice that whatever the ex- 

 planation, the facts support our theory so far as this, that only 

 one factor of the physical environment regulates the growth 

 9(S)0fi (32) 



