LIMITING FACTORS TO GROWTH MEASUREMENTS. 311 



Observations were made in this way on the following occa- 

 sions and at the following places : — 



(1) On Agave Morrissii in the Hakgala Botanic Gardens 

 from March 9 at 6 a.m. to March 10 at 6 p.m. Elevation 

 5,600' ; highest temperature reached 21-2° C. (70-2° P.). 



(2) On Agave Americana at Hakgala from June 1 at 8 a.m. 

 to June 2 at 6.30 p.m. Elevation 5.600'; highest temperature 

 reached 21-8° C. (71-2° F). 



(3) On Furcraea gigantea at Kandy from June 11 at' 8 a.m. 

 to June 12 at 6 p.m. Elevation 1,700'; highest temperature 

 27-8° C. (82-0° P.). 



The curves of growth and shade temperature observations 

 are given in Plate 22. 



Growth limited by Temperature. 



It is obvious from these curves that the growth throughout 

 shows a remarkable agreement with the temperature of the 

 surrounding air. In every case the rate of growth increases 

 toward the middle of the day, slackens in the evening, and then 

 gradually decreases through the night. Perhaps the most 

 instructive example is the case of Agave Morrissii at Hakgala. 

 The first day was bright and fine with hot sun, and the air 

 temperature reached 21-2° C. (70-2° P.). The second day was 

 dull and cloudy with rain, and the temperature did not rise 

 above 17-2° C. (62-9° P.). The difference in the rate of growth 

 was most marked. On the first day the greatest growth, that 

 between 12 noon and 2 p.m. , was 1*95 cms. On the second day 

 it was only "95 cms. between 2 p.m. and 4 p.m. 



The lowest temperature ever recorded in this set of experi- 

 ments, 9-6° C. (49-2° P.), occurred at 6 a.m. on March 9, and 

 though the temperature rose by 8 a.m. to 16-3° C. (61-3° F.). 

 the resulting growth in the two hours from- 6-8 a.m. was only 

 • 1 5 cms. Throughout the three curves shown on Plate 22 a close 

 correspondence between growth and the temperature of the 

 surrounding air is obvious. Other observations taken in 

 addition to those shown fully confirm this general conclusion. 



