LIMITING FACTORS TO GROWTH MEASUREMENTS. 313 



AtHakgala, on March 9th, the sun shone full on the plant 

 from sunrise at 6.5 a.m. to about 1.30 p.m. It will be noticed 

 what a rapid increase of growth takes place at this period. The 

 fact that between 2 and 4 p.m. the growth curve falls more rapid- 

 ly than the temperature may perhaps be correlated with the fact 

 that after 1.30 p.m. the greater part of the flowering shoot was 

 in the shade. 



The difference between the growth of Purcraea at Kandy on 

 June 11th, a day when the shoot was exposed to bright sun- 

 light, and on June 12th, when the sun was obscured till about 

 3.30 p.m., is very striking and is greater than we should expect 

 on the assumption that the temperature of the surrounding 

 atmosphere is ' the governing factor in the growth of the 

 plant. 



These facts point to the idea that the factor which really 

 regulates the growth of the shoot is the internal temperature 

 of the growing region of which the temperature of the surround- 

 ing air is only a more or less rough measure. It becomes there- 

 fore of importance to find out the temperature of the growing 

 region. Owing to the difficulty of obtaining shoots at the 

 right stage it was thought advisable to use all the available 

 plants for growth measurements so as to have a large number 

 of observations on which to base deductions. The insertion 

 of a thermometer into the growing region would possibly have 

 injured the plant and rendered the growth measurements 

 unreliable, so that no records of internal temperature for Agave 

 or Furcraea flowering shoots were obtained. 



Failing this a number of observations of internal tempera- 

 ture were made in the case of the Giant Bamboo (Dendrocala- 

 mus giganteus) , of which species a large number of growing 

 culms was available at Peradeniya, Kandy, and Anuradhapura. 

 A small hole was cut into the growing region and a thermo- 

 meter bulb inserted. A continuous set of readings every two 

 hours was taken with several culms, of the temperature shown 

 by the thermometer in the culm and at the same time the 

 temperature of the surrounding air in the shade. As the culm 



9(8)06 (28) 



