120 GROWTH 



Eucalyptus regnans shows the greatest rate of increase of 

 area between the age of forty and fifty years.* The measure- 

 ment of the diameter of the annual rings indicates that the 

 growth rate falls off with time. In many instances growth in 

 thickness is periodic through the seasons on account of climatic 

 factors, thus at Peradeniya Hevea brasiliensis shows no growth 

 in thickness during the dry season January to March. From 

 the end of March to the beginning of October, the wet season, 

 growth is uniform ; whilst during the dry season October to 

 December growth is considerably less and sometimes may 

 cease altogether.! In instances such as this it would appear 

 that climate is all-important, especially as regards the provision 

 of adequate supplies of soil water. Thus the erratic growth of 

 Hevea observed during the second dry season may be pre- 

 determined by the amount of rainfall during the previous wet 

 season. 



In more temperate climates, soil temperature is a significant 

 factor; the observations of McDougallJ show that the root 

 growth of forest trees begins in the early part of the year when 

 the soil temperature reaches a degree sufficiently high for 

 absorption of water to take place, and stops in the autumn 

 when the soil becomes too cold. There is no inherent tendency 

 for periodicity, and when a resting period obtains during the 

 summer months, its cause may be found in the decreasing 

 water supply. There is, however, as Fetch has shown for Hevea, 

 a personal as well as a specific physiology of plants 3 an almost 

 untrodden field, and many of the results observed may be 

 ascribed to personal peculiarity rather than to this or to that 

 factor. 



Allusion has been made to the importance of the previous 

 history of the conditions and of the plant in determining the 

 activity of the plant at the present moment. In the present 

 connexion, the important and extensive work of Balls on the 

 cotton plant must be mentioned. From a long series of 

 observations, Balls concludes that the different behaviour of the 

 plants, as indicated by the crop of cotton, are the inevitable 



*Patton: " Proc. Roy. Soc. Victoria," 1917, 30, i. 



f Fetch: "Ann. Roy. Bot. Gard. Peradeniya," 1916, 6, 77. 



McDougall : " Amer. Journ. Bot.," 1916, 3, 384. 



Balls: " Phil. Trans. Roy. Soc./' Lond., B., 1917, 208, 157. 



