1 86 GROWTH 



than the percentage increase in the dry weight of the offspring. 

 An extreme case is presented when calcium is lacking, for 

 although the number of offspring is more than double the 

 number of the parent plants, the dry weight of the offspring is 

 37*5 per cent, less than the original dry weight of the parent 

 plants. 



The growth rate of a plant varies with the age of the or- 

 ganism and also may show periodic and seasonal variation. 



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 examples 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 pro- 

 vision 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 McDougall % 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 

 to 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 Petch has shown for 

 Hevea, a personal as well as a specific physiology of plants, 



* Patton : " Proc. Roy. Soc. Victoria," 1917, 30, 1. 



j Petch : "Ann. Roy. Bot. Gard. Peradeniya," 1916, 6, 77. 



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



