LIMITING FACTORS TO GROWTH MEASUREMENTS. 367 



case of growth from reserve. Besides the conditions previous- 

 ly mentioned, viz.: — 



Temperature of the Growing Organ, 

 Water supply to the Growing Organ, 

 Light (of subordinate importance), 

 we must also have conditions favourable to — 

 Assimilation and Transpiration. 



Now as Blackmail and Matthaei (3) (13) have shown, assimi- 

 lation is itself a process which depends on 3 factors : Tempera- 

 ture, Light, and C0 2 supply, each of which may become 

 limiting and check the whole process. In the same way 

 transpiration may be limited by darkness, by saturation of 

 the atmosphere, by low temperature of the soil, or by absence 

 of water from the soil. 



The case of the growth of Tea I take to be an example of the 

 failure of one of these conditions, a failure which would proba- 

 bly not take place if the plant grew from reserve material. It 

 is a well known fact in Ceylon that during a period of very wet 

 and dull weather when the atmosphere is saturated with 

 moisture or almost so both day and night, the growth of the 

 young shoots of tea falls off very markedly, i.e., there are very 

 few young growing shoots to be found. The average tempera- 

 ture does not fall as a rule in these periods, for though the days 

 are cooler the nights are warmer than in drier periods, so that 

 the cessation of growth is not due to temperature. It is also 

 obviously not due to lack of water supply, for the humidity 

 is high and the rainfall great. It is often supposed that it is 

 due to the lack of organic food since the dull days check assi- 

 milation. This explanation can scarcely be accepted since 

 Blackman and Matthaei (3) have shown that the supply of 

 CO 2 in the air is always so small that it limits the assimilation 

 to a much slower rate than the light energy is capable of per- 

 forming. It does not seem probable that even on dull days the 

 intensity of the light is so low as to check an assimilation which 

 is always limited to a very slow rate by the small amount of 

 CO 2 available in the air. 



