54 PKESIDKXTIAL ADOKESS SECTION C. 



the immediate aims of their investigations. To some api)lications 

 which are of ecological interest 1 will refer later. 



1 wish first to outline very briefly and without attempting; 

 to enter into detail some of tlie main results. 



Blackman's principle of limiting? factors* still holds the field 

 in spite of criticisms from various quarters. It states that a 

 process is limited in rate by that factor which is present in least 

 relative concentration or intensity. 



For carbon assimilation, under any given set of conditions 

 of light intensity, temperature and carbon dioxide concentration, 

 only one of these will ordinarily be the limiting factor, and the 

 rate of assimilation can be raised only by increasing this factor. 

 Increasing the othei's is without effect as they are already in 

 relative excess. 



Thus, under the conditions ordinarily occurring in nature, the 

 low percentage of CO2 in the atmosphere is the limiting factor on 

 a sunny day. The light is more intense and the temperature 

 higher than is necessary to enable a gi-een leaf to assimilate the 

 whole of the CO.^ that can reach the chloroplasts. When the COo 

 supply is artificially increased the rate of assimilation rises pro- 

 portionately. Increasing the light intensity on the other hand 

 is without effect, while raising the tempera'ture not only does not 

 increase the rate, but, if carried far, reduces it by lowering the 

 efficiency of the leaf. 



In the early morning and late evening, the low intensity of 

 light does not supply sufficient energy for the assimilation of the 

 CO2 as fast as it could reach the chloroplasts. Under these 

 conditions it is light which acts as a limiting factor. 



In very cold weather, the low temperature reduces to a low 

 level the maximum rate at which a leaf can work and tempera- 

 ture is then the limiting factor. In this case even an increase in 

 the supply cf COo is without effect. 



Examples illustrating similar relations under experimental 

 conditions are already numerous in the literature. The experi- 

 ments of Blackman and Matthaei with natural illuminationf . of 

 Brown and Escombe]: on the effect of varying partial pressures of 

 COo, of Blackman and Smith on water plants^ , and more recently 

 those of Willstatter and StoU on assimilation in velalinn to 

 temperature and light|| may be cited in this connection. 



There is one aspect of this principle of limiting factors whicli 

 has not. I believe, until quite recently been clearly stated, but 

 which must be taken into consideration if the principle is to be 

 appreciaU'd at its true value. 



According to the M'ell known cliemical " Law of Mass 

 Action." if two substances react together the rate of reaction is 



* Annals of Botany. 19. 1905. p. 281. 



+ Proo. Rov. Soc, B. 7fi. 1905, p. 402. 



t Ptoc. Rov. Roc. B. 70. 1902. p. ,397. 



§ Proc. Rov. Soc. B. 8.3. 1911. p. 389. 



H UritersucluitisT(Mi liher dip Assiniilatioii der Kolilensanre. Berlin, 1918. 



