140 THE BIOLOGY OF FLOWERING PLANTS 



know any details. As the assimilation process goes on its 

 rate is affected, at one stage or another, by such factors 

 inside the plant as the rate of diffusion of raw materials, 

 and of finished products, and the flow of water, and by such 

 external factors as the supply of carbon dioxide, and of 

 light, and by the temperature. 



Our knowledge of the principles underlying the regula- 

 tion of assimilation — and of many other functions — by 

 external conditions is due to F. F. Blackman, who has 

 published, along with his collaborators, a series of papers 

 on this and allied subjects. The principles involved are 

 discussed in Blackman 's paper on " Optima and Limiting 

 Factors " (1905). Extensive data and discussions are given 

 by Blackman and Matthaei (1905), Blackman and Smith 

 (191 1), and Matthaei (1904). 



There are according to Blackman " five obvious control- 

 ling factors in the case of a given chloroplast engaged in 

 photosynthesis : 



(i) the amount of carbon dioxide available ; 



(2) the amount of water available ; 



(3) the intensity of the available radiant energy ; 



(4) the amount of chlorophyll present ; 



(5) the temperature in the chloroplast." 



Of these, two may be called " internal " — the amount of 

 chlorophyll, with which we have already dealt, and the water 

 available ; for the water in the chloroplast is regulated by the 

 water relations in the internal leaf cells. To these factors 

 we must add, in view of the recent work of Briggs (1923), 

 the supply of nutrient salts. 



Water Supply.— The question of the influence of water 

 supply is a difficult one. Probably the actual assimilating 

 mechanism in the chloroplast is always abundantly supplied 

 with water ; but we do not know this, and its relations must 

 be complex. The chlorophyll exists in the chloroplast, 

 a body of colloidal nature, suppHed with its water from the 

 cell sap either directly or through the protoplasm. The 

 condition of the plastid colloids must be altered by the 

 changes in the amount of water and of dissolved substances 



