50 PLANT PHYSIOLOGY 



can be opened for the free diffusion of gases, we see that the 

 absorbent power of leaves is still greater. According to 

 Brown's measurements it is fifty times that of a strong 

 potassic hydrate solution of equal surface.* That the dif- 

 fusion will keep pace with this absorption is conceivable 

 when it is known that the rate of diffusion through an 

 opening one millimeter in diameter is forty times as fast as 

 through an opening ninety millimeters in diameter. The 

 smaller the opening, within certain limits, the more rapid 

 the diffusion through it. Hence the movement of carbon- 

 dioxide molecules through the stomata of an insolated leaf 

 must be very rapid. 



The absorption of needed carbon-dioxide by green plants 

 is accomplished, then, by purely physical means, by diffu- 

 sion. The continued absorption of carbon-dioxide, and of 

 all other food-materials also, is accomplished by the same 

 means ; but in order that there should be continued absorp- 

 tion, continued inward diffusion, there must be continued 

 removal of what is absorbed. If this were not so, there 

 would presently result from the diffusion equal proportions 

 of the food-materials within and without the plant, and the 

 diffusion would then cease. The construction of foods out 

 of the food-materials absorbed removes the food-materials, 

 prevents their accumulating, and continues the absorption 

 by maintaining the conditions for diffusion. 



If we now exEumine the conditions and the means necessary 

 for the elaboration of carbon-dioxide, itself innutritions 

 though a food-material, into nutritious matters or foods 

 proper, we shall at the same time come more clearly to 

 understand how it continues to be absorbed. The condi- 

 tions and the means, besides the presence of carbon-dioxide, 

 are warmth, water, light of definite composition, chloro- 

 phyll, and living protoplasm. Warmth is necessary mainly 

 as one of the general conditions for life, for at any tempera- 

 ture at which protoplasmic activity is possible diffusion and 

 chemical combination are also possible. Water is necessary, 

 for its component atoms become chemically combined with 



* Brown. H. T. The fixation of carbon by plants. Nature, Sept. 14, 

 1899. 



