PLANT STRUCTURE 11 



than in the experiments of Brown and Escombe so that for 

 a given A/na ratio diffusion will be a greater fraction of the 

 maximal value. The effect of wind is to reduce the diffusion 

 path up to the septum, i.e. the term nR/\^ and only to a less 

 extent to alter the extra resistance due to the septum itself. 

 In consequence wind will have a greater proportionate effect 

 on diffusion to a free surface than to or from a multiper- 

 forate septum. 



Thus Brown and Escombe's experimental work and sub- 

 sequent studies have established that the relatively imperme- 

 able cuticle with its evenly spaced pores of small dimension 

 permits a high rate of diffusion whilst affording considerable 

 protection to the mesophyll cells from external agencies. 



THE RATIO OF THE RATE OF PHOTOSYNTHESIS TO THAT OF 

 TRANSPIRATION IN LAND LEAVES 



The stomata then are the main pathway for gaseous dif- 

 fusion, and we next consider the magnitude of the concen- 

 tration of carbon dioxide in the intercellular spaces of the 

 leaf in terms of the external concentration. The internal 

 concentration will be determined by (i) the diffusion path 

 and (2) the net rate of consumption of carbon dioxide. 

 Heath (1951) and Gabrielson (1948) have both concluded 

 that the concentration of carbon dioxide in the intercellular 

 spaces is about one-third that in the air. An independent 

 method of measuring this concentration is available from 

 measurements of the water vapour lost per gram of carbon 

 dioxide assimilated. For it follows that as the stomata per- 

 mit the rapid diffusion of carbon dioxide into the leaf they 

 also necessarily result in the loss of water vapour from the 

 leaf. Penman and Schofield (195 1) found that during 

 summer growth an acre of turf lost 1 1,000 tons of water and 

 yielded 130 cwt. of dry matter. The ratio of the rate of trans- 

 piration to that of photosynthesis will be under any given 

 set of conditions, as may be seen from equation 2.1, a 

 measure of the ratio of the difference in concentration 

 multiplied by the diffusion constant for water to that for 

 carbon dioxide, because we may assume that the area and 

 equivalent length of the diffusion path between the external 



