92 THE BIOLOGY OF FLOWERING PLANTS 



can absorb from ordinary moving air about 0"i28 c.c. 

 carbon dioxide per square centimetre per hour. These 

 investigators also show that under ordinary favourable 

 conditions a sunflower leaf absorbs from 0*029 ^^ 0*044 c.c. 

 carbon dioxide per square centimetre per hour. Other 

 investigators have found higher figures ; Thoday (19 10) 

 finds assimilation to proceed at a rate equivalent to the 

 absorption of o'i49 c.c, and this is probably the most 

 accurate determination made for a (detached) leaf in 

 ordinary conditions. At the lowest estimate, therefore, the 

 assimilating leaf can absorb carbon dioxide at one fourth 

 the rate of a free absorbing surface of sodium hydroxide, 

 instead of at only one thirty-third that rate ; at the highest 

 estimate it is rather more efficient. 



Taking the case of transpiration we find a similar 

 discrepancy. Thus Bakke (1914) found that the loss of 

 water from a sunflower leaf might be as much as three fifths, 

 while from the dahha it might be nine tenths, of that from a 

 freely evaporating water surface. Such high values are 

 not infrequent, though one quarter to one half are more 

 usual ratios. 



The existence of cuticular transpiration does not 

 explain this difference ; it is much too small. To carbon 

 dioxide exchange the cuticle is, as we have seen, an almost 

 complete barrier. The explanation is given by the physical 

 laws which govern the diffusion of gases through small 

 openings. The fundamental work was done by Browne 

 and Escombe (1900), primarily for the case of carbon 

 dioxide. They also considered the application of their 

 results to transpiration ; Renner (19 10) has since extended 

 the work on transpiration. 



Browne and Escombe determined the rate at which 

 carbon dioxide diffuses through a pore, by a method simple 

 in principle. A nickel covering was sealed to the mouth 

 of a wide glass tube, at the bottom of which was an absorbing 

 layer of sodium hydroxide. The nickel was pierced by a 

 pore of the size it was desired to study, and the rate at which 

 carbon dioxide diffused through the pore from the air was 



