CHAPTER XII. 



PHYSIOLOGY OF THE LEAF (continued} TRANSPIRATION 

 AND RESPIRATION. 



Transpiration Experimental proof Conditions affecting trans- 

 piration Not mere evaporation Regulation by stomata 

 Chloro-vaporisation Significance of transpiration Oxygen 

 respi ration Experiments. 



PERHAPS the most obvious function of the leaf is its 

 transpiration i.e. the giving off of water-vapour into the 

 surrounding atmosphere a process for which it is evi- 

 dently adapted by its thinness, expansion, the numerous 

 stomata, and the copious ramification of the vascular 

 bundles (venation) and intercellular spaces. 



It is easy to demonstrate the fact of transpiration by 

 observing the loss of weight undergone by a leaf placed 

 in a balance, or the bedewing of a cold belJ-jar held over 

 a living leaf; or, more exactly, by carefully fitting a leaf 

 so that its petiole passes through a cork into one end of a 

 U-tube filled with water, and noting the movements of 

 the water in the distal leg of the tube, and the decrease 

 in weight of the whole apparatus suspended on the balance, 

 as water evaporates from the lamina. Similar experiments 

 with whole plants prove the same. 



That variations of temperature and moisture in the 

 atmosphere affect the rapidity of this transpiration is also 



