ETHERIAL OILS : NUMBER OF STOMATA 185 



eucalyptus, wormwood, and pine are all shrubs or trees 

 which occur naturally in dry stations. So great is the 

 amount of ethereal oil given off from the rue that it is said 

 to be possible to set it in flame on hot days. It has long 

 been supposed that this feature has some biological sig- 

 nificance. Dixon (19 1 4) has shown experimentally that 

 diff^usion takes place more slowly in an atmosphere laden 

 with such vapour. It is also known that less heat is radiated 

 through these vapours. Experiment has so far failed to 

 show, however, that transpiration is appreciably diminished 

 by this means. 



§ 26. Number of Stomata 



Reduction in transpiration may also occur through re- 

 duction in the number of stomata. The variability of 

 stomatal number in a single plant may be great, and is often 

 marked between two individuals in different stations. Neger 

 has collected some instances which show that, comparing 

 related species, the number tends to be less in those grow- 

 ing in dry stations, e.g. in the sedges and poplars. For 

 a single species, however, the exposed leaves have more 

 numerous stomata. Yapp found the number much greater 

 in the sun leaves of Spiraea, but the stomata were also 

 much smaller. As we have seen, sun leaves have in general 

 more numerous stomata than shade leaves. In alpine 

 plants the stomatal number seems to increase with eleva- 

 tion and exposure. An exhaustive study taking account of 

 stomatal size and of conditions of assimilation has still to 

 be made. The exact effect, too, of the light and heat 

 reflecting action of polished leaves has yet to be studied. 



§ 27. Reduction of Leaf Surface and Transpiration 



Lea! Fall. — We have dealt so far with various features 

 which tend to reduce diffusion through the stomata and the 

 epiderm. Transpiration may also be reduced by the 

 reduction of the evaporating surface and such reduction 



