56 The Ohio Journal of Science [Vol. XX, No. 3, 



Jost (2) seems to consider hairs as effective in retarding air 

 currents' near the surface of the leaf: "Development of hairs 

 full of air can also effectively retard transpiration, since such 

 a covering protects the plant from the effects of air currents, 

 producing a superficial region free from atmospheric move- 

 ments." 



Gager (3) gives practically the same statement: "Certain 

 structural features of the plant operate to reduce transpiration. 

 The epidermal hairs, as for example on mullein leaves, tend to 

 retain the more humid air near the surface of the leaf, even 

 when the wind blows. " 



Cowles, in his text book (4) gives a somewhat different 

 aspect given to the whole matter: "Hairs commonly are 

 believed to have an important role in the retardation of trans- 

 piration, but the evidence for this view is not abundant. 

 Probably they are much inferior in this respect to cutin or 

 even waxy coats. Wooly felt would seem to be most efficient. 

 Removal 'of hairs in Stachys Janata results in an increase of 

 20% to 50% in transpiration. Evaporating surfaces covered 

 with hairy felt have been shown to lose much less water than 

 without the hairy coverings. Similar effects might be looked 

 for in scale covered leaves or leaves with branched hairs, when 

 the hairy coat is dense enough to retard water vapor. In most 

 plants that are hairy, however, the hairs are erect and more or 

 less scattered, so it is difficult to see how they can appreciably 

 retard water vapor, though their presence may reduce the 

 evaporating surface. It must be admitted that the known 

 uses of leaf hairs are small in comparison with their abundant 

 development. While the discovery of advantages now unknown 

 is possible, it is much more likely that most of such hairs are 

 of little or no advantage to the plant." 



Little actual experimentation has been done on the relation 

 of hairy coverings to transpiration. Wiegand (5) sums up 

 this experimentation in his paper with the results of his own 

 experiments. " Kerner (6) took two raspberry leaves and 

 covered the bulbs of two thermometers with them, having the 

 hairy side outermost in one and the smooth side in the other; 

 the smooth one showed 2-5° F. higher than the other, which 

 would indicate increased transpiration. Vesque (7) found by 

 cultural experiments that with certain plants, when dryness 



