REDUCTION OF TRANSPIRATION BY HAIRS 127 



occur in many Arctic species and in a number of the plants that inhabit 

 the paramos (alpine steppes) of the Venezuelan Andes ; here the risk of 

 desiccation arises not from the heat of the sun, but from violent winds, 

 which in the Arctic regions are in addition extraordinarily dry. 



There are at present no exact experimental data available with 

 regard to the effectiveness of a woolly or felted hairy covering as a 

 means of reducing transpiration. In order to obtain the requisite 

 quantitative information, at any rate in a single case, the author carried 

 out an appropriate experiment with Stachys lanata. The leaves of this 

 plant are thickly covered with woolly hairs on both sides : the density 

 of the covering will be appreciated when it is stated that there are no 

 less than 120 hairs per sq. mm. on the upper side, or 156,000 on an 

 entire leaf with a superficial area' of 13 sq. cm. For the experiment 

 two opposite leaves were selected, which were fully grown and approxi- 

 mately equal in size. After both had been painted on the under surface 

 with cacao-wax, the hairs were carefully removed from the upper side 

 of one leaf with the aid of curved scissors. The leaves were then allowed 

 to transpire for twenty-four hours in the shade at a temperature of 

 20-25C, their short stalks being sealed into the necks of small 

 flasks filled with water. Under these conditions the intact leaf lost 

 646 grams by transpiration, the shaved leaf "915 grams; the ratio of 

 these values is as 1 : 1*42. The leaves were next exposed to sunlight 

 for an hour (during twenty minutes of which period, however, the sun 

 was slightly obscured by clouds) ; the losses of weight were now found 

 to be *08 g. for the intact and '167 for the shaved leaf, giving a ratio 

 of 1 : 2 - 09. 73 In the second case, therefore, the reduction of trans- 

 piratory activity effected by the hairy covering amounted to more 

 than 50 per cent.: as in this plant the upper surface of the leaf is 

 provided with numerous stomata, the measurements quoted refer 

 mainly to stomatic transpiration. It is quite evident that a dense 

 woolly coat of hairs reduces transpiration very considerably, when the 

 organ concerned is exposed to direct insolation ; its influence in this 

 respect is less powerful, though by no means negligible, in diffuse 

 light, where its action consists mainly in the retardation of surface- 

 ventilation. 



No doubt there are also other ways in which a dense coat of hairs 

 may afford protection to the plant. The hairy coverings of plants 

 which inhabit deserts or steppes may serve to diminish the loss of heat 

 produced by nocturnal radiation ; in the case of young leaves which 

 are just unfolding the hairs may, as Wiesner suggests, protect the 

 developing chlorophyll-apparatus against the injurious action of sun- 

 light. These juvenile or transitory hairy coverings, which are usually 

 confined to the upper side of the leaf, often disappear completely at a 



