576 



ECOLOGY 



many instances, even the production of hairs seems unrelated to external factors, as 

 possibly in Coreopsis lanceolata, where both hairy and smooth forms grow in 

 similar habitats. Perhaps the reference of structures to inherent causes is but an 

 expression of ignorance that may be eliminated upon adequate experimentation. 



The role of epidermal hairs. Hairs commonly are believed to have 

 an important role in the reduction of transpiration, but the evidence for 



this view is not abun- 

 dant. Probably they 

 are much inferior in this 

 respect to cutin or even 

 to wax coats. The most 

 efficient form of hair pro- 

 tection would seem to be 

 that afforded by a woolly 

 felt ; it has been shown 

 that the removal of such 

 a felt in Stachys lanata 

 results in an increase of 

 twenty to fifty per cent 

 in the transpiration. 

 Evaporating surfaces 

 artificially coated with 

 FIG. 823. A portion of a branch of the river hairy felts have been 



grape (Vitis vulpina), illustrating the structural trans- shown to lose much less 

 formation that a plant may undergo when attacked fa fa 



by a gall-forming insect; the attack of a gall-fly (Ceci- 



domyia V itis-pomum) induces the development of en- hair covering. Similar 



tirely new organs, and parts that otherwise are smooth results mav be looked 



become woolly-pubescent. r , , 



for in scale-covered leaves 



or in leaves with appressed or branched hairs, where the hairy coat 

 is dense enough to retard the escaping water vapor. In the cinerarias 

 the hairs at first grow erect and then horizontally, producing a 

 chambered layer, while in Espeletia there are two or more such layers; 

 the retarding effect of these layers upon escaping water vapor is 

 not difficult to understand. In most hairy plants, however, the 

 hairs are erect and more or less scattered, so that it is difficult to 

 see how they can appreciably retard escaping water vapor, though 

 their presence may to some extent reduce the evaporating surface. 

 Hairs often are most abundant on those parts that most need protection, 

 as on young leaves and on the stoma-bearing surface of adult leaves. 



