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 rSle 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 Stack ys lanata 

 results in an increase of 

 twenty to fifty per cent 

 in the transpiration. 

 Evaporating surfaces 

 artificially coated with 

 hairy felts have been 

 shown to lose much less 

 v/ater than without the 

 hair covering. Similar 

 results may be looked 

 for in scale-covered leaves 



Fig. 823. — A portion of a branch of the river 

 grape ( Vilis vulpina)^ illustrating the structural trans- 

 formation that a plant may undergo when attacked 

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

 domyia VUis-pomum) induces the development of en- 

 tirely new organs, and parts that otherwise are smooth 

 become woolly-pubescent. 



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. 



