574 



ECOLOGY 



poplar, fig. 820). In many leaves there is no obvious relation between 

 habitat and hair production, mesophytes frequently and hydrophytes 

 rarely (as in Pistia and Salvinia) being conspicuously hairy (fig. 897). 

 The influence of external factors upon the production and form of 

 epidermal hairs. — Perhaps the most striking variations in hairiness 

 within the same species are found in amphibious plants, such as Jussiaea 

 repens and Polygonum amphibium. Aquatic 

 individuals of the latter have smooth leaves 

 (fig. 821), while land forms have leaves covered 

 with numerous stiff and long hairs (fig. 822). 

 Indeed, so different are the two forms of Poly- 

 gonum amphibium, that the hairy form has been 

 regarded as a different species, P. Hartwrightii, 

 although it is possible to find both forms on the 

 same plant at the edge of a pond. In a number 

 of land species with mesophytic and xerophytic 

 forms, the latter are the more hairy, and in 

 some plants (as Artemisia canadensis) winter 

 leaves are more hairy than are the summer 

 leaves. Certain species (as Convolvulus sepium) 

 are much more hairy in maritime than in inland 

 situations. From analogy with the excretion 

 of wax and the formation of cutin and of 

 palisade cells, hair formation ^^'ould appear to 

 be facilitated by increased transpiration, yet 

 it is difficult to see in just what way the loss 

 of water can produce a complex new structure 

 like the hair of Polygonum. Possibly water is 

 a factor which inhibits hair development, trans- 

 piration acting rather as a releasing stimulus 

 (see p. 952). 



Fig. 820. — A cross sec- 

 tion through a leaf of the 

 silver poplar (Popidus alba), 

 showing a tangled felt of 

 wooUy hairs on the under 

 and stoma-bearing surface, 

 the upper epidermis (e) 

 being smooth ; note that the 

 stoma (i) has a well-defined 

 inner ridge, the inner ves- 

 tibule thus being deariy 

 marked off from the sto- 

 matal cavity; this leaf has 

 a representative mesophytic 

 chlorenchyma with two rows 

 of palisade cells (p) and a 

 loose sponge tissue (/) ; con- 

 siderably magnified. 



Some remarkable cases of hair production occur on insect galls. The leaves of 

 Vitis aestivalis when young and of V. Labrusca throughout life are covered with a 

 tawny tomentum, while the leaves of V. vulpina and V. cordifolia are smooth. 

 However, insect galls occurring on the latter species are covered with a hairy coat 

 like that of ordinary leaves in the former species- (fig. 823). It would appear that 

 the smooth-leaved species are potentially hairy, needing only the insect stimulus to 

 induce hair production. Similarly, stellate hairs, resembling those usually found 

 on hairy-leaved oaks, may be produced on smooth-leaved oaks through insect 

 activity; in like manner, thorns may be produced on the otherwise smooth leaves 



