BIOLOGY OF HAIRINESS i8i 



and pointed, and markedly but not excessively hairy. 

 There is undoubtedly a causal connection between dry 

 atmosphere and hairiness, the mechanism of which we do 

 not know. 



Yapp (19 1 2) has analysed the conditions for Spircea 

 Ulmaria, the meadow-sweet, in which variation of hairiness 

 is found not only in plants of different habitats but in the 

 younger and older leaves of the same plant, which, in 

 meadow vegetation, exist under quite different conditions. 

 The leaves may be thickly clad on the lower surface with 

 fine hairs (pubescent), or hairless, or partly hairy ; the 

 differences between the bare green leaf surface and the silvery 

 felt are very marked. The rather complex changes may best 

 be given in Yapp's own words : " This pubescence is 

 subject to a kind of periodicity, and appears only under 

 certain definite conditions. The chief rules governing the 

 appearance of the hairs are as follows : {a) The seedlings, 

 also all leaves formed during the first year, are glabrous. 

 {h) On the erect flowering shoots of adult plants there is a 

 regular succession of glabrous, partially hairy and completely 

 hairy leaves. The earliest radical spring leaves are glabrous, 

 the cauline leaves hairy, {c) The non-flowering shoots of 

 adult plants produce only radical leaves. The earliest of 

 these are glabrous as in {h). Subsequently the successive 

 leaves exhibit increasing hairiness up to June or July, 

 after which they are decreasingly hairy until glabrous 

 leaves are once again produced in autumn, {d) The distri- 

 bution of pubescence on the partially hairy leaves is 

 interesting. The terminal leaflet is invariably hairy, and 

 there is a regular decrease of hairiness from above down- 

 wards. Individual partly hairy leaflets generally possess 

 a marginal band of hairs, with sometimes additional bands 

 running inwards between the main veins." 



In addition to the differences in hairiness the upper 

 leaves show the "sun leaf" characters, the lower and 

 radical leaves the " shade leaf " characters which we have 

 already studied. The shade leaves are larger, thinner, 

 with fewer, but much larger, stomata than the sun leaves ; 



