36 LEAF-SURFACE [CH. 



of the Mistletoe, and many Conifers. In the narrow leaves 

 of the latter, e.g. Pines, Firs, Cedars, &c., and in some small 

 leaves of other plants there is only one vein, the midrib. 



These matters are by no means trivial, and the student 

 should understand that not only does venation yield 

 valuable species-characters, as we shall see further on, but 

 that good service has been rendered to paleontology by 

 applying what is known regarding living species to the 

 impressions of fossil-leaves found in the rocks, and so at 

 least helping the avoidance of error. 



The surface of the leaf, apart from microscopic charac- 

 ters, usually presents itself as hairy or devoid of hairs; 

 in the latter case it is termed glabrous. This term does 

 not necessarily mean smooth, in the sense of being shiny 

 or glossy, but simply devoid of visible hairs, and when the 

 glabrous surface is also especially smooth and shining, the 

 terms polished (Ivy, Holly, &c.), or varnished (Coffee, Salix 

 pentandra) may be employed. 



As a general rule the upper and lower surfaces of a 

 leaf differ in respect to these matters : the latter being 

 usually duller and paler in hue, and often hairy when the 

 upper surface is glabrous ; or covered with a waxy bloom, 

 giving it a sea-green (glaucous) appearance, while the 

 upper surface is deep or bright green. It is also a common 

 event to find the venation prominent below, and scarcely 

 visible above ; while in other cases the softer parts of the 

 leaf are puckered up between the veins so that the latter 

 appear sunk in the tissues above and protrude below, as in 

 the Sallows, when the leaf is termed rugose, or wrinkled. 



The different forms of hairs and degrees of hairiness 

 have already been referred to in Vol. I. pp. 85 92. The 

 principal varieties met with in the leaves of such trees 

 and shrubs as we are here concerned with are, pubescent 

 or slightly and softly downy as in Red Currant ; silky as 



