354) OLD AND YOUNG LEAVES. 



buds. The leaves of a great number of plants are only hairy during the commence- 

 ment of development. Long hair-cells may be seen inserted by their narrow bases 

 between the flattened epidermal cells; these at an early stage shrink up close to 

 their origin, and then break off. They may remain hanging to the leaf for a little 

 while, but afterwards are thrown or pushed off by the enlargement and expansion 

 of the leaf -blade, or are frequently blown away by the wind. The leaflets, which 

 were originally quite thickly clothed with hairs, then appear partially or entirely 

 smooth and green on both sides. A remarkable instance of this is furnished by 

 Amelanchier vulgaris, whose foliage, early in the spring, is folded along the midrib 

 and covered with snow-white wool, reminding one strongly of the Edelweiss, while 

 in the summer no trace of the covering remains. The White Poplar (Populus alba), 

 pear-trees, and mountain-ashes behave in like manner. Horse-chestnut leaves, 

 when they make their way through the brown, loosened bud-scales, are thickly 

 covered with wool, but during the spring they lose this so completely that only 

 here and there on the fully-formed leaves can remnants still be observed clinging 

 to the leaf. It is, however, not only woolly coverings that are later either partially 

 or wholly thrown off as superfluous. On the foliage-leaves of the already- 

 mentioned Wayfaring Tree (ViburnuTn Lantana) appear felted stellate hairs 

 which fall off as soon as the epidermis is sufficiently thickened. In a species of 

 Rhubarb {Rheum Ribes) brittle, candelabra-like, short-branched trichomes are 

 situated on the edge of the leaf, which is much crumpled at an early stage, and 

 later, when of no further use, they break away in pieces and fall off. Again, in 

 many mulleins (e.g. VerbascuTn pulverulentum and granatense), there are branched, 

 shrub-like hair-structures which become detached from the surface of the fully- 

 developed leaves, and are carried away in loose flakes by the wind. 



The covering of the young leaves of the Beech (Fagus silvatica) consists of 

 silky hairs, and the way in which these are arranged and utilized is so peculiar that 

 it is worth while to inquire further into the details. At first sight, the under 

 surface of the young beech-leaf appears to be entirely covered with silky hair; 

 on a closer examination, however, it is seen that the hairs are only inserted on 

 the margins and on lateral veins, and that the green portions of the leaf are in 

 reality perfectly smooth and free from hairs. Since the green portions of the 

 leaf are thrown into deep folds (see figs. 92* and 92^), and the veins are still 

 close to one another, while the tops of the silky hairs springing from these veins 

 reach far beyond the vein next to them, all the furrowed depressions caused by 

 the folding are completely covered over. Each groove is bridged over by the 

 hairs, which are regularly arranged, side by side, parallel to one another; thus 

 the leaf appears to be clothed completely in a delicate silken coat. There can 

 be no doubt as to the function of these hairs. The green tissue overspanned by 

 them is protected from the sun until its epidermis is sufficiently thickened, and 

 when this is the case the folds flatten out (fig. 92^) and the leaf assumes a 

 horizontal instead of a vertical position, thus turning the lower surface away 

 from the sun, and rendering the hairs of no further use. They have become 



