OLD AND YOUNG LEAVES. 



353 



the sun and wind. The ribs themselves are composed of cellular structures which 

 are not open to the danger of over-transpiration, and the epidermis which covers 

 them is entirely devoid of stomata. When the leaves at the ends of the young 

 twigs are opposite, erect, and concave, and their margins are in contact, they form 

 an actual capsule round the apex of the shoot. This occurs in the Wayfaring Tree 

 (Viburnum Lantana\ illustrated in fig. 90 5 . The small folds of green tissue 

 project into the interior of the capsule, and the still closely-pressed lateral veins 

 form the outer wall, and at the same time furnish a protective covering for the 

 enlarging green portions of the leaf. As soon as these are fully developed, and the 



Fig. 92. Unfolding of Beech-leaves. 



i The brown bud-scales have been loosened, and the membraneous stipules surrounding the foliage-leaves are visible above. 

 2 Further stage of development, the folded foliage-leaves being visible between the stipules. The same twig further 

 developed. * Lower surface of a young folded leaf. 6 Portion of the same leaf ; the depressions caused by the folding 

 are bridged over by silky hairs. Surface view of an unfolded leaf ; the stipules are withered and about to fall. 

 i Vertical section of a leaf at right angles to the midrib. Vertical section parallel with the midrib. 



epidermal cells are correspondingly thickened, the projecting folds become smooth, 

 the veins separate from one another, and the whole leaf becomes flat, assumes a 

 horizontal instead of a vertical position, and turns the upper instead of the lower 

 surface to the incident light (see fig. 90 6 ). 



It has already been repeatedly stated that coats of varnish as protective 

 coverings are especially to be met with on young leaves, which they guard from 

 over-transpiration and desiccation during their development, and that when the 

 leaf-laminas become provided with a cuticularized epidermis, these coats disappear. 

 It has also been pointed out, incidentally, that coats of hairs are of great use as 

 protections and screens to the young foliage-leaves when they first emerge from the 



VOL. L 23 



