LEAVES AND LEAF-BEARING AXES. 



-^5^ 



belonging to the dermatogen, as in Utricularia according to Pringsheim. But in 

 Cryptogams the dermatogen becomes differentiated only after the formation of the 

 leaf; and hence the hairs are always at a greater distance from the apex than the 

 youngest leaves (Fig. ii6); the superficial cell of the stem, which in Cryptogams 

 becomes the apical cell of a new leaf, is not an epidermal cell, since its origin dates 

 long before the differentiation of the tissue into epidermis and periblem. 



(5) The Tissue of the mature Leaf is continuous in its formation with that of the 

 Stem. It is impossible, histologically, to find a boundary line between the stem 

 and the base of the leaf, although such a boundary line must be assumed theo- 

 retically. If the surface of the stem is imagined to be continued through the 

 base of the leaf, the transverse section thus caused is called the Insertion of the 

 Leaf 



The continuity of the tissue is especially observable in vascular plants, where 

 the well-developed leaves^ consist, like the stem, of epidermal and fundamental 

 tissues and fibro-vascular bundles. The 

 cortical layers of the stem bend out with- 

 out interruption into the leaf, and consti- 

 tute its fundamental tissue; in the same 

 manner the epidermis passes over from 

 the stem into the leaf; the fibro-vascular 

 bundles of the leaves have, in Phanero- 



FiG. 119.— Longitudinal section through the apical region 

 of an upright shoot oi Hippiiris vulgaris; s apex of the stem ; 

 b, b, b the verticillate leaves ; A /6 the buds in their axils, which 

 all develope into flowers ; g g the first vessels (the dark parts 

 of the tissue indicate the inner cortex with its intercellular 

 spaces). 



gams and many Cryptogams, the appear- 

 ance of being the upper ends of the * com- 

 mon' bundles which ascend in the stem 

 (Fig. 119); and where this is not the 

 case, as in Lycopodiacese, the basal por- 

 tions of the foliar bundles and the fibro- 

 vascular mass of the stem are nevertheless 

 in continuity. 



The main cause of the continuity of 

 tissue between stem and leaf is that the 

 leaf arises from the cone of growth of the stem, where it still consists entirely 

 of primary meristem; in vascular plants the young leaf appears as a luxuriant 

 development of its outer layers (the dermatogen and inner layers of periblem, see 

 Sect. 19). And as vascular bundles (at first in the form of procambium) become 

 differentiated in the central tissue of the stem or plerome, similar bundles also 

 appear in the tissue of the growing leaf, in such a manner that the two are in 

 connection with one another. This connection may be such that the foliar 

 bundles appear as the upper prolongations of those of the stem; thus arise the 

 ' common ' bundles of Phanerogams, the portion that nms through the stem 

 being termed the Leaf trace (see Sect. 18). But in some Vascular Cryptogams, 

 as Lycopodiaceae and Equisetaceae, the procambium bundles which are differentiated 

 in the tissue of the young leaf are so connected with the young fibro-vascular 



^ Leaves which wither early, or which persist as small scales, like all the leaves of Psilotnm, and 

 many small leaf-scales of Phanerogams, have no fibro-vascular bundles. 



