LEAVES AND LEAF-BEARING AXES. 
155 
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 ihe tiialure Leaf is conti7iuons in its for??ialion 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 j. 
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- 
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 Lycopodiaceae, the basal por- 
tions of the foHar 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 runs through the stem 
being termed the Leaf-trace (see Sect. 18). But in some Vascular Cryptogams, 
as Lycopodiaccce and Equisetacese, the procambium bundles which are differentiated 
in the tissue of the young leaf are so connected with the young fibro-vascular 
Fig. 119.— Long-itudinal section through the apical region 
of an upright shoot oi Hippuris vitlgaris ; s apex of the stem ; 
b, b, b the verticillate leaves; k k 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). 
^ Leaves which wither early, or which persist as small scales, like all the leaves of Pülotnm, and 
many small leaf-scales of Phanerogams, have no fibro-vascular bundles. 
