134 EXTERNAL CONFORMATION OF PLANTS. 



leaf never has its origin in the interior of the tissue of the stem, and is never 

 covered by layers of tissue of the stem, like roots and many shoots. In Cryptogams 

 it is usually one superficial cell (?'. e. superficial before the differentiation of the 

 epidermis) ^vhich forms the foliar protuberance. In Phanerogams a mass of tissue 

 bulges out as the rudiment of the leaf, consisting of a luxuriant growth of the 

 periblem covered by dermatogen (sect. 9, Fig. 103). By this means the leaf is 

 immediately distinguished from the hair in its most rudimentary state. The hair is 

 an outgrowth of the epidermis ; but since in Phanerogams the primordial epidermis 

 (dermatogen) covers the whole punctum vegetatiojiis above the leaves, hairs may 

 also spring up higher in position than the youngest leaves from single dermatogen- 

 cells (as in Utricularia according to Pringsheim). But in Cryptogams the dermatogen 

 only becomes diff"erentiated after the formation of the leaf; and hence the hairs are 

 always at a greater distance from the apex than the youngest leaves (Fig. 106); 

 the superficial cell of the stem, which in Cryptogams becomes the apical cell of a 

 new leaf, is not an epidermis-cell, since its origin dates long before the dififerentialion 

 of the tissue into epidermis and periblem. 



(5) The Tissue of the Leaf is contimwus iji 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 ideally. 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 Insertioji of the Leaf. The con- 

 tinuity of the tissue of stem and leaf is an evident consequence of the early 

 origin of the leaf below the apex of the piinctiim vegetatioins before the differen- 

 tiation of its tissue began. An inner mass of tissue is usually formed close be- 

 neath the apex of the stem before the formation of the leaf, and to this mass may 

 be applied in the case of Mosses, Equisetaceae, and other Cryptogams, the term 

 Plerome, which Hanstein has proposed for Phanerogams (sect. 19). This takes 

 no share in the origin of the leaf, and the continuity of its tissue is brought 

 about by the outermost layers of the primary meristem, in which also the fibro- 

 vascular bundles usually originate. When, however, the inner stem-tissue (ple- 

 rome) is itself transformed into a fibro-vascular body, as in Hippuris (Fig. 109) 

 (and notably in many Mosses), a continuity is subsequently brought about between 

 the fibro-vascular bundles of the leaves and this innermost tissue of the stem 

 (Fig. 109). When fibro-vascular bundles are formed in the stem having no con- 

 nexion with the leaves, they are termed by Nageli ' cauline bundles'*; but in Pha- 

 nerogams it is usually the case that each fibro-vascular bundle describes a curve 

 beneath a leaf-insertion, one branch of which bends into the leaf, while the other 

 branch runs downwards into the stem (Fig. 109,^^); the latter is called by Han- 

 stein the inner leaf-trace, and the whole bundle is a ' common ' one, i. e. common 

 to both stem and leaves ; both common and cauline bundles may run through the 

 same shoot (as in Ferns, Cycadese, and Piperacccc). In the fully developed shoot 

 the cortical layers of the stem, at least the outer ones, bend outwards into the leaf 

 without obvious interruption, and form its fundamental tissue ; in the same manner 

 the epidermis passes over continuously from the stem to the leaf. When the stem 

 produces fibro-vascular bundles, the leaves are usually also provided with them ; 

 they remain without vascular bundles only when they are arrested early in their 



