PHANEPvOGAMIA : AXIAL ORGANS. 237 



quently lie either liber-bundles, or liber-bundles carrying milk-sap, 

 actual milk-vessels, or milk-sap passages. Since these often respect- 

 ively exclude each other, while often there is no trace of any of 

 them, the liber certainly cannot be called an essential constituent 

 of bark (as the innermost layer) ; still more inaccurate would it be 

 to term the cambium-layer, which much rather appertains to the 

 vascular bundles, the innermost layer of the bark. 



In trunks (Stammen) the epidermis sooner or later forms corky 

 substance, which is either gradually cast off in layers, as at first in 

 the Birch, often merely becomes destroyed gradually by atmospheric 

 influences, and thus, in part, acquires considerable thickness, as in 

 the Oak, or, as often happens, the outer part of the inner layer of 

 the bark and the outermost liber layer are thrown off together, and 

 never reproduced. In the last case, new liber and internal cortical 

 layers are formed annually, but with a peculiar form of cells resem- 

 bling corky tissue ; and the outermost are in like manner annually 

 thrown off, as, for instance, in the Vine. 



The pith, lastly, is composed of parenchyma alone, which, in its 

 older condition, becomes thick-walled and porous. It often con- 

 tains, also, solitary ramified liber-cells (Rhizophora Mangle), milk- 

 vessels, reservoirs for peculiar secretions, &c. 



The vascular bundles originate after the cellular tissue, in the 

 same order as the latter ; or rather, as the cellular tissue is gradually 

 formed, a part of it passes gradually into vascular tissue. The 

 direction of the vascular bundles wholly depends, therefore, upon 

 the direction of the organising force. On account of this, also, the 

 distinction between developed and undeveloped internodes, ex- 

 plained in 126., forms the chief basis for the course of the vascular 

 bundles. In the former, when the process of formation proceeds 

 from below upward, in horizontal discs, the vascular bundles are 

 straight, tolerably parallel to the axis of the internode, e. g. in Tra- 

 descantia, Tropceolum ; where, on the contrary, one hollow cone is 

 set upon another, as it were, in the terminal bud, the vascular 

 bundles at their first development hold a course from the base to 

 the apex of the cone, therefore from the circumference of the in- 

 ternode to its centre, and afterwards, as new internodes are super- 

 posed, the vascular bundles of the first cone are developed forward 

 through the succeeding ones, to the circumference, where they enter 

 the leaves or buds They make, therefore, a curve convex toward 

 the interior, the length and convexity depending on the form of 

 the terminal bud. The curve is very convex in Yucca, Mammillaria, 

 &c. ; more elongated in the Palms, Drac&na, Iris, &c. Since all 

 new portions in the axis are formed outside the primary vascular 

 bundles, whether they be increase of thickness to the vascular 

 bundles, as in Dicotyledons, or the rudiments of new bundles, in 

 Monocotyledons, the older and deeper vascular bundles, running 

 towards the periphery to the leaves and buds, must necessarily cross 

 the more superficial, ascending bundles, or their developing masses, 

 which have originated outside them. This condition is naturally 



