236 MORPHOLOGY. 



in the Grasses, UmbelUfercR, &c. Every axis is originally clothed 

 with epidermis or epiblema ( 29.), according to the medium in 

 which it vegetates. On this, therefore, are frequently formed all 

 the appendages of epidermoid tissue, especially glands, hairs, &c. 

 and corky matter ( 29.). The varieties resulting from these 

 are so manifold, that, at present, it is only with great difficulty, if 

 at all, that they can be treated generally ; the varieties which re- 

 sult from the different arrangement and nature of the vascular 

 bundles, are of more importance, and may be subjected to a more 

 general examination. All vascular bundles are usually separated 

 from their fellows by parenchyma ; more rarely they form a per- 

 fectly closed circle. The separate bundles are either placed in 

 a single circle (as in most Dicotyledons), or scattered in the paren- 

 chyma. The latter, again, collectively form a circle, which, like 

 the preceding, encloses a definite portion of cellular tissue (pith) 

 in the centre (e. g. in most Grasses, many UmbellifercB, Nyctaginece, 

 Chenopodiacece, Amarantacece), or such an arrangement does not ma- 

 nifest itself (in cane-like Palms, Grasses with solid stems). The 

 latter distinction seems to me very unimportant, since it varies in 

 closely related plants, in one and the same family, e. g. in Mays 

 (vascular bundles scattered throughout the parenchyma) and Pha- 

 laris (scattered vascular bundles enclosing a pith). In all cases 

 where the arrangement of vascular bundles indicates such a boun- 

 dary between included and excluded parenchyma, the inner is called 

 pith (niedulla), the outer bark (cortex). The portions of cellular 

 tissue between the vascular bundles, maintaining the connexion 

 between the pith and bark, are termed great medullary rays. In 

 the simplest plants merely a central vascular bundle occurs, or a 

 perfectly closed ring of elongated (vascular bundle) cells, like that 

 in the Mosses, which, however, again encloses parenchyma in the 

 centre (e. g. in Ceratophyllum). In flat stems, as in Spirodala, Ruscus, 

 they lie in one plane (in a line on the transverse section). Con- 

 sequently they both have merely bark and no pith. 



The bark is composed, in addition to epidermis, of cellular tissue, 

 in which we can in general distinguish merely an uniform paren- 

 chyma, but sometimes, especially in perennial axes, two layers ; 



1. the outer, which consists of elongated cells, with thick but 

 almost gelatinous and generally porous walls, the boundaries be- 

 tween which are often quite undistinguishable, and the intercellu- 

 lar spaces of which are filled with intercellular substance ; and 



2. the inner layer, which is generally formed of roundish, thin- 

 walled, lax parenchyma. In the latter alone occur reservoirs for 

 secretions, milk-vessels, special forms of cells with special contents ; 

 in the former, scarcely anything but cells, containing homogeneous, 

 colourless or red juices, and sometimes crystals. The two layers 

 occur most distinctly defined in the trunks of which the cuticle 

 does not form cork until a late period (as in the Cactacece) ; in other 

 trunks and steins they very often pass gradually into each other. 

 In front of the vascular bundles, in the inner layer of the bark, fre- 



