PHANEROGAMIA : AXIAL ORGANS. 243 



distinction to those tissues in which there is no longer normally any 

 process of cell-formation going on. The latter consist in such cases 

 either of living parenchymatous cells, in which, under favourable in- 

 fluences, such a process of cell-formation may recur, as, for instance, in 

 the germination on Monocotyledonous leaves, &c., or of relatively dead, 

 very woody cells, in which such a process of development can never be 

 revived, as, for instance, in the older part of a vascular bundle, wood, &c. 



2. In all cases we find that the new cells in the cambium-layer, and the 

 new vascular bundles, or the thickening mass of the older vascular bundles, 

 are developed from the base towards the apex of the axis, from the older 

 into the new internode, and from the main to the secondary axes, but 

 never the reverse. 



3. Only in the Gymnospermce, Monocotyledons, and Dicotyledons does 

 a cambium- layer occur in the circumference of the axis ; and where this is 

 present it forms the limits between bark and pith or medullary cellular 

 tissue. In the Monocotyledons this cambium-layer only occurs as an 

 exception, and is, of course, independent of the separate, invariably 

 definite, vascular bundles ; in the Gymnospermce and Dicotyledons this 

 layer is never wanting, and is so constituted that the cambium of each 

 separate vascular bundle of the simple or outermost ring also belongs to 

 the general cambium-layer, and is connected by the cambium masses, in 

 front of the medullary rays, into one continuous layer. Moreover, every 

 individual, unconnected, isolated vascular bundle, has its own cambium. 



4. The main difference must still be deduced from the nature of the 

 vascular bundle, which corresponds with the great natural divisions of 

 plants. Other distinctions that have been advanced are untenable, and 

 are based upon deficient observation of all existing conditions. 



5. All asexual Gymnosporce can grow upward only, owing to the de- 

 ficiency of a cambium-layer. All Gymnospermce and Dicotyledons grow in 

 thickness as well as height. In the Monocotyledons we find both condi- 

 tions, sometimes manifested exclusively as terminal growth, and then, 

 again, the latter combined with continuous increase in thickness. From 

 this we cannot, however, deduce any classification of the Monocotyledons, 

 since both conditions are met with in the same family, as, for instance, in 

 the Liliacece and in branched Palms (?). 



I. Asexual Gymnosporcp,. 



These all agree in having simultaneous vascular bundles, and, as far as 

 I know, in that no cambium-layer occurs in any stem, by which the latter 

 might be further thickened when once formed. The structure is in general 

 very simple. Liverworts and Mosses have only a simple central vascular 

 bundle, without so-called vessels. The Lycopodiacea have only a central 

 vascular bundle, generally forming an irregularly-lobed figure in the 

 transverse section, this being occasioned by the arrangement of the vessels. 

 The vascular bundles, going into the leaves, run for a time upward in the 

 bark before they enter the leaf. The stem of Isoetes is formed in a very 

 different manner, and here we find a ring of vascular bundles, undeveloped 

 internodes, and a constant and successive dying off from below. Mohl has 

 made very exact observations on this subject. 



Ferns have an extremely deficient stem-formation, exhibiting sometimes 

 developed, and sometimes undeveloped, internodes ; in all, however, there 

 is but a simple ring of vascular bundles. The vascular bundles rise 

 vertically in the developed internodes, and, at the starting point of the 



R 2 



