394 H. von Mohl on the Cambium-layer 



parts of its course, depended partly on the nature of the ma- 

 terial at ray disposal, which afforded me the means of studying 

 the developed stems of a great number of Palms, but not the 

 deep investigation of the history of development of the stem, — 

 partly on the necessity of laying particular stress on this point, 

 because it afforded the most striking evidence against the doctrine 

 of the endogenous growth of the Monocotyledons, at that time 

 universally received, and adopted by DeCandollc for the system- 

 atic division of the Phanerogamia. But even now the matter is 

 not to be put aside so simply as is done by Schleiden ; for this 

 different course of the vascular bundles is a very characteristic 

 mark of the Monocotyledonous stem, and the result of a peculiar 

 mode of development. 



In the majority of the Dicotyledons the young vascular bun- 

 dles lie side by side in the cambium-layer, and ascend, without 

 suffering any curvature towards the centre of the stem, up to 

 the point where they turn outwards to the leaf; and the cam- 

 bium-layer traverses in an uninterrupted circle the individual 

 vascular bundles and the parenchymatous cells lying between 

 them, so that the liber-laycrs of all the vascular bundles are 

 situated outside the cambium -layer, in the bark. If new vascular 

 bundles are formed, these are developed in the same cambium- 

 cylinder which had given birth to the older vascular bundles, and 

 between the latter. But since, in the interval between the forma- 

 tion of the older and younger vascular bundles, the former have 

 been somewhat advanced in their development, and their woody 

 portion thickened in the direction of the radius of the stem, the 

 medullary cellular tissue of the stem having also grown outwards 

 in like proportion, and the cambium-ring being thus forced 

 outwards, the inner part of the wood of the younger bundles 

 lies at a somewhat greater distance from the centre of the stem 

 than the corresponding part of the older bundles, yet without 

 the intermediate and outer parts of the new bundles being 

 pushed further out than the older bundles. 



In the Monocotyledons matters are essentially different. The 

 vascular bundle formed by a direct transformation of a part of 

 the cambium-layer lies, as in the Dicotyledons, in its whole 

 length within the cambium-cylinder, — or rather, since the bud is 

 always drawn to a point at the jmnctum vegef.ationis, within a 

 hollow cone {Kegclmantel) , forming a continuation of the cam- 

 bium-cylinder. Simultaneously, and not only beside it but 

 also on the peripheral side, parenchymatous cellular tissue is 

 formed from the cambium, and through this the constantly 

 renewed cambium-cone is moved outwards from the vascular 

 bundle towards the circumference of the stem. This j)roduction 

 of ccUuIhi tissue outside the vascular bvmdlc is almost or quite 



