406 On the Camhium-layer of the Stem of the Phanerogamia. 



cently-developed parenchyma. Some distance down only does 

 it fall into transverse lamellse, which are mostly convex down- 

 wards, and which, on account of the far more vigorous develop- 

 ment of the intercellular passages containing air, appear alter- 

 nately bright and dark, and of which the one set of strata corre- 

 sponds to the future nodes and the other to the internodes. 

 About this time the vascular bundles of the stem are developed 

 in the cambium-sheath; and the cellular tissue of the medulla 

 has lost the transparency of cambium, on account of the inter- 

 cellular passages. Then only is formed, in the laraellse corre- 

 sponding to the nodes, a secondary cambium, in consequence 

 of which the cambial tissue re-assumes its transparency ; and in 

 this cambium are developed the much-entangled vascular bundles 

 which become interwoven with the vascular bundles of the stem. 

 Lastly, a third law enunciated by Schacht says that no new 

 vascular bundles are formed in the cambium-ring of the Dico- 

 tyledons, but this is only in a condition to form parenchyma- 

 cells, while the formation of wood-, liber-, and vascular-cells only 

 proceeds from the cambium of the wood-bundles (p. 251). This 

 axiom, which Schacht founded chiefly on the examination of the 

 stem of Urtica dioica, is equally untenable with the foregoing. 

 The ordinary organization of our native trees is even opposed to 

 this. As is well known, the medullary rays run through the 

 whole internode, longitudinally, between the vascular bundles in 

 the young shoots of these, while the woody layers subsequently 

 formed by the lateral ramification of the vascular bundles stand 

 in connexion, and the medullary rings then only fill up the short 

 meshes of the reticulation. This ramification of the vascular 

 bundles running over the original medullary rays can only be 

 brought about by a part of the cells of the medullary rays 

 becoming transformed into wood-cells and vascular tubes, and 

 converted into woody bundles, which, uniting with the product 

 of the cambium-layer lying in the vascular bundles, form the 

 connected secondary woody layers. But there are still more 

 striking instances. I will not refer to the observations on tro- 

 pical climbers made by others, since I cannot test them, and 

 shall only call attention to one among the observations on this 

 point made by Karsten (p. 140) on Bannisteria nigrescens, as 

 this appears to me particularly convincing. In this, Karsten 

 found that woody portions, presenting themselves in the form of 

 rouuded-off radii strongly projecting at the circumference of the 

 wood, are not produced by the development of the primary vas- 

 cular bundles, but consist of wood-bundles formed over the me- 

 dullary rays. We may ascertain, even in plants which are at 

 our disposal in a fresh state, that the medullary rays have the 

 power to form woody bundles. This occurs in such plants as 



