of the Stem of the Phanerogamia. 401 



and the production of new cells in the boundary-layer is alto- 

 gether at an end, as in the Monocotyledons. This is the case 

 particularly in the stems of Cucurbita, Cucumis, Lagenaria, &c, 

 also in the stems of Basella alba and B. rubra. Here also, as in the 

 Monocotyledons, we find, instead of a cambium-layer connecting 

 the cambium-regions of all the separate vascular bundles into a 

 general closed cylinder, a defined external boundary to the me- 

 dullary tissue, which undergoes no further alteration with time. 

 How little indicative is the distinction between this structure of 

 the stem and that of an uninterrupted passage from medulla 

 into rind, is shown by the portions of the same stem situated 

 below the cotyledons, where, in Basella, the boundary-region 

 just referred to is certainly in some degree marked, by its cells 

 being of somewhat less diameter than those of the rind and 

 medulla, yet forming no definite boundary-line, — while in the 

 cotyledonary internode of the Cucurbitacese mentioned above, 

 the cellular tissue of the rind and medulla is of quite uniform 

 structure, and no trace can be found of any line of demarcation 

 between them. Under these circumstances, it does not seem 

 warrantable to apply to the peripheral region of the medullary 

 parenchyma, formed last out of the cambium of the Monocoty- 

 ledons, the term 'wood-cylinder/ since (not to dwell upon the 

 vessels in this situation) a portion of the stem to which the name 

 of wood should apply, ought to present at least a decided con- 

 trast to the parenchyma of the stem; here, however, such a 

 contrast either has no existence, or depends on mere thickening 

 of the walls of parenchymatous cells, or upon a slight alteration 

 of form (elongation) of the medullary cells, which takes place 

 with a gradual transition. These boundary-cells are structures 

 of arrest or limitation (Hemmungsbildung), with which the medul- 

 lary cells cease their reproduction on the outer side. 



The case is different in Dracaena. Here the formation of new 

 tissue does not cease ; the product of the continuously developed 

 cambium-layer, however, does not consist of mere parenchyma- 

 cells, but, as in the antecedent formation of the vascular bundles, 

 of two kinds of tissue — parenchyma-cells and fibrous bundles. 

 Here there exists a clear analogy to the normal formation of the 

 wood of Monocotyledons, although the bundles are imperfect, con- 

 taining no vessels. In accordance with the type of the Mono- 

 cotyledonous stem, the new layers of wood are not formed in 

 continuous concentric layers, but in the form of isolated, although 

 freely anastomosing, bundles. If, with Karsten, we apply the 

 name wood-cylinder to these external bundles, the essential dif- 

 ference of their organization is a reason for not extending the 

 name to the boundary-layer of the other Monocotyledons con- 

 sisting simply of cellular tissue ; for in the latter the formative 



