400 H. von Mohl on the Cambium-layer 



without intending to indicate the import of the lignified tissue, 

 as to whether its cells were wood-cells or liber-cells (p. 100). 

 In Dracaena and allied plants, the cell-forming activity of the 

 cambium is not exhausted by this conversion into lignified cells, 

 but endures during the whole life of the stem, and gives rise to 

 the production of woody layers, as in the Dicotyledons, — the 

 bundles of which must not, however, be regarded as the inferior 

 prolongations of the new vascular bundles formed above (p. 99), 

 but may be compared with the annual rings of the Dicotyledons 

 (p. 103). 



In this interpretation of the boundary-line between medullary 

 and cortical tissue, distinctly marked in many Monocotyledons, 

 but altogether imperceptible in others, as a wood-cylinder, two 

 questions arise : — whether this boundary-layer, composed of 

 homogeneous cellular tissue, is to be compared with the fibrous 

 layer of Dracaena, which is continually thickened throughout 

 life ; and whether this last structure corresponds to the annual 

 rings of the Dicotyledons. 



There is no doubt that the said boundary-line is formed in 

 the Grasses, Asparagus, Ruscus, Iris, &c, in the following way : 

 that in the cambium-layer, the further it is developed outwards, 

 and the more the formation in it of vascular bundles approaches 

 its close, the production of the medullary parenchyma-cells side 

 by side with the bundles undergoes an alteration — the cells, as 

 they come to lie more externally, becoming of smaller diameter 

 and mostly of greater length, until the formation of new cells at 

 last entirely ceases. In the first place, in spite of the outer- 

 most of these cells having far thicker walls and a much greater 

 length than the inner medullary cells and the cells of the rind, 

 they are no analogue of the wood, but merely a modification of 

 the medullary parenchyma: and this is the more clear since 

 the difference of length and thickness of the walls is by no 

 means a constant character; for, as Karsten truly remarks, in 

 the Palms this layer of cells cannot be distinguished from the 

 cells of the medulla and rind. We find similar conditions re- 

 curring when we examine the corresponding region of the Dico- 

 tyledons. In trees we usually find the same condition as in 

 Palms, the cellular tissue of their medullary rays passing into 

 the cellular tissue of the bark without displaying essential varia- 

 tions in the organization of the cells ; and the only distinction 

 existing is, that at the limit between medulla and rind, the cells 

 are capable of multiplying by division. In other Dicotyledons, 

 on the contrary, we find a sharp line of demarcation between 

 medullary and cortical parenchyma, similar to that in the rhi- 

 zome of Iris, &c., and indeed with the same peculiarity, that the 

 boundary-line lies a little outside the circle of vascular bundles, 



