differences in the Structure of Stems. 245 



curved course of the vascular bundles as a primary distinction 

 between the Monocotyledons and Dicotyledons^ for this is de- 

 pendent on two other relations, that of the closed vascular 

 bundles and the undeveloped internodes; consequently it 

 would on the one hand be present in the Dicotyledons if they 

 had closed vascular bundles, and on the other would not be- 

 long generally to the monocotyledonous stem, but only to that 

 with undeveloped internodes. 



D. Now from a combination of the circumstances related 

 under A. and C. there originates in the simple closed circle 

 of vascular bundles and proportionally large leaf-bases, for the 

 closed vascular bundles, the form of the Fern-stem, and for the 

 unclosed that of the Cactean stem : the latter repeats nearly 

 all the relations of the Fern-stem, only always above the earth. 



4. In the dicotyledonous structure of stems many diversities 

 still result from the hypertrophy of the pith, the bark, or both, 

 as for instance in the Euphorbia, Cacteae, many tubers, e. g, So- 

 lanum tuberosum, and particularly also the CycadecB, the struc- 

 ture of whose stem has only the most superficial resemblance 

 to that of the Palms, and is certainly more nearly allied to that 

 of the Fern stems, but differs essentially from them by the 

 unlimited vascular bundles, and approaches far more to the 

 Cactece. 



5. Lastly, the modification of the cells composing the 

 woody bundles, whether originally or at a subsequent stage of 

 development, differs extremely, much more indeed than hi- 

 therto believed. The light wood of the Avicennics consists al- 

 most wholly of porous vessels ; the equally light and soft wood 

 of Bombax pentandra consists almost entirely of parenchyma, 

 spiral, circular and reticulate vessels, and rarely of prosen- 

 chyma occurring in the exterior part of the annual rings. 

 The wood of the Melocacti, Mammillarice and Echinocacti 

 consists entirely of peculiar short broad thin-walled cells, 

 terminating above and below in an obtuse conical form, with 

 very thick annular or spiral fibrous cells (deposited on their 

 narrow margin), like those which Meyen has represented in 

 his ^ Phytotomie' of Opuntia cylindrica, where they occur, as 

 in most of the Opuntice, though in less abundance, at the co- 

 arctations of the joints. It is well known that in the ConifercB 



