PHANEROGAMIA : AXIAL ORGANS. 255 



rhizomes. The leafless Euphorbiacece, Carica, Theophrasta, Nym- 

 phcea, and Nuphar, as well as many Cactacece, afford excellent material. 

 I at present know of no other researches in reference to this point, ex- 

 cept my own very imperfect ones into the stems of Cactacece, especially 

 Mammillaria, Echinocactus, and Melocactus. The vascular bundles at 

 first make an arc of considerable curvature ; by the gradual develop- 

 ment of the pith the curvature becomes almost effaced, and it only re- 

 mains in the upper part, where the vascular bundles pass off to the leaves. 

 The first succeeding layer developed in the vascular bundle is applied 

 over and ftp beyond this, dividing at the point where the primary vas- 

 cular bundle goes off to the base of the leaf, and uniting again above to 

 pass up to the base of a leaf situated higher up. The next layer of 

 structure forms in the same way, by splitting and reuniting, two meshes, 

 one for the primary vascular bundle, and one for the portion of the first 

 layer of increase running to the upper leaf, then above this it runs up 

 to the base of another leaf. This structure is continued up throughout 

 the whole stem, which thus possesses a form of wood exhibiting perfectly 

 regular meshes or areolae, which appear to be formed by an alternating 

 superposition of vascular bundles, and each give passage to a bundle 

 coming from the innermost part of the wood. Of course, there is here 

 a perfect crossing of the vascular bundles going to the lower leaves by 

 all the subsequently formed portions of vascular structure, and by a 

 little care we may make preparations not very unlike the structure of a 

 Monocotyledonous stem with undeveloped internodes. The whole struc- 

 ture bears great similarity to that of the arborescent Ferns, allowing 

 for the different nature of the vascular bundles and the difference of 

 dimension. 



Many interesting varieties in the structure of the wood occur here 

 also ; and the wood of the Mammillarice and Melocacti, composed en- 

 tirely of peculiar spiral-fibrous cells, is particularly worthy of notice. 



The stems of the Rhizanthece (Blume) appear to be altogether aber- 

 rant and irregular in their structure ; I cannot say anything about them, 

 since I have no material, and I refer to the researches of Unger and 

 Goppert presently to be named. 



Even Moldenhauer * remarked, that one and the same vascular 

 bundle varied in its structure in different parts of its course. As a 

 general rule, we may say that in the Monocotyledons the vascular 

 bundles are simplest in their lower part, often, for instance, in the 

 Palms, composed at that part solely of elongated parenchyma (liber) ; 

 in the middle becoming more complicated from within outward, exhi- 

 biting almost all the forms corresponding to the varied expansion of the 

 cell ; above, they become simpler again, particularly where they pass off 

 into a leaf or branch, and consist frequently merely of such elements as 

 correspond to a considerable expansion in the longitudinal direction after 

 the appearance of layers of thickening. In the Dicotyledons the vas- 

 cular bundles appear to have a tolerably uniform structure below and in 

 the middle, but toward the upper end the onward developing portion of 

 each older bundle passes into the form of a primary bundle, or, in other 

 words, every primary vascular bundle of a new internode appears as the 

 immediate prolongation, not of the primary bundle of the preceding 

 internode (which rather runs to a leaf), but of the layer of increase of 



* J. J. P. Moldenhauer, Beitrage, &c. 



