88

THE PALM-STEM :

bundles run down to the base of the stem in the form ofa close fibrous network, and that in consequence of thecontinued deposition of new fibrous layers, the stem ex-hibits an uninterrupted growth in thickness, while in thePalms the lower fibrous extremities of the vascularbundles, as a general rule, remain simple, and do not rundownwards to the base of the stem. The case observedby me in Cocos, of the solution of the vascular bundlesinto a number of slender fibres (p. 9) is to be regardedas an approximation, in the Palms, to the structure of thisfibrous reticulation. These differences occurring betweenthe stem of Yucca and that of the Palms are undoubtedlynot of sufficient importance to allow of our supposing anessential difference to exist in the mode of developmentof the vascular bundles. Since, then, in every Monoco-tyledonous stem vascular bundles appear, in consequenceof the development of a branch, which, without the for-mation of that branch, would not have originated,—sincethese vascular bundles form a greater mass, and spreadso much more widely over the stem from the base of thebranch, the older this latter becomes,—since in Yucca thecourse of the vascular bundles exhibits such mechanicalconditions in the vicinity of a wound on the stem, asmust result from a downward growth of the vascularbundles on the stem, it is quite justifiable for me to de-clare that Mirbel’s view, that the vascular bundles of thePalms grow from below upwards, is an opinion opposedto the phenomena of the growth of the Monocotyledons;and to presume, on the contrary, that the lower portionof these bundles is developed in the direction from abovedownwards.

Meneghini came to the same result through a series ofdeductions altogether different from mine. He had alreadystated, in his first paper on the structure of Monocotyledons(Richerche sulla Struttura del Caule nelle Piante Monoco-tiledoni, 1836, p. 77), that the formation of vascularbundles was caused by definite currents of nutrient sap,without, however, carrying out this view more minutely.