DEVELOPMENT OF THE VASCULAR BUNDLES. 395 



and Periblem (p. "7), was unknown to Sanio, although he himself first clearly 

 described this phenomenon for some special exceptional cases ; Russow opposes 

 this view, and especially the further conclusions drawn by Hanstein, chiefly on the 

 ground of the phenomena in Equisetum. 



Hanstein, in his paper of the year 1868, founded the doctrine that the leaf-trace 

 bundles of the typical Dicotyledonous stem are formed in the periphery of the 

 plerome-cylinder, while its central part becomes the pith ; and that the periblem 

 (together with the dermatogen) forms all the lateral members, especially the leaves 

 and the outer cortex, as well as the portions of the bundles, which pass through the 

 latter on their way to the leaves and branches. According to this, every leaf-trace 

 bundle is derived in its cauline portion from the plerome-cylinder, in the portion 

 going out into the leaf, from the periblem mantle. Considering the contradiction 

 between this view and that of Sanio and Russow, and in view of the fact already 

 mentioned at p. 8 that the differentiation of plerome and periblem in the growing- 

 points of stems cannot always be traced up to the extreme apex, it might at first be asked 

 whether Hanstein's view of the matter be not incorrect, in so far that the boundary 

 between his plerome and periblem might run, not between the primary meristematic 

 layers above-mentioned, but between those which are only secondarily severed from 

 one another, which Russow calls Peristem and Mesistem. This question or con- 

 jecture is however negatived, even wholly apart from stems with an axial bundle 

 ending in a sharply defined plerome-apex, as Hippuris (p. 8), on inspecting longi- 

 tudinal sections of many growing-points of stems. 



In the growing-point of Berberis vulgaris, where the plerome is well distinguished 

 from the single-layered periblem, Schmitz has recently investigated the origination of 

 the leaf-trace bundles, and proved that this takes place in the outer layers of the 

 plerome, but not even at its outer boundary, as the one or two outermost layers take 

 part in the formation of the external cortex. In the apex of the shoots of Meni- 

 spermurn canadense, according to the same observer, it is the external surface of the 

 plerome bundle, here also surrounded by an originally single-layered periblem, in 

 which the vascular bundles originate. In Ephedra, according to Schmitz, a certain 

 decision of the present question is not attainable, on account of the less sharp 

 original severance of the primary layers of meristem. 



There are thus indubitable cases among stems of the Dicotyledonous type where 

 the portion of the leaf-trace bundle running through the stem arises from the 

 plerome, and these cases are without doubt numerous. Although, considering the 

 different behaviour of Equisetum, it may be open to objection to base a principle of 

 general application on the existing results, before more numerous special observations 

 have been undertaken, yet provisionally, and in reservation of further researches, a 

 generalisation of the results obtained for cases with a similar type of structure and 

 growth is demanded. For the Dicotyledons in question, therefore, the origin of the leaf- 

 trace bundles, and of the bundle-ring generally, will have to be referred to the outer 

 part of the plerome strand ; and, according to Schmitz's observations on Meni- 

 spermum and Berberis, this will also apply to the fibrous strands and rings which 

 accompany the bundles. As however the bundle-rings of the Dicotyledonous type 

 may no doubt be regarded as everywhere morphologically homologous — at least 

 until the contrary should be proved — and as further the vascular strand, or, which is 



