DEVELOPMENT OF THE VASCULAR BUNDLES. 393 



logical region of development of the vascular bundles in the stem ^ For the axial 

 bundles, as has repeatedly been said above, it is established that their initial strand is 

 the plerome cylinder ; the bundles running from them into the leaves are formed out 

 of the surrounding periblem. For the cortical bundles the latter also holds good, 

 at least in the great majority of cases. The same has already been stated at p. 22 

 for the entire bundle-system of the stem of Equisetum. 



It follows first of all from these facts, in accordance with the principle more 

 generally enunciated in the Introduction, that in the stem the formation of vascular 

 bundles is not everywhere connected with one and the same primary layer of meristem. 

 This, however, does not yet answer the question how far other bundles or systems of 

 bundles, occupying a definite position in the stem, may arise from definite zones 

 of differentiation of the primary meristem. This is especially the question in the 

 case of the bundles of the ring and cylinder of the stems typical of Dicotyledons and 

 Monocotyledons. For reasons given at p. 20, the Ferns must be passed over for the 

 present. 



In stems of the Dicotyledonous type the bundles obviously have their origin in an 

 annular zone corresponding to their definitive arrangement. This zone, when the 

 formation of the vascular bundles begins in it, is distinguished by the rapidly 

 succeeding longitudinal divisions, which give rise to them, and by the small 

 width of the cells, at least at definite points, corresponding to the commencement 

 of the bundles. In the adjoining zones, which become converted into pith and 

 cortex, the longitudinal divisions, from an early period of development onwards, 

 take place more rarely, and cease sooner, if those cases where groups of sclerenchy- 

 malous fibres are formed be left out of consideration ; the cells, which for the most 

 part become developed into parenchyma, follow the general growth in the transverse 

 direction chiefly by increase of volume without divisions ; and this in general takes 

 place in such a manner that the more considerable increase of volume begins in the 

 middle of the pith. The rapid longitudinal divisions of the bundle-ring always 

 begin, on any cross-section, at those points where, in correspondence with the 

 general rules of succession, the first vascular bundles originate : thus, for example, in 

 a young internode, in the position of the single, or of the median trace-bundle going 

 to the next leaf above. Here there first appears in cross-section a small group of 

 narrow cells, proceeding from the division of two, or not many more, original cells, 

 which gradually increases by means of farther divisions corresponding at each time 

 to the thickness of the initial strand, the cross-section of which is represented by 

 this group. 



At the side of, or between the first initial bundle-groups of any cross-section 

 the commencements of new initial bundles then appear, in the same form, and in the 

 order corresponding to the general rules for the succession of bundles, and to the 

 course of the leaf-trace in the particular case, until the definitive number holding good 

 for the bundle-ring in question is complete. In particular cases the rapid longitudinal 



* Sanio, Botan. Zeitg. 1863, p. 356 &c. ; 1864, p. 192 &c.; 1865, p. 165 &c. — Hanstein, Die 

 Scheitelzellgruppe, &c. (1868); cf. p. 7. — Russow, Vergl. Untersuchungen, p. 177 &c. — Vochting, 

 Melastomeen und Rhipsalideen ; compare pp. 259 and 261. — Schmitz, Entwickelung d. Sprossspitze 

 der Phanerogamen, Halle, 1874. — Falkenberg, Monocotyledonen, I.e. 



