Mor;pliology of Vegetable Tissues. By W. H. Gilburt. 803 



In its earliest condition the procambium can only be distin- 

 gnislied from the surrounding tissue, when examined in transverse 

 section, by the comparatively small size of the cells composing it. 

 This similarity, however, speedily disappears, as most of its cells 

 divide into two, three, or four; the cell-plates being laid down 

 indifferently as to order or relative position. This is well shown 

 in Fig. 3, which represents a portion of the procambium of the 

 Lombardy poplar. This division of the cells does not take place 

 simultaneously throughout the whole of the tissue, but commences 

 at certain spots which correspond with the position to be occupied 

 by the vascular bundles, whether primary, secondary, or of a 

 higher order. 



In some species the next step in development consists in 

 certain cells at more or less definite distances, growing most 

 rapidly, and dividing in a radial direction, that is, with regard to 

 the axis of the stem ; this being the commencement of the primary 

 medullary rays. In others, however, these are not to be distin- 

 guished till much later. 



In Figs. 6 and 7, which are from a section of Ftcus carica, 

 the primary medullary rays are present, and it will be seen that by 

 them the procambium is separated into well-marked bundles, and 

 while on their inner side there is no distinct line separating them 

 from the medulla, yet that on the outside their limits are well 

 defined. 



The next stage is, that the layer of procambium-cells next the 

 medulla, which have remained unaltered, and which I would call the 

 generating layer, becomes divided by septa which are invariably 

 tangential to the circumference of the stem. As the differentiation 

 of the phloem elements commenced at certain points which agree 

 with the position and order of the future vascular bundles, so with 

 the septation of these cells ; commencing at certain definite points 

 the process continues till we have a closed cambium ring or a 

 series of bundles separated from each other by the primary 

 medullary rays. In Fig. 1, which represents a portion of the 

 procambium of the laburnum, it will be seen that there are three 

 cells, which show this septation at the earhest stage in which I 

 have observed it. In each cell there are two septa, the outermost 

 one of which is the thinnest, and therefore the last laid down. 

 In Fig. 2 is shown part of a bundle from the same section, of 

 rather earlier origin, in which we have a radial series of four or 

 five cambium-cells ; the fact of their origin being from a single cell 

 is still plainly indicated by the greater thickness of the mother-cell 

 wall which stiU seems to surround them. 



The same process in every important particular holds good in 

 the Lombardy poplar, three stages in the development of 

 which are represented in Figs. 3-5. 1 have, however, occasionally 



3 H 2 



