702 MICROSCOPIC STRUCTURE OF PHANEROGAMIC PLANTS 



towards the surface of the stem, and assist, by their interlacement 

 with the outer bundles, in forming that extremely tough investment 

 which the lower ends of these stems present. New fibro- vascular 

 bundles are being continually formed in the upper part of the stem, 

 in continuity with the leaves which are successively put forth at its 

 summit ; but while these take part in the elongation of the stem, 



they contribute but little to the increase 

 of its diameter. For those which are 

 most recently formed only pass into the 

 centre of the stem during the higher 

 part of their course, and usually make 

 their way again to its exterior at no 

 great distance below ; and, when once 

 formed, they receive no further additions. 

 It was from the idea formerly enter- 

 tained that these successively formed 

 bundles descend in the interior of the 

 stem through its entire length until they 

 reach the roots, and that the stem is thus 

 continually receiving additions to its 

 interior, that the term endogenous was 

 given to this type of stem-structure ; 

 but, from the fact just stated regarding 

 the course of the nbro-vascular bundles, 

 it is obvious that such a doctrine cannot be any longer admitted. 



In the stems of dicotyledonous phanerogams, on the other hand, 

 we find a method of arrangement of the several parts which must 

 be regarded as the highest form of the development of the axis, 

 being that in which the greatest differentiation exists. A distinct 

 division is always seen in a transverse section (fig. 540) between three 

 concentric areas the pith, the wood, and the bark the first (a) being 



FIG. 540. Diagram of the first 

 formation of an exogenous 

 stem : a, pith ; b, b, bark ; c, c, 

 plates of cellular tissue (me- 

 dullary rays) left between the 

 woody bundles d d. 



FIG. 541. Transverse section of stem of Clematis: a, pith; b, b, b, woody bundles 

 c, c, c, medullary rays. 



central, the last (b) peripheral, and these having the wood interposed 

 between them, its circle being made up of wedge-shaped bundles 

 (d d), kept apart by the medullary rays composed of unchanged cel- 

 lular tissue (c, c) that pass between the pith and the bark. The pith 

 (fig. 541, a) is almost invariably composed of cellular tissue only, 

 which usually presents (in transverse section) an hexagonal areolation. 

 When newly formed it has a greenish hue, and its cells are filled with 



