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 nbro-vaseular 

 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 110 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 



fc i<;. 540. Diagram of the first . . . 



formation of an exogenous continually receiving additions to its 

 stem : a, pith ;&, Z>, bark ; c, e, interior, that the term endogenous was 



given to this type of stem-structure : 

 but, from the fact just stated regarding 

 the course of the fibro-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) bet \\eeii three 

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



dullary rays I left between the 

 woody bundles d d. 



Fit,. 541. Transverse section of stem of ( 'Ir/nutis : a, pith: l>, 1>, b, woody bundles 



r, r, <. medullary rays. 



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

 I iet ween them, its circle being mad*' up of wedge-shaped bundles 

 (il '/). kept apart by the nn-il nllitri/ r<ii/n composed of unchanged cel- 

 lular tissue (c, c) that pass between the pit li and the hark. The pith 

 (lii;. ~> 1 I . ") is almost invariably composed of cellular tissue only. 

 u Inch usuall v pi events (in transverse sect ion) an hexagonal areolation. 

 When newly Conned it has a greenish hue. and its cells are filled with 



