THE FIBRO'VASCULAR BUNDLES. 



95 



(as in the roots of radish, tubers of the potato, &c.). The elements of the fibro- 

 vascular bundles, as far as they consist exclusively of procambium, are prosenchy- 

 matous or at least elongated in the direction of the axis of growth of the bundle. In 

 open bundles there arise also in the cambium, with the increase of their thickness, 

 horizontally extended rows and layers of cells disposed radially, by which the later- 

 formed xylem- and phloem-layers of the bundle become arranged in a radial fan-like 



Fig. 82.— Transverse section of a fibro-vascular bundle in the mature elongated hypocotyledonary portion of the stem oi Ricinus 

 cotnnutnis ; r cortical parenchyma; 7n parenchyma of the pith; * bast : y phloem portion with thin-walled cells; c cambium ; 

 g g large pitted vessels ; t t smaller pitted vessels with wood-cells between them ; cb continuation of the cambium into the 

 parenchyma lying between the bundles; the parenchyma-cells are repeatedly divided by tangential walls. (Between the cortex 

 (r) and the phloem of the bundle lies a layer filled with compound starch-grains, the bundle-sheath, or starch-bearing layer.) 



manner ; these horizontal elements mostly assume the character of parenchymatous 

 cells, and may be generally designated as rays ; within the xylem they are called 

 xylem-rays, within the phloem, phloem-rays. 



The position of the layers of phloem and xylem in the transverse section of a 

 bundle varies according to the class to which the plant belongs and the organ in 

 which they are found ; in the open bundle in the stem of Dicotyledons and Conifers 

 the former lies towards the circumference ^ the latter facing the axis of the organ ; 

 between the two lies the cambium -layer (Fig. 82). But it sometimes occurs that a 



^ In Dicotyledons bundles also occur exceptionally within the circle of wood proper (in the 

 pith), where the phloem portion is surrounded by wood as by a sheath (in the rachis of the inflo- 

 rescence of Ricinus) ; in Heterocentron roseum the medullary bundles have, according to Sanio, their 



