DICOTTLEDONES. 



185 



tissue within -which constitutiDg the pith, and that without, 

 the cortex. The thin paren- 

 chyma here and there persist- 

 ing between the bundles, and 

 radiating from the centre, 

 forms the Medullary Kays. 

 The cambium of each bundle 235 296 



joins that of the adjacent bundles on either side, and thus 

 arises a cambium ring, or zone, sometimes called " zone of 

 life.'' From this cambium, xylem, or wood, is formed on 

 the inner side; and phloem, or bast, on the outside. The 

 new layer of wood produced during the year (annual 

 ring) has generally larger pores, or ducts, in that portion, 

 found early in the season, and much smaller ones formed 

 later. To this fact largely is due the phenomenon of well 

 marked annual rings, whose number, as counted in cross 

 section (Fig. 297), indicates the age of the tree. 



igo. There is great diversity in the general disposition 

 or arrangement of the fibro-vascular bundles, as may be 

 observed in the study of different young Dicotyledonous 

 plants before they have become so numerous, or fused 

 laterally so as to form a solid cylinder. There are very 

 few " cauline bundles," that is, such as have no connection 

 with the leaves ; they are mostly " common bundles," or 

 extend both up into the leaf and down 

 .^.^ into the stem. There may be several enter- 

 ^^ ing the leaf, and these descending in the 

 $^Sh stem may for a time remain isolated, but 

 ^'^'^ further down unite laterally with those 

 297 from other leaves. K the bundle-cylinder 



Figs. 295, 296. Fibro-vascular bundles in young sterns^ diagrammatit^lly repre- 

 sented. Fig. 297. Transverse section of an exogenous stem, showing pith, annual 

 rings, medullary rays, and bark. 



