COLLATERAL BUNDLES. 



121 



When corresponding bundles of plants of different groups are 

 compared together, some diversities as regards the arrangement 

 of the wood and liber elements are exhibited ; but most of the 

 cases can be referred without difficulty to the class of 



366. Collateral bundles (see 313) of the ordinary type ; 

 namely, those having liber on the external aspect and wood on 

 the. internal aspect. In some cases, however, this order may be 

 exactly reversed ; e.g., in the cortical fascicles of Calycanthacese. 

 The wood-elements in collateral bundles are generally arranged 

 in radial series ; the inner ducts or their equivalents (tracheids) 

 being more slender and having more closely coiled spiral mark- 

 ings than those nearer the periphery of the bundle. The radial 

 series ma}- be in close contact, separated by very thin plates 

 of parenchyma, or 

 may have a large 

 amount of this tis- 

 sue between them. 

 In dicotyledons, as 

 a rule, the ducts 

 at any given dis- 

 tance from the cen- 

 tre of the stem have 

 a noticeable uni- 

 formitv, so that a 



^ 7 



cross-section of 

 the primaiy tissue 

 shows a number of 

 concentric circles of 

 ducts of the same 

 size. Sometimes, 

 however, the ducts 

 in a radial series 

 ma}' be reduced to 

 one. In stems of monocotyledons there is less regularity in the 



/ * j 



arrangement of the wood-elements, but there is a substantial 

 likeness in their structure in any group. They are generally in 

 the form of a blunt wedge, the apex towards the centre of the 

 stem, the space between the inclined sides of the wedge being 

 mostly occupied by small ducts, wood-cells, and fibres. 



FIG. 98. Transverse section of a collateral flbro-vascular bundle of the stem of Indian 

 corn: p,p, conjunctive parenchyma; a, outer face; /. inner face of the closed fibro-vas- 

 cular bundle, which consists of a xylem portion ((/, g, two large pitted ducts; s, spirally 

 thickened duct; r, isolated ring of an annular duct; /, aeriferous lacuna, caused by 

 splitting resulting from growth) and a phloem portion, r, r. The whole bundle is sur- 

 rounded by a bundle-sheath of thick-walled cells. (Sachs.) 



