COLLATERAL BUNDLES. 



121 



When corresponding bundles of plants of different groups are 

 compared together, some divei'sities as regards the arrangement 

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

 cases can be referred witliont diffltulty 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 maj- be 

 cxactlj- reversed ; e.(j.,m 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 closel}' coiled spiral mark- 

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

 series maj- be in close contact, separated by vei-y thin plates 

 of parenchyma, or 

 ma3' 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- 

 form it}-, so that a 

 cross-section of 



the primary 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 monocotj'ledons there is less regularity in the 

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

 lil<eness 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 

 mostl}' occupied by small ducts, wood-cells, and fibres. 



Fig. 98. Transverse section of a collateral fibro-vascular bund'e of the stem of Indian 

 corn : p,p, conjunctive ptirencliyma ; a, outer face; i, inner face of tlie closed flbro-viis- 

 cular bundle, wbicb consists of a xylem portion {g, g, two lar^e pitted ducts; s, spirally 

 tbickened duct; r, isolated ring of an annular duct; ^ aeriferous lacuna, caused by 

 pjilitting resulting from growtli) and a pldnem portion, v, v. Tlio whole bundle is sur- 

 rounded by a buudle-sbeatb of tbick-walled cells. (Sacbs.) 



