. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 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-reve
. Physiological botany; I. Outlines of the histology of phænogamous plants. II. Vegetable physiology. Plant physiology; Plant anatomy. 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, sp
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