. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. SECONDARY WOOD 121 can be related to the peculiar form of the cambial segments from \vhich the vessels are derived. The perforations are varied (cf. p. 35 and Fig. 17, D-F), but the type in which a number of cross-bars remain is commoner than in the primarj' wood. Tracheids (cf. p. 37) differ from vessels in being derived from single segments of the cambium, which show no open perforations in the end-walls ; they are usually much shorter than the vessels, but of about the same \\idth. The vessels and tracheid
. An introduction to the structure and reproduction of plants. Plant anatomy; Plants. SECONDARY WOOD 121 can be related to the peculiar form of the cambial segments from \vhich the vessels are derived. The perforations are varied (cf. p. 35 and Fig. 17, D-F), but the type in which a number of cross-bars remain is commoner than in the primarj' wood. Tracheids (cf. p. 37) differ from vessels in being derived from single segments of the cambium, which show no open perforations in the end-walls ; they are usually much shorter than the vessels, but of about the same \\idth. The vessels and tracheids are the water-conducting elements of the secondary wood, and the former are generally much more numerous than the latter; in fact in some cases there are practically no tracheids ( ^^'illow). The typical wood-fibres closel}' resemble those de- scribed on p. 33, having pointed ends and more or less thickened walls, which are provided with oblique slit-shaped simple pits (Fig. 60, /). Such fibres are connected by transitions with others which bear bordered pits with oblique slits ( Beech), and differ but little from the fibrous tracheids of the Conifers (see p. 340). In the typical wood-fibres the walls are lignified and the contents dead; but ,fibre-like cells with living protoplasts occur in the secondary wood of the Sycamore and of many herbs. Such fibres are occasionally septate ( Vine) and constitute transitions to wood-parenchyma. Where vessels are in contact with the ordinary mechanical fibres, pits are not developed on the Fig. 59.—Transverse section of the outer part of the stem of the Horse Chest- nut IJEsculus) showing the secondary phloem (Sec. ph.) and the cambium (Cam.), , companion cells ; /, fibres of pericycle; ^, medullary ray ; , primarj- phloem ; , Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these il
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