SECONDARY WOOD 



121 



can be related to the peculiar form of the cambial segments from 

 which 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 primary 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 width. 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 (e.g. Willow). 

 The typical wood-fibres 

 closely 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 



am. 



FIG. 59. Transverse section of the outer 

 part of the stem of the Horse Chest- 

 nut (JEsculus) showing the secondary 

 phloem (Sec. ph.) and the cambium 

 (Cam.), c.c., companion cells ; /, 

 fibres of pericycle; M.Y., medullary 

 ray ; P.ph., primary phloem ; s.t., 

 sieve-tube. 



with others which bear 

 bordered pits with oblique 

 slits (e.g. 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 (e.g. Vine) and constitute transitions 

 to wood-parenchyma. Where vessels are in contact with the 

 ordinary mechanical fibres, pits are not developed on the walls s 



