112 



MORPHOLOGV OF TISSUES. 



Monocotyledons these are still more conspicuous, especially if the bundle-sheath 

 of lignified prosenchyma, which often occurs in them, is taken into account (see 

 Fig. 92). Among Ferns, Lycopodiaceae (with isolated bundles^), and Rhizocarpeae, 

 the xylem lies in the centre of the bundle, while the phloem forms a soft succulent 

 sheath round it (Fig. 67, p. 80, and Fig. 94). 



According to the relative position of the two principal constituents of fibro- 

 vascular bundles, they may be classified under two varieties: — (i) Collateral bundles, 

 in which the phlo6m and xylem run parallel to one another, like the two halves of a 



Fig. 93.— Part of a transverse section from the. mature hypocotyledonary portion of the stem (tigellum) o( Ricmtcs com- 

 ■mtmis; r parenchyma of the primary cortex, m of the pith, both belonging- to the fundamental tissue ; between r and b is the 

 simple bundle-sheath containing starch-grains, also belonging to the fundamental tissue. The fibro-vasculnr bundle consists 

 of the phloem b, y, the xylem g; t, and the cambium c c, and is therefore an open bundle. The cambium of the bundle 

 cc\s also continued into the fundamental tissue that lies between it and the neighbouring bundles in the form of interfascicular 

 cambium cb, formed by subsequent divisions of large parenchymatous cells (secondary meristem). In the phloem are bast- 

 fibres b b, the soft-bast jt'jv (partly parenchyma and partly sieve-tubes); in the xylem narrow pitted vessels it, broad pitted 

 vessels g-g^, and between them wood-fibres. 



rod bisected lengthwise, as occurs in Phanerogams and Equisetaceae ; and (2) 

 Concentric, when the xylem is completely enclosed within a layer of phloem, as 

 in Ferns, the simple bundles of Lycopodiaceae ^, &c. The latter kind are always 



^ The bundle in the stem of Lycopodium Chamcecyparissus, Sec, is clearly a union of several 

 fibro-vascular bundles, such as occurs in the axial cylinder of roots (see Book II,, Lycopodiacese). 



^ See Russow,VergleichendeUntersuchungen liber die Leitbiindelkiyptogamen. Petersburg 1872, 

 p. 159 et seq. The terminology , employed in this paper does not seem to me happy, with the 

 exception of the term ' collateral bundles.' 



