100 



BOTANY 



PART I 



when this tissue is peripheral. The bundles in most Pteridophytes 

 (Fig. 119) are of the former type, those in the rhizomes or stems of 

 some Monocotyledons (Fig. 118) of the latter. 



In the Pteridophytes the narrow elements of the protoxylem (sp) lie in groups 

 in the strand of xylem, peripherally, centrally, or among the later-formed vessels. 

 The xylem is surrounded by a sheath of parenchyma (Ip). Outside this comes a 

 zone composed of sieve-tubes (o) and parenchyma (s), the narrow protophloem 

 elements being situated at the outer edge of this. 



In COLLATERAL vascular bundles (Fig. 120 A), which consist of a 

 strand of xylem and as a rule a single strand of phloem, the xylem 

 lies beside or rather behind the phloem. The median plane of the 



bundle is always placed radially 

 in the stem, the xylem being 

 directed inwards and the phloem 

 outwards. The protoxylem in 

 collateral bundles is usually 

 placed at the inner edge of the 

 strand of xylem, the proto- 

 phloem at the outer edge of 

 the phloem, as the bundle is 

 seen in transverse section. Such 

 collateral bundles are character- 

 istic of the shoots of the Sper- 

 maphyta and the Horsetails. 

 BICOLLATERAL bundles, in 

 which the xylem is accompanied 

 by a strand of phloem on the 



FIG. 118. Concentric vascular bundle with external inside as Well as On the OUtside, 

 xylem from the rhizome of Convallaria vwjalis. a | so occurs as f or example in 

 ph. Phloem ; x, t, xylem ; s, protoxylem. (After , , ~ , . _ 



ROTHERT.) the stems of Cucurbitaceae. In 



Monocotyledons the collateral 



bundles, like the radial and concentric vascular bundles, are closed, 

 i.e. the whole bundle consists of permanent tissue, the xylem abutting 

 directly on the phloem (Fig. 120 A). In Gymnosperms and 

 Dicotyledons, on the other hand, the bundles are usually open, i.e. 

 the xylem and phloem remain separated by a layer of meristematic 

 tissue called the CAMBIUM (Fig. 121). 



In all vascular bundles the strands of xylem are mainly composed 

 of narrower or wider lignified elements that serve for the conduction 

 of water. These may be tracheides and tracheae, or only tracheides. 

 They occur singly or in groups without intercellular spaces among 

 narrow, living, elongated and often unlignified cells of the conducting 

 parenchyma (xylem parenchyma), or are surrounded by a sheath of 

 this tissue (Fig. 119 Ip). Sclerenchymatous fibres are sometimes 

 present in addition. In the Pteridophyta only tracheides are present, 

 while in the bundles of Phanerogams both tracheae and tracheides 



