V2i 



BOTANY 



PART I 



this the xyleni is accompMuied by phloem both on the outside and 

 inside. Such bicollateral bundles are characteristic of the Cucur- 

 bitaceae. The xylera and phloem of roots generally form separate 

 strands (Fig. 134 s, v), and the xylem strands are differently oriented ; 

 while in stems the narrow vessels are nearer the centre and the wider 

 nearer the circumference, in roots this order is exactly reversed. The 

 xylem and phloem strands make up what is spoken of as a radial 

 bundle. 



The " CONCENTRIC " vascular bundles of the Pteridophy ta (Fig. 

 135) contain tracheides {sp), and only in exceptional cases tracheae (sc). 

 The latter are as a rule wanting in Pteridophyta, although this group 

 goes by the name of Vascular Cryjjtogams (^'•^^). The water-conducting 

 elements exhibit scalariform thickenings, only the narrowest having 

 spiral markings (sj)) ; they are surrounded by xylem parenchyma (Ip). 

 Outside this comes a zone of tissue consisting of sieve-tubes (r) and 

 phloem parenchyma (s). 



A number of vascular bundles of this kind are present in the stems of most Ferns 

 and species of Selac/inella. In Lycopodium, they are fused into a single central 

 cylinder. In the stem of Equisetaceae vascular bundles of collateral structure appear. 



The vascular bundles are developed from strands of meristematic 

 tissue which are called procambium strands. Within each strand 



a zone of tissue commences to 

 divide tangentially, and behav- 

 ing as a primary meristem pro- 

 duces to both the inside and 

 the outside new cells in radial 

 order. If the whole meristema- 

 tic tissue of a procambium strand 

 is exhausted in this process, the 

 vascular bundles are said to be 

 CLOSED ; but if any of the 

 meristematic tissue remains in 

 an undifferentiated condition 

 between the xylem and phloem 

 portions, the bundles are spoken 

 of as OPEN (Fig. 1334 The 

 Pteridophy tes have, almost with- 

 out exception, closed bundles : 

 in Monocotyled on s also the 



Fio. 134. — Transverse section of central portion of 

 the rout oi Acorua Calamvjs. in, Medulla ; .<, xylem; 

 c, phloem ; p, pericycle ; c, endodenni.s ; i-, cortex. 

 (x 90.) 



bundles f\rt^. always r.lngpd (Fig. 



131); Gyi^nosperms and Dic o- 

 tylfidnns (Fig. 133), on the contrary, have open b undles. 



In those portions of plants which are still actively growing in length, the pro- 

 cambium strands remain unditferentiated, except at definite jioints, where single 

 rows of cells lose their meristematic condition and form narrow, annular, and spiral 



t 



