120 



THE BIOLOGY OF A PLANT. 



lateral view of the rhizome. From this network bundles are 

 given oft' which extend on the one hand into the roots and on 

 the other into the leaves, branching in the latter to form the 

 complicated system of veins to be described hereafter (p. 129). 



Each bundle consists of a number of different tissues which, 

 broadly speaking, have the function of conducting sap from one 

 part of the plant to another. 



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FIG. 52. Highly magnified cross-section of a fibro-vascular bundle surrounded by 

 the fundamental parenchyma, /.p. f, scalariform tracheids ; 7>.s, bundle-sheath ; 

 p.s, phloem-sheath ; ib./, bast-fibres ; s.f, sieve-tubes ; p.p, phloem-parenchyma ; 

 U\PI wood (xy lem) parenchyma ; s. r, spiral vessel. 



These tissues have the following definite arrangement. Beginning with 

 the outside of a bundle, we find (Figs. 52, 53)- 



1. Bundle- sheath .; a single layer of elongated cells enveloping the 

 bundle, probably derived from and belonging to the fundamental system. 



2. Plrtoem-sheath ; a single layer of larger parenchymatous cells con- 

 taining starch in large quantities. 



3. Bast-fibres ; soft, thick-walled, elongated, pointed cells containing 

 protoplasm and large nuclei. 



4. Sieve-tubes; larger, soft, thin-walled, elongated cells containing 

 protoplasm and having the walls marked by areas perforated by numerous 

 fine pores (panelled). They join at the ends by oblique panelled partitions 

 (shown in Figs. 52 and 53). 



