78 



THE BIOLOGY OF A PLANT. 



open network (Fig. 38), which can only be seen in a lateral 

 view of the rhizome. From this network bundles are given off 

 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. 87). 



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. These tissues have the following 



}-~xv ::- 



~ -'-. 

 _ ' >'* ' ' ' ' ' 



dia^^ ; - -'-' - : 



k -.' 



/^ 





FIG. 39. Highly magnified cross-section of a fibro-vascular bundle surrounded by the fun- 

 damental parenchyma, f.p. t, scalariform tracheids; b.s, bundle-sheath; p..?, phloem- 

 sheath; b.f, bast-fibres; s.t, sieve-tubes; p.p, phloem-parenchyma; u\p, wood (xylem) 

 parenchyma; s.v, spiral vessel. 



definite arrangement. Beginning with the outside of a bundle, 

 we find (Figs. 39, 40)- 



1. Bundle sheath a single layer of elongated cells envelop- 

 ing the bundle, probably derived from and belonging to the 

 fundamental system. 



2. Phloem-sheath ; a single layer of larger parenchymatous 

 cells containing starch in large quantities. 



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

 taining protoplasm and large nuclei. 



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



