v 

 VASCULAR BUNDLES. 13 



The pith of trees seems mainly a structure of temporary utility 

 to the plant, and the function of the cortex is chiefly protective ; 

 but as the main function of the stem is to convey liquid nourish- 

 ment from the root to the leaves, and to carry back, also in a 

 diffusible form, the material elaborated in the leaves to growing 

 parts, it is one of the most noticeable characters of the bundles 



Fir;. 7. Transverse section of the stem of the Jerusalem Artichoke (Heli- 

 dnthus tuberosus). From The Elements of Botany, by Mr. Francis Darwin, by 

 his permission and that of the Syndicate of the Cambridge University Press. 

 c, cortex ; /, bast fibres ; c.c, companion-cells ; i.cb, interfascicular cambium ; 

 d.v, pitted vessel ; p.x, spiral vessel of protoxylern ; e, endodermis ; s.t, sieve- 

 tube ; cb, cambium ; m.r.p, pith-ray ; x.f. wood fibre ; p.p, pith. 



that they are largely composed of vessels, elongated tube-like 

 structures formed by the absorption of the transverse, or top and 

 bottom, walls of rows of long cells placed end to end. For this 

 reason they are often spoken of as vascular bundles. They also 

 contain, however, cells which have not been thus fused into 

 vessels, such cellular tissue, when its constituent cells are not 



