VASCULAR BUNDLES U 



In the first year the xylem and phloem are formed directly by 

 the modification of the inner and outer parts respectively of the 

 procambium-strand ; but subsequently all wood, bast, and pith- 

 rays originate in the cambium. Accordingly the xylem and 

 phloem of the first year are termed 'primary, and that formed 

 from the cambium secondary (Fig. 6). 



The pith of trees seems mainly a structure of temporary utility 

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



[ — ch 





1} 





D 



N 



' ^^ f 



Fig. 7.— Transverse section of the stem of the Jerusalem Artichoke (Helidnthus 

 tuberosus). From The Elements of Botany, by Mr. Francis Darwin, by his permis- 

 sion and that of the Syndicate of the Cambridge University Press, c, cortex ; /, bast 

 fibres; c.c, companion-cells; i.cb, interfascicular cambium; dv, pitted vessel; 2?.a;, 

 spiral vessel of protoxylem ; e, endodermis ; s.t, sieve-tube : cb, cambium ; m.r.p, 

 pith-ray ; x.f, wood fibre : p.p, pith. 



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 

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

 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, 



