FIBRO-VASCULAR BUNDLES. 55 



thick and lignified ; when pitted like vessels they are known 

 as tracheide fibres ; when unpitted or with small slit-like 

 pits, libriform fibres. Water containing food-substances 

 in solution taken in by roots, travels along the cells of the 

 young wood-fibres to reach the leaves. 



(3) Wood-parenchyma. The cells have thin walls furnished 

 with large, simple pits, and still contain protoplasm; 

 frequently absent. 



Most frequently the wood-fibres constitute the greatest 

 bulk of the xylem. 



Phloem.— {\) Sieve-tubes with thin side walls, but thick 

 transverse perforated septa, the sieve-plates already described. 

 Contents : albuminous substances. 



(2) Bast-fibres, usually much elongated prosenchymatous 

 cells with tapering ends ; walls very thick but remaining 

 flexible ; that is, not lignified. 



(3) Phloem parenchyma consists of thin-walled parenchy- 

 matous cells. 



In open vascular bundles new additions are made to 

 both the phloem and xylem of the bundle by the cambium 

 lying between the two. Cambium cells are elongated with 

 oblique ends, and by the differentiation of their daughter- 

 cells the vascular bundle increases in size, those on the 

 inner side of the cambium being transformed into xylem 

 elements, those on the outside into phloem ; the central 

 portion of the cambium remaining unchanged ; and from 

 its daughter-cells similar additions to the elements of the 

 vascular bundle are made during every season of growth. 

 From the above account it is seen that the xylem increases 

 in size by additions to its outer . surface, the phloem by 

 its inner surface. The periodical additions to the phloem 

 are very much thinner than those of the xylem. In 



