FIBRO-VASCULAR BUNDLES. 



103 



the exception of the parenchyma, all these elements are elongated 

 and are arranged in various sorts of fascicles or bundles, whence 

 the name, the fascicular system. Since fibres and vessels play 

 such an important part in the composition of this system, it has 

 been also called the fibro- vascular system, and the bundles, fibro- 

 vascular bundles. 



309. When reduced to its lowest terms, a fibro-vascular bun- 

 dle consists of two tissue elements, namely, cribrose-cells and 

 tracheal cells, the latter being sometimes replaced either wholly 

 or in part by ducts. 



310. The two elements are usually associated with some 

 parenchyma and with a considerable proportion of long bast- 



fibres ; but, while preserving a general uniformity of structure 

 throughout, a bundle ma}- become considerably changed in com- 

 position during its course. This is well shown by comparing 

 sections taken at some distance from each other ; for instance, 



FIG. 83. Longitudinal radial section of a collateral fibro-vascular bundle, from the 

 stem of a dicotyledon : b i, wood; in, liber; the wood comprises, b, a narrow annular 

 duct, c, wider spiral duct, rf, a. duct with septum, e, woody parenchyma, /, woody fibre, 

 g, wide duct with areolated pits, h, septate woody fibres; the liber comprises, n, liber- 

 fibres, m, short parenchyma, /, cribrose-cells, i, cambium, A% long parenchyma or 0:1111- 

 biform. (Kny.) 



