FIBUO-VASCULAR BUNDLES. 



103 



tho exception of the parencln-ma, all these elements are elongated 

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

 the name, the fascictdar system. Since fibres and vessels pla)'. 

 such an important part in tho composition of tliis sj-stem, it has 

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

 vascular bundles. 



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

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

 tracheal cells, the latter being sometimes replaced either whollj' 

 or in part b^- duets. 



310. The two elements arc usuall}- associated with some 

 parench^-ma and with a considerable proportion of long bast- 



flbres ; but, while preserving a general uniforraitj- 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. Longitiidiiiiil radial section of a collateral libro-vasciilar bundle, from the 

 stem of a dicotyledon : h—i, wood ; i — n, liber ; the wood comprises, b, a narrow annular 

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

 j7, wide duct with areolatad pits. A, septate woody fibres; the liber comprises, n, liber- 

 ilibres m, sliort parenchyma, /, cribrose-cells, i, cambium, k, long parenchyma or cam- 

 biform. (Kny) 



