INTERNAL STRUCTURE. 



[CHAP. 



bundle ; and another (smaller) system (the phloem system) 

 on the side of the bundle towards the circumference of 

 the .stem, characterised by the 

 presence of thin-walled tubes, 

 of which the transverse septa 

 and, in definite areas, the 

 vertical walls separating similar 

 contiguous tubes, are finely 

 perforate : with these is very 

 often present a varying pro- 

 portion of thick-walled bast- 

 fibres, 

 "the tissues in a fibro-vas- The figure (69) represents 



cular bundle ;/ the phloem, a fibrO-vaSCular bundle CUt 

 with a few thick-walled bast- , . , 



fibres, c cambium-layer, w across, showing at c the layer 



wood-ceils, -v wide vessel of of delicate cells, called the 



cambium-layer, the cells of 



which divide and give off new 



cells on each side on the inner side chiefly wood-cells 

 and vessels, on the outer side fibrous liber-cells and 

 sieve- or lattice-tubes. In Dicotyledons these bundles 



FIG. 69. Diagram represent- 

 ing a common arrangement 

 of t" 



. 70. Diagram showing the arrangement of the fibro-vascular bundles 

 (each of them wedge-shaped in cross-section), in the stem of a young 

 Dicotyledon. The pale circle passing through each bundle near its 

 thicker end indicates the future cambium-cylinder. /. the pith. 



are arranged in the undifferentiated cellular tissue of the 

 stem in such a manner, at a very early stage of its growth, 

 that the cambium-cells of the bundles, which are side by 



