ii6 VASCULAR BUNDLES. [c:ta.p. 



a continuous ring around the pitn, and constantly increase 

 in thickness during the successive working seasons of the 

 tree by organically continuous additions to their outer side : 

 so that in Monocotyledons the bundles are closed or definite ; 

 in Dicotyledons continuous or indefinite. 



But the nature of this difference you will appreciate 



better when you understand the'^com- 



position or arrangement of the tissues 



forming these fibro-vascular bundles. 



Each bundle contains at first a layer of 



cells ot extreme delicacy, which cells 



are capable of undergoing division and 



enlargement; and it is by means of 



Fig. 89. Diagram repre- this layer Only that the bundle can 



orth^tiUues^^'nTfiSo- increase in thickness. This layer of 



Uber"^^J cambium-il>t! activc cells is cuclosed bctwccn two 



3;eToodr''''^^''''''^°''^«iistinct systems; one system, on the 



side towards the centre of the stem, 



consisting of long, thick- walled cells and vessels, formxing 



the proper wood of the fibro-vascular bundle, and another 



(smaller) system, on the side of the bundle towards the 



circumference of the stem, characterised by thick-walled, 



tapering cells, forming the liber system. 



The figure (89) represents a fibro-vascular bundle cut 

 across, showing at c the layer of delicate cells, called the 

 ca77iMum layer, the cells of which divide and give off new 

 cells on each side — on the inner side wood-cells (2^) and 

 vessels (7^), on the outer side fibrous liber-cells (/). In 

 Dicotyledons these bundles are arranged in 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 side, 

 coalesce, and thus form one continuous cylinder of multi- 

 plying and enlarging cells. The consequence is, that in 



