VASCULAR CYLINDER OR STELE ii; 



If we select an individual bundle in the internode of almost 

 any dicotyledon and trace it upwards it will be found to pass 

 out of the stele across the cortex and into the leaves, where it 

 branches and forms the veins. Bundles of this kind common 

 to both leaf and stem are termed common bundles, that part of 

 each present in the stem being spoken of as the leaf-trace of the 

 bundle. From each leaf one or several bundles may enter the 

 stem, and on being followed downwards they are found to 

 descend perpendicularly through one or more internodes, finally 

 uniting with bundles which have entered the stem from older 

 leaves lower down. The bundles in their descent all keep about 

 the same distance from the centre, so that in a transverse section 

 they appear arranged in a circle. 



Great variation exists in the manner and amount of branching 

 and union of the bundles in different plants, but the arrange- 

 ment is always such that the vascular bundles of the leaves, 

 stems and roots form a continuous conducting system of tissues 

 specially adapted to facilitate rapid and easy transmission of sap 

 to all parts of the plant. 



In this type of stem each vascular bundle consists of three 

 kinds of tissue, namely : — 



(i) xylem or wood (n, i. Fig. 55); 



(2) phloem or bast (d) ; and 



(3) a thin-walled meristem tissue termed the cambium of 



the bundle {c). 



These tissues are arranged side by side in such manner that 

 in a transverse section of the stem a radius drawn from the 

 centre to the outside passes through all three ; the cambium 

 lies between the wood and the bast, the wood being nearest 

 to, and the bast farthest away from the pith. 



Bundles in which the wood and bast lie on the same radius 

 are termed collateral bundles; when as in dicotyledons they 

 also possess cambium they are said to be open. 



(a) Wood or xylem. — The elements met with in the wood 



