100 



BOTANY FOR BEGINNERS 



CHAP. 



The general arrangement of the elements in an open vascular 

 bundle is shown below in a tabular form. 



Near Pith. 



Reticulate vessels 

 Pitted vessels . . 



Cambium cells. . 



Sieve-tubes 

 Companion cells 



Bast fibres 



I | Soft bast . 

 Hard bast. 



Xylem 



Cambium 



Phloem. 



An 



Open Vascular 

 Bundle. 



Bundle Sheath (PericycJe. ) 



The Monocotyledonous Type of Vascular Bundle. 

 If a vascular bundle of a monocotyledonous plant be examined, 

 there will be found two kinds of tissue present. They are : 



(1) The Xylem, which always points towards the centre of 

 the stem. 



(2) The Phloem, which is turned towards the exterior of the 

 stem. 



The structure of such a vascular bundle is much the same as 

 in the dicotyledonous type, only the variety of vessels is not so 

 great. The bundle is surrounded by a special sheath of thick 

 walled cells. 



The Course of Vascular Bundles. If the bundle passes 

 from the stem into the leaf, as most bundles do, it is called a 

 common bundle, because it is common to both stem and 

 leaf. The portion of the bundle in the leaf is termed a leaf- 

 trace. In a few cases the bundle never passes from the stem, 

 and it is then spoken of as a cauline bundle. 



The arrangement of the bundles in the stem depends upon 

 the phyllotaxis (p. 36). If the arrangement of the leaves 

 is |, the bundle which proceeds from the leaf will pass 

 through five internodes before it joins on to the bundle below, 

 as in the Wallflower. 



If the leaves are decussate there will be four rows of leaves 

 on the stem, and the bundle from any leaf will have to pass 

 through two internodes only before it joins on to the bundle of 

 the leaf below. Thus, the bundle which proceeds from a leaf 

 will pass inwards for a short distance, then bend and pass down 



