HISTOLOCN 



IQ 



CHAPTER IV. 



HISTOLOGY OF THE VEGETATIVE ORGANS. 



The shoots and roots of grasses conform in their internal 

 structure to the monocotyledonous type. In all grasses numerous 

 threads are found running longitudinally within the stem and some 

 of these pass into the leaves, at the nodes, and run as nerves in the 

 blades of the leaves. These threads are the vascular bundles. 

 The rest of the tissue of the stem and leaves consists of thin-walled 

 parenchymatous cells of different sorts. 



The general structure of these bundles is more or less the same 



in all grasses. A vascular 

 bundle consists of only 

 xylem and phloem, without 

 the cambium, and so no 

 secondary thickening can 

 take place in the stems of 

 grasses. Such bundles as 

 these are called closed 

 vascular bundles to distin- 

 guish them from the dicoty- 

 ledonous type of vascular 

 bundles which are called 

 open vascular bundles on 

 account of the existence of 

 the cambium. 



The component parts and 

 elements of which the vas- 

 cular bundles in grasses are 

 composed may be learnt 

 by studying the trans- 

 verse and longitudinal 

 sections of these bundles 

 in any grass. The cross 

 and longitudinal . sections 

 of a vascular bundle of the 

 stem of Pennisetum cen- 

 chroides, are shown in figs. 

 22 and 23. In the figure of 

 the transverse section the 

 two large cavities indicated 

 by the number 3 and the two small circular cavities with thick 

 walls lying between the larger ones and indicated by the numbers 

 I and 2 are the chief elements of the xylem. 



By looking at the longitudinal section it is obvious that these 

 elements are really vessels, the larger being pitted and the smaller 

 annular and spiral vessels. These vessels together with the 



Kig. 22. — Transverse section of a vascular 

 handle. x 250 



1. Annular vessel; 2. spiral vessel; 3. 

 pitted vessel : 4. phloem or sievp tubes : 

 5, sclerenchyma. 



