STRUCTURE OF THE VASCULAR BUNDLE. 



317 



between bundle-ends and bundle-trunks, although a sharp limit between the two 

 cannot be shown in any case. 



To the category of bundk-trunks belong chiefly the bundles which pass through 

 the stem, roots, leaf-stalks, and thick nerves of the leaf. The description of their 

 structure must start from these organs. 



Most bundle-trunks, however different in details, possess the composition which 

 has above been indicated in a general way. In comparatively few cases their 

 structure is simplified by essential organs disappearing or remaining rudimentary. A 

 distinction must therefore be drawn between incomplete and complete bundle-trunks. 

 Here we shall first speak of the latter. 



I. Bundle-trunks. 



Sect. 100. The essential tissue-elements of the complete bundle are tracheae 

 and sieve-tubes.- The two are always so arranged that one longitudinal portion of 



Fig. 147. — Acorus Calamus ; cross-section through 

 the periphery of the flower-stalk (145). e epidermis; b 

 small vascular bundles, with a sclerenchymatous sheath 

 on the outside. In the middle is a large vasculctr bundle; 

 ■ui its phloeiQ ; g outer lai^e trachcEe of the xylem ; / 

 intercellular passage at the inner side of the latter. The 

 cross-section through the leaf shows the same structure. 



Fig. 14S.— Cross-section through the 

 concentrically arranged lower end of a 

 bundle of the leaf-trace in the stem (145). 

 The delicate and small-meshed phloem 

 occupies the middle, and is surrounded, 

 by a ring of scalariform netted tracheides ; 

 outside this is parenchyma. 



the bundle includes all the tracheae, while another, or in rarer cases more than one 

 other, includes all the sieve-tubes. Thus in every bundle we have to distinguish 

 between that part which contains the tracheae (tracheides or vessels) and that 

 which contains the sieve-tubes, or expressed more shortly, between the xylem and 

 the phloem. 



In both parts the characteristic tissue-elements are as a rule not present alone, 

 but are placed between rows or layers of cells (cf. p. 5) in such a way that all or 

 most trachese or sieve-tubes are in contact with the latter at least at one point. It 

 is true that in the case of very small bundles this intercalation is not uncommonly 

 absent in the xylem ; but then the few tracheae of which this consists, border, for 

 the most part at least, on the cells which encircle the bundle. More rarely masses of 



