346 CONDUCTING SYSTEM 



III. THE STRUCTURE OF VASCULAR BUNDLES. 



Having made ourselves acquainted with the several structural 

 elements that are employed for purposes of conduction, we must next 

 proceed to review the various ways in which these elements may be 

 combined so as to form composite conducting strands [or vascular 

 bundles]. In the present chapter we shall confine our attention strictly 

 to the primary conducting tissues. 



A. SIMPLE CONDUCTING STRANDS. 



The simplest possible form of conducting strand is entirely made 

 up of water-conducting elements. The delicate vascular anastomoses of 

 leaves and stems, for example, often consist of nothing beyond a few 

 tracheides, and thus represent minute channels serving exclusively for 

 the conduction of water. Most frequently, indeed, these tracheides are 

 enclosed within a glucose-conducting parenchymatous bundle-sheath, in 

 which case the anastomoses do not, strictly speaking, pertain to the 

 category of simple strands ; in other instances, again, the sheath is 

 fibrous, as, for example, in certain thick Monocotyledonous leaves (Rhapis, 

 Vanda furva) and in the wing-like inflorescence-bract of the Lime. As 

 a further example of the purely water-conducting type of bundle, we 

 may once more mention the strand of rudimentary tracheides which 

 occupies the centre of the stem in Mosses. 



Of far more frequent occurrence are simple bundles consisting 

 exclusively of protein-conducting elements, in the form of sieve-tubes 

 accompanied by companion-cells and leptome-parenchyma (cambiform 

 cells). Floral axes, and other organs in which the translocation of 

 protein compounds assumes large proportions, often contain isolated 

 leptome-strands in addition to the ordinary vascular bundles. Such 

 accessory leptome-strands are found, for instance, in the perimedullary 

 region of the stem in many Campanulaceae and Cichoriaceae, and in 

 species of Solanum. In the scape of Plantago lanceolata they are 

 arranged in tangential series between the larger and smaller vascular 

 bundles. In many Cucurbitaceae they occur in the extracambial 

 parenchyma, both on the inside of the mechanical cylinder, and outside 

 the latter, between the collenchyma and the ordinary cortical paren- 

 chyma ; occasionally they may even run immediately beneath the 

 epidermis. In Cucurbita all the accessory leptome-strands are linked 

 up by cross-connections ; they serve in all probability, as A. 

 Fischer has suggested, to supply the developing mechanical tissues 



