CHAPTER III. — TISSUE SYSTEMS. 



!75 



The cells composing it, therefore, for the most part, have 

 thickened walls, and they are elongated in the direction of the 

 length of the organ which bears them. They belong to the 

 prosenchymatous series chiefly, although other tissues are 

 commonly included. 



In some plants, as in the stem of Indian Corn and the petioles 

 of the Plantain, the fibro-vascular bundles may be readily 

 separated in the form of tough, stringy masses from the softer 

 surrounding tissues. 



d...-~. 



Fig. 435. Fig. 436. 



Fig. 435. — Radial fibro-vascular bundle from root of Buttercup, a, two of the four 

 xylem rays ; b, two of the four phloem masses : c, endodermis or bundle-sheath ; d, pericam- 

 bium. Magnified about 100 diameters. After De Bary. 



Fig. 43C — Diagram of open collateral bundle, x, xylem, mainly composed of ducts 

 and wood cells; p. the phloem, containing bast-fibers and sieve-cells; c. cambium layer, 

 composed of small, thin-walled and imperfectly developed ceils; r, medullary ray; m, por- 

 tion of medulla or pith. 



Although most of the different kinds of tissues described in 

 the last chapter may be found in a fibro-vascular bundle, only 

 two of them are really essential, ducts (or tracheids, which may 

 be regarded as imperfectly formed ducts) and sieve -cells. These 

 and their associated tissues always constitute separate longitu- 

 dinal portions of the bundle. The portion to which the ducts 

 belong is called the xylem, and that to which the sieve-cells 

 belong, the phloem. Sometimes the bundle is sharply marked 

 off from the surrounding tissues by a sheath, or endodermis, com- 

 posed of a single row of cells different both from those exterior 

 and from those interior to it, Fig. 435, c. In many cases, how- 



