THE FIBBO-VAaCULAB. SYSTEM. 



10? 



sues, any of which may be wanting in, or separated a little 

 space from, the bundle. In short, the elementary tissues, 

 particularly tracheary, sieve, fibrous, and parenchymatous 

 tissues, are to be considered as the units, and the term Fibro- 

 vascular Bundle as little more than a convenient expression 

 of the usual condition of aggregation of these units.* 



The general structure of fibro-vascular bundles will be 

 more readily un- 

 derstood after 

 the examination 

 of a number of 

 examples. Those 

 which follow are 

 not in any sense 

 typical ; they are 

 only illustrative. 



136.— The fi- 

 bro-vascular bun- 

 dle of the stem of 

 Pteris aquilina 

 is composed of 

 tracheary and 

 sieve tissues, par- 

 enchyma, and a 

 small amount of 

 poorly developed 

 fibrous tissue. In 

 transverse s e c - 

 tion the bundle 

 has usually an 

 elliptical outline. 

 The great mass 

 of the bundle is made up of large scalariform vessels, 

 which occupy its interior {(J,g,g, Fig. 96). Enclosed in 

 the scalariform tissue are masses of parenchyma and a few 



* By considering the Fibro-vascular Bundle to be one of tbe struc- 

 tural units of the higher plants a serious mistake has been made, 

 leading to profitless discussions and speculations as to its typical struc- 

 ture, and diverting attention from the study of its actual structure, 



Fig. 96. — Part of a transverse section of the fibro-vas- 

 cular bundle of tbe stem of Pteris oquUina ; s, spiral ves- 

 sel; j7, !7, scalariform veppels ; ^/5, sieve tissue; o, fibrous 

 tissue (protophloSm of Eussovp) ; sg. bundle sheath; p, 

 starch-bearing parenchyma : JT, K^ thickened angles or 

 scalariform vessels. — After Suchs. 



