THE FIBRO-VASCULAR SYSTEM. 



107 



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 Pibro- 

 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- 

 bre- vascul ar 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 c 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 intei-ior {g, g, g. Fig. 96). Enclosed in 

 the scalariform tissue are masses of parenchyma and a few 



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

 tural units of tbe bigber plants a serious mistake bas been made,, 

 leading to vrofitless discussions and speculations as to its typical struo. 

 ture, and diverting attention from tbe study of its actual structura 



, — Part of a fransvei'se section of the fibro-vas- 

 cular bundle of the ptem of PleHs aquUina ; s, spiral ves- 

 sel ; (7,(7, scalariform vech-els : ^y?, sieve tissue; &, fibrous 

 tissue (protophloeni of Eussow) ; sg, bundle sheath; p^ 

 starch-bearing parenchyma: K, A", thickeneii angles or 

 scalariform vessels. — After Sachs. 



