170 PART II. THE INTIMATE STRUCTURE OF PLANTS. [ 33. 



The Interfascicular Conjunctive Tissue consists typically of 

 parenchymatous cells with thin walls and protoplasmic contents ; 

 but it is frequently sclerenchyraatous where it abuts on the 

 vascular bundles, thus contributing to the formation of a more or 

 less complete sheath of sclerenchyma round them (e.<j. many 

 Monocotyledons). 



The various systems of sclerenchymatous supporting-tissue (stereom) 

 described above, the hypodermal, the cortical, the pericyclic, the interfascicular, 

 and the medullary, may be connected with each other in various combinations. 

 Thus, the hypodermal and the cortical systems may be continuous ; or the 

 hypodermal, cortical, and pericyclic; the pericyclic and the interfascicular, 

 etc. ; as the mechanical couditions of the member may render necessary (see 

 Fig. 124). 



33. The Vascular Tissue-System. The primary vascular 

 tissue is differentiated from the procambium or primary desmogen 

 (see p. 151) of the stele in the form of strands or bundles, vascular 

 bundles. The vascular tissues of the bundles are either tracheal 

 tissue (p. 133), which is always lignified, and is termed wood or 

 xylem ; or sieve-tissue (p. 135), which is termed bast or phloem. 

 A vascular bundle may consist, either exclusively of wood or of bast ; 

 or of both wood and bast, when it is said to be a conjoint bundle. 

 It is generally the case that a varying proportion of sclerenchyma 

 (stereom) is associated with the vascular tissue ; hence the bundles 

 are frequently spoken of as fibro-vasLular bundles. 



As a rule, an equal number of wood-bundles and of bast-bundles 

 are differentiated in a stele, whether they be isolated or conjoined ; 

 there may be only one of each (e.g. finer branches of the dicho- 

 tomous roots of most Lycopodiums) or there may be a very 

 considerable number (e.g. stems of Monocotyledons). There are, 

 however, some exceptions to this rule : thus, the stele of the root 

 of some Lycopodiums (L. inundatum and Selago) aud Ophio- 

 glossums (0. vulgatum and lusitanicum) contains one bast-bundle 

 and two wood-bundles ; again, in the roots of Selaginella and 

 Isoetes, the stele encloses one wood-bundle and two bast-bundles. 



The primary vascular bundles differentiated in a stele (whether 

 a monostele or a polystele) are frequently distinct from each other 

 for a considerable distance, for instance, throughout an internode 

 of a stem ; but very commonly they are found to fuse at shorter 

 intervals {e.g. stem of Lycopodium), or they may be altogether 

 fused into one or more vascular masses ; a stele presenting this 

 intimate connexion of the bundles is said to be yamodesmic. An 



