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PART II. ANATOMY AND HISTOLOGY. 



[32 



cells with thin walls and protoplasmic contents; but in many 

 cases sclerenchyma is differentiated in it. 



The most important fact with regard to the parenchyma of the 

 pith is that, in many cases, the cells forming the central portion 

 of the pith soon die, or even the whole of them (e.g. Elder). When 

 this is the case, the dead cell-walls freqviently undergo disorganisa- 

 tion, so that the stem becomes hollow. 



The sclerenchyma of the pith may consist of scattered strands 



(e.g. stems of some Palms) ; or it may form a ring connecting the 



inner ends of the bundles of the hollow vascular cylinder (e.g. 



Bongainvillea spectabilis, woody Piperacese. 



The bulk of the pith varies very much. It is relatively very 



large in tuberous 

 shoots, such as 

 Potato, Apios, etc. 

 The Interfasci- 

 cular Conjunc- 

 tive Tissue con- 

 sists typically of 

 p a r e n chy matous 

 cells with thin 

 walls and proto- 

 plasmic contents ; 

 but it is fre- 

 quently scleren- 

 chymatous where 

 it abuts on the 

 vascular bundles, 

 thus contributing 

 to the formation 

 of a more or less 

 complete sheath of sclerenchyma round them (e.g. many Mono- 

 cotyledons) : in woody plants the cells of the medullary rays 

 become lignified. 



The various systems of sclerenchymatous supporting-tissue (stereom) 

 described above, the hypodermal, the cortical, the pericyclic, the inter- 

 fascicular, 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 conditions of 

 the member may render necessary (see Fig. 98). 



FIG. 98. Diagram (after Schwendener) illustrating the dis- 

 tribution of the supporting-tissue or srereom, as seen in trans- 

 verse section of stems: A of Arum moculatum having isolated 

 cortical stereom-strands ; B of Allium vineale, with continuous 

 pericyclic stereom-ring ; C of Juncws glaucus (hollow), with 

 hypodermal stereom-strands and conjunctive stereom-strands ; 

 / vascular bundles ; 8 stereom-strands ; 1 air-cavities. 



