TISSUE SYSTEMS 343 



stele. It is often found to contain sclerenchyma, either in 

 isolated patches, or in the form of sheaths surrounding the 

 vascular bundles (ficj. 728). The part of the stele within the 

 pericycle may consist of a solid vascular cylinder containing 

 no conjunctive tissue at all, or only a little dipping in from the 

 pericycle in the intervals between the bundles, as in many roots. 

 In Dicotj'ledons isolated vascular bundles sometimes occur in the 

 pith. Where a large pith exists the cells of it tend after a time 

 to break up and disappear, forming hollow stems as in the 

 TJmhrUifcrce. These hollow stems are extreme instances of 

 lysigenous formations. 



In polystelic stems each stele, in addition to its envelope of 



Fig. 729. 



..pe: 



pXi 



Fig. 729. Stele of stem of Feru, composed of three fused concentric bundles. 

 PH. Endodermis. pp. Pericycle. ph. Phloem, px. Groups of protoxylem. 



endodermis, is surrounded by a layer of pericycle. Such steles 

 are usually solid vascular tissue, and do not show intrastelar 

 ground tissue, both pith and interfascicular ground tissue being 

 absent {fig. 729). 



The Vascular Tissue System. 



• 



The Stele. — We have seen that the centre of the tissue of 

 the growing point, which has been named the plerome, develops 

 into a solid strand of complex character, forming the stele. 

 Usually this remains single throughout the axis, and the plant 

 is termed monostelic. The arrangements of its parts are 

 different in the stem and the root, but there is a single central 

 cylinder running throughout the whole axis. Separate portions. 



