STliUCTlJliE OF rilE MONOCOTYLEDONOUS STEM 149 



111 sonic stems the i)ith or medulla (lisii])])eiirs more or less eom- 

 ])letely after a time, lea\'iiif^ a eylindrieal hollow cavity. This may be 

 eontinuoiis throiif^h the nodes orse[)arated at those jjoiiits by transverse 

 ])artitions. 



Structure of the Monocotyledonous Stem. — In monoeotyledons (Fig. 

 121) we have the Closed bundle, in which the one element surrounds 

 and encloses the other. In all medicinal monocotyledonous stems 

 possessing such bundles, it is the xyleni wiiich encl()S(>s the ])hloem. If 

 the two cylinders thus formed have a connnon center, which form is 

 not very common, it is called a Concentric bundle. It is clear that in 

 the last two forms a cambium cylinder, such as distinguishes the stele, 

 possessing the form i)re\i()usl>' considered, cannot be formed. In such 



-a 

 ■6 



4H. 



l''if^. 424. TninHvorMo .si'i-linri of inonucut^lt^donouH wtrm: a, cloHcd buiullcy ncatl.ciod through piiren- 

 oh>'iiin; /i, iiiicknis Hhi^ath, or I'luloduriiiiH. 



plants indefinite growth in thickness of the bundles obviously cannot 

 occur, and the same is true of the entire stele, unless new bundles shall 

 dexelop in it. Usually this does not occur, but if the upper portion of 

 tlu> i)lant shall branch and continue to extend its leafy surface, meristem 

 tissue will then fornr towiird the outer portion of the stele, and from 

 this new bundles will suceessixcly arise, so that the thickness of the 

 trunk will kee]) ])ace with the (>xtension of the crown, notwithstanding 

 that the indixidual biuidles do not increase in thickness after the com- 

 ])letion of their primary structure. In stems ])ossessing this form of 

 bundles the hitter (Fig. 124, «.) are found more or less scattered through 

 the fundamental or medullary tissue, though there is commonly more 

 or less of a coneeutration of thcin in some one region, usually toward 

 the perii)hery of the stele. 



