STRUCTURE OF THE MOXOCOTYLEDOXOUS STEM 119 



Til sonic stems the ])illi oi' incdull.-i (lis;ii)|)c;irs more or less eom- 

 ])Ietely after a time, lea\in<j; a eyliiidrieal liollow cavity. This may he 

 eoiitiiiiious throii,y;h the nodes or sejjarated at those i)oiiits hy transverse 

 partitions. 



Structure of the Monocotyledonous Stem. In monoeolyledons (Fij;. 

 4lM) \\(> ha\-e the Closed hnndle, in which the one element surrounds 

 and encloses the other. In all medicinal monocotyledonous stems 

 possessing such hundles, it is the xylem which encloses the phloem. If 

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

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

 the last two forms a caml)inm cylinder, such as distinguishes the stele, 

 possessing the form i)revi()usly considered, cannot be formed. In such 



Fig. 424. Transverse section of monocotyledonous stem: a, closed hundles scattered through paren- 

 chyma; 6, nucleus sheath, or cndodermis. 



plants indefinite growth in thickness of the hundles ohviously cannot 

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

 develo]) in it. Usually this does not occur, hut if the ui)i)er portion of 

 the plant shall l)ranch and continue to extend its leafy surface, meristem 

 tissue will then form toward the outer i)ortion of the stele, and from 

 this new hundles will successively arise, so that the thickness of the 

 trunk will kee]) ])aee with the extension of the crown, notwithstanding 

 that the individual hundles do not increase in thickness after the com- 

 pletion of their primary structure. In stems i)ossessing this form of 

 hundles the latter (Fig. 424, a) are found more or less scattered through 

 the fundamental or medullary tissue, though there is commonly more 

 or less of a concentration of them in some one region, usually toward 

 the i)eriphery of the stele. 



