XIX] 



ANATOMY 



59 



the protostele has a solid xylem: a pith arises higher up by development 

 of parenchyma in its centre. Dififerent plants or regions exhibit successive 

 stages in medullation: first there is an inner xylem with scattered paren- 

 chyma, then a mixed pith with parenchyma predominating centrally, and 

 finally a small pith free from tracheides. Thus the pith is of intra-stelar 

 origin. The stele is dilated upwards till in the adult stem it appears as a 

 solenoxylic ring, bounded externally by endodermis and phloem. Finally 

 in large rhizomes a second endodermis lines the xylem internally (Fig. 351). 

 Lang has greatly extended the interpretation of this adult structure. The 

 stele can be followed to the apex of the stem be>'ond the \'oungest leaves 



Fig. 351- Helminthoslachys zcylanica. Transverse section of 

 part of the stele and cortex of the rhizome : O.E., outer endo- 

 dermis; I.E., inner endodermis; ^.7"., sieve-tubes. (After 

 Farmer and F"reeman.) 



and roots, and hence it is actually cauline. It gives off sectors of itself as 

 leaf-traces obliquely right and left from its upper side (Fig. 352), and the 

 upper region is thus common to stem and leaf But the lower ventral 

 portion does not give off leaf-traces at all, and it is cauline. The xylem-ring 

 is mesarch: it is distinguished as consisting of an outer and an inner ring 

 with the protoxylem elements between them. The leaf-trace which is always 

 undivided shows variet)- in its mode of departure. In large rhizomes it passes 

 off as a mesarch portion of the stelar tube (Fig. 352), but the inner xylem 

 is scanty, and dies out as the trace passes outwards. In smaller rhizomes 



