33] CHAPTER II. THE TISSUES. 127 



said to be closed. In the stems of most G-ymnosperms and Dicgty- " ' 

 ledons, on the other hand, the whole of the procambium is not 

 converted into the primary wood and bast of the collateral conjoint 

 bundle, but a portion of it persists as an embryonic merismatic 

 tissue, the cambium, forming a transverse zone- Jbetween the wood 



Fio. 103. Transverse section of a conjoint, collateral, closed, vascular bundle of the stem 

 of a Monocotyledon (Zea Mais) : a outer or peripheral end of the bundle ; i inner or central 

 end ; p conjunctive tissue, the portion immediately investing the bundle being sclerenchy- 

 matous ; I lysigemus intercellular space ; g r spiral and annular vessels constituting 

 the protoxylem ; g g large pitted vessels, between which lie the smaller pitted vessels of 

 the wood ; v v v sieve-tubes of the bast with intervening companion-cells ; just outside the 

 bast, and within the sclerenchymatous sheath, the remains of the protophloem are visible. 

 (After Sachs : x 553.; 



on, the inner (central) side and the bast on the outer side (see 

 Figs. 97, 105). Such a bundle is said to be open. 



