178 



PART II. THE INTIMATE STRUCTURE OF PLANTS. 



[ 33. 



The protophloem is in all cases external : and though the proto- 

 xylem is also generally external, it is sometimes internal (as in 

 the bundles in the petiole of Cycads, in the stem of Isoetes, and in 

 the concentric bundles of stems and petioles of many Ferns), being 

 more or less surrounded by the rest of the primary xylem. 



FIG. 137. Transverse section of a conjoint, collateral, closed, vascular bundleof the stem 

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

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

 matous ; I lyeigenous intercellular space ; s r spiral and annular vessels constituting 

 the protoxyhm; 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: x563.) 



In members, whether monostelic or polystelic, in which the pri- 

 mary bundles or the steles are arranged in one or more circles (or* 



