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PART II. ANATOMY AND HISTOLOGY. 



[33 



bicollateral bundle is, in fact, a structure intermediate between 

 the collateral and the concentric bundle. The former type of con- 

 centric bundle occurs in the rhizomes of various Monocotyledons 

 (Acorus, Iris, Cyperus, Carex, etc.), and in the medullary bundles 

 of the stem of some Dicotyledons (Rheum, Statice, Ricinus, Piper, 

 etc.). The latter type is rare in Phanerogams (e.g. the cortical 



FIG. 100. Transverse section of an open, collateral, conjoint, vascular bundle of the 

 stem of Ranunculus repent : s spiral vessel of the protoxylem at the inner (central) end of 

 the wood j m pitted vessel of the wood ; c cambium ; v a sieve-tube of the bast with 

 adjacent granular companion-cells; vg sheath of sclerenchymatous conjunctive tissue. 

 (After Strasburger: x 180.) 



and medullary bundles of the Melastomacese) ; but it prevails in 

 the Filicinse and in Selaginella, when the bundles (two_or jnore) of 

 each stele of the polystelic stem, form a central mass of wood 

 completely, or nearly completely, surrounded by a ring of bast. 



