SECT. I 



MORPHOLOGY 



123 



(Figs. 131, 132), and in the lianunciilaceae among the Dicotyledons 

 (Fig. 133), the phloem consists solely of sieve-tubes and companion 

 cells ; in the other Dicotyledons parenchymatous elements are also 

 present, and these are accordingly distinguished as phloem 

 PARENCHYMA ; no companion cells are found in Gymnosperms and 



Pig. 133. — Transverse section of a vascular bundle from a stolon of Banmieulus repms. s, Spiral 

 tracheides ; m, vessel with bordered pits ; c, cambium ; v, sieve-tubes ; vg, sheath. ( x ISO.) 



Pteridophytes, and in addition to sieve-tubes the phloem contains 

 only phloem parenchyma. 



The bundles of the Phanerogams (Gymnosperms and Angiosperms) 

 are generally collateral in structure, that is, the xylem and phloem 

 are in contact on one side only. In stems the most usual arrange- 

 ment of the two portions of a collateral bundle is that in which the 

 xylem lies nearer the centre ; in leaves the xylem portion lies nearer 

 the upper, and the phloem portion nearer the lower surface. Closely 

 allied to the collateral type is the bicollateral type of bundle. In 



