104 



BOTANY 



nearer the centre, and the wider nearer the circumference, in roots 

 this order is exactly reversed. Closely allied to the collateral type is 

 the bicollateral type of bundle. In the latter the xylem is accom- 

 panied by phloem on each side, both on the outside and inside. Such 

 bicollateral bundles are characteristic of the Cucurbitaceae, but occur 

 also in several other families of Dicotyledons. 



The vascular bundle strands of the Pteridophytes (Fig. 121), 



Fig. 119. — Transverse section of a vascular bundle from a stolon of Ranunculus repens. s, Spiral 

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



although usually termed concentric bundles, cannot be compared 

 with the single vascular bundles of the Phanerogams, but correspond 

 rather to an aggregated complex of such bundles. The centre of such 

 a vascular bundle strand consists of tracheids (sp), and also, in special 

 instances, of trachea? (sc). These elements show typical scalariform 

 markings, and only the very smallest are spirally thickened (sp). The 

 xylem parenchyma (Ip) surrounds the tracheal elements, while both 



