434 STUDIES IN FOSSIL BOTANY 



As the leaf-trace passes upwards and outwards through 

 the cortex, it loses its secondary tissues, and divides up 

 repeatedly (Fig. 165, Lf) to form a number of smaller 

 bundles, each of which eventually assumes a collateral 

 structure. These ultimate leaf-trace bundles have their 

 protoxylem directed outwards, adjoining the phloem 

 (see Fig. 166, in which two exceptionally well-preserved 



Fig. 166. — Medultesa auglica. Two leaf-trace bundles from the outer cortex of the stem, 

 in transverse section, showing collateral exarch structure. j4.r, protoxylem ; x, 

 centripetal xylem ; //;, phloem. X 35. Phil. Trans. S. Fromja photograph. S. 

 Coll. 579. 



collateral bundles are shown) ; their xylem usually 

 consists of spiral and finely scalariform elements only, 

 the pitted tracheides, which are abundant in the un- 

 divided leaf-traces, disappearing as the ramification of 

 the bundle goes on. 



The cortical tissues of the stem require no detailed 

 description ; they consist of short-celled parenchyma, 

 traversed by numerous secretory canals, much resembling 

 the gum-canals of recent Cycads. 



