THE FOSSIL OSMUNDACE^E. 225 



The Leaf-trace. 



The leaf-trace departs from the stele of the stem in a perfectly protostelic manner. 

 The protoxylem groups previously described (fig. 11) at the periphery of the xylem 

 ring are all decurrent from the leaf-traces. The first sign of preparation for the de- 

 parture of the leaf-trace is the appearance of a slight prominence on the periphery of 

 the xylem ring opposite one of the mesarch protoxylems (figs. 12 and 13, prx.). Followed 

 upwards, this develops into a broad, bluntly rectangular projection into which the 

 protoxylem group passes (fig. 13). This projection gradually increases in size and 

 eventually separates off as the xylem strand of a leaf-trace (fig. 14). While still enclosed 

 within the phloem of the stem, the protoxylem of the leaf- trace is seen to be deeply 

 mesarch, with a large amount of centripetal xylem on its adaxial side (fig. 14). Even 

 after the leaf-trace as a whole has become completely free from the stele and has advanced 

 some distance into the cortex, a considerable number of centripetal elements are still 

 present (fig. 15, a and b). 



In the close neighbourhood of the stele the transverse section of the xylem of the 

 leaf- trace is oblong in outline, while the leaf- trace itself is oval (fig. 16). As it passes 

 outwards both the xylem strand and the whole leaf-trace become curved round aclaxially. 

 This curvature gradually increases until in the outermost trace in our specimens the 

 xylem strand has attained the form of a semicircle, while the leaf-trace itself is reniform 

 (fig. 17). As the leaf- trace passes out the centripetal xylem gradually disappears, and 

 by the time that the curve of the xylem strand has become at all pronounced it has 

 entirely gone and the protoxylem is perfectly endarch (fig. 16, prx.). In a leaf-trace 

 still further out the protoxylem group is seen to have divided, and in the outermost 

 leaf- traces two or three protoxylem strands are present. No doubt the leaf-trace became 

 still more curved and the protoxylems still more numerous in the outer region of the 

 sclerotic cortex that is, however, wanting in our specimen. 



The xylem of the leaf-trace is surrounded on all sides by a parenchymatous sheath 

 two to four layers thick, and similar in appearance to the xylem sheath of the stem 

 (fig. 16. xy. sh.). This tissue is followed by a continuous and well-developed zone of 

 phloem consisting of large sieve-tubes and a few cells of phloem parenchyma. On the 

 other hand, the closest examination failed to reveal any protophloem whatever, even on 

 the abaxial side of the trace. While still in close proximity to the stele of the stem 

 the leaf-trace is limited by a zone of some three or four layers of cells exactly similar 

 in appearance to those that have been referred to as the pericycle in the stem. As the 

 leaf-trace passes outwards, the contents of the inmost cells of this zone become less and 

 less conspicuous, while those of the outermost layer become more dense and prominent. 

 In fact, this layer gradually assumes the appearance of an endodermis (fig. 16, en.). 



The Root. 

 The roots arise singly or in pairs upon the stem stele at points immediately below 

 the departure of certain of the leaf-traces. Many traces, however, are not provided 



