328 STUDIES IN FOSSIL BOTANY 



connection with B. hirsuta was known. The three 

 prominent points (occasionally reduced to two) mark 

 the position of the spiral protoxylem-elements. In 

 some specimens the phloem encircling the wood is 

 exquisitely preserved. 



Where the leaf-trace joins on to the vascular cylinder 

 of the stem, its spiral tracheae are directed inwards. 

 As, however, the bundle left the stele, it swung round, 

 as it were (as often happens in recent Ferns also), 

 taking up the position shown in Fig. 121. In M. 

 Renault's type the form of the petiolar bundle is 

 essentially the same as in the English species, but the 

 structure is less simple and the three xylem-points are 

 much longer, so that he compares the sectional form of 

 the wood with that of the Greek omega (<u). 



The structure of the cortex of the stem shows no 

 striking differentation ; in the petiole the outer cortical 

 layers constitute a continuous sclerotic zone (Fig. 121). 



The stem, in all the species, gave rise to very 

 numerous diarch adventitious roots (Fig. 121, r l , r 2 ), 

 resembling those of many living Ferns. 



In the petiolar bundle the position of the protoxylem 

 is shown, not only by the presence of spiral tracheides 

 at the prominent points, but also by direct developmental 

 evidence, such as is rarely to be found in a fossil plant. 

 In several cases a petiole or rachis has fortunately been 

 preserved when still young, with its xylem only partially 

 lignified, so that the small, thick-walled elements of the 

 protoxylem, already differentiated, contrast sharply with 

 the rest of the wood, in which the walls are still 

 unthickened (see Fig. 122). 



As the leaf-trace approached the stele its three 



