4(50 



R. KTDSTON AND D. T. GAVYNNE-VAUGHAN ON 



in the xylcni ring of the stem, and the modern Osmundas, where the ring is broken 

 through immediately the xylem of the leaf departs. 



The Structure of the Leaf-trace and Petiole. 



As seen in transverse section, the xylem of the leaf-trace departs from the stele of 

 the stem as a stout and slightly curved band of almost even width. At its point of 

 departure it possesses a single median protoxylem strand which is distinctly mesarch 

 on the adaxial side of the petiole (figs. 15 and 16). Often, indeed, there is a very con- 

 siderable amount of centripetal xylem to be seen, which does not entirely disappear 

 until after the trace has entered the sclerotic cortex. Just as the sclerotic ring of the 

 petiole separates off from the sclerotic cortex of the stem the protoxylem strand 

 divides into two, and usually at the same time becomes truly endarch. In some cases, 



Fig. 4. — Osmundites Kolbci. Diagram of stipular base of petiole in the outermost region of our material. 



however, a few centripetal elements are still present even so far out as this (figs. 17 

 and 18). In the stipular base of the petiole the leaf- trace becomes more and more 

 elongated, and its ends curl inwards until it attains the customary curved form 

 characteristic of Osmundaceous petioles. In some of these outer traces the softer 

 tissues of the meristele form a distinct zone surrounding the xylem (fig. 19, a). They 

 are delimited by a layer of cells with dark contents (fig. 19, b), probably representing 

 the endodermis, but the whole is too imperfectly preserved for further detail. 



The stipular wings of the leaf-bases are all concrescent in the neighbourhood of the 

 stem ; but further out the outlines of the separate stipules, although still closely 

 ad pressed, become quite distinct, their limits being defined by dark lines (fig. 20). The 

 stipular wings are very short and stout, and in most cases consist entirely of parenchyma 

 without any sclerotic strands. The absence of such strands may be due to the fact 

 that our sections did not reach far enough up the petiole. Indeed, some of the outer- 

 most appear to have a few sclerotic strands arranged as in text fig. 4. The sclerotic 

 ring consists of fibrous elements and is homogeneous throughout. 



