762 MR R, KIDSTON AND MR D. T. GWYNNE-VAUGHAN ON 



The Base of the Petiole. 



Once the leaf-trace has passed into the petiole it increases rapidly in size and becomes 

 much more curved, taking successively the form of a crescent, a semicircle (PI. III., fig. 14), 

 an arch, and finally of a horse-shoe with deeply incurved ends. The median protoxylem 

 group becomes broader and more prominent as it passes out. Finally it divides into 

 two, and then into several separate groups (PI. III., fig. 15), twenty or more being 

 present in the outermost leaf-bases. The phloem is nowhere preserved, and only in a 

 few of the outer leaf-bases are any indications to be found of the thin-walled cells that 

 occupied the space between the leaf-trace and the sclerotic ring of the petiole (PI. III., 

 fig. 14). A number of isolated strands of sclerenchyma occur in this tissue, scattered 

 on all sides of the leaf-trace and also in its concavity (PI. VI., fig. 3). They are very 

 irregular in size and form, but two of them are very much larger than the rest and are 

 constant in position. They lie in the two bays formed by the incurved ends of the 

 leaf-trace, and in immediate contact with it. These strands are increasingly conspicuous 

 in the outer leaf- bases (PI. I., fig. 2, scl.). 



In the actual living plant the sclerotic ring of each leaf-base was surrounded by 

 thin-walled tissue, which was prolonged on either side of the petiole so as to form two 

 stout and wide stipular wings. This tissue does not exist as such in the fossil, but 

 is represented by the matrix that fills up the spaces between the sclerotic rings of the 

 petioles. The stipules also contained a very large number of thick-walled fibrous 

 elements, which were embedded in this parenchyma, and these still remain in situ. They 

 occur for the most part as isolated fibres, but they are also grouped together to form so 

 many strands of various shapes and sizes, which are scattered irregularly throughout the 

 substance of the stipule (PI. III., fig. 16). A general idea of the distribution of all this 

 sclerenchyma may be obtained from PL VI., fig. 3, which is a diagrammatic 

 reconstruction of a section of a leaf-base situated at some distance from the stem. 

 Towards below the leaf-bases all appear to become concrescent by their parenchymatous 

 stipules. The sclerotic strands of the stipules are already present even in this region, 

 but those lying within the sclerotic ring of the petiole do not appear until further out. 



The smaller leaf-bases that are supposed to represent scale-leaves do not differ from 

 the rest in structure, except that the xylem of the leaf-trace is very poorly developed. 

 It would seem, indeed, that only a very few tracheides in immediate contact with the 

 protoxylem elements ever become fully differentiated, the rest of the metaxylem 

 remaining permanently thin-walled. This actually proved to be the case in the scale- 

 leaves of Osmunda Claytoniana and (). cinnamomea. A full-sized leaf-trace is laid 

 down by the meristem of the leaf-rudiment, but their protoxylem elements alone appear 

 to be sufficient to meet the diminished water-supply needed by the abortive leaf through- 

 out its life, and except at the very base of the scale the metaxylem elements remain 

 permanently unthickened and unlignified. This metaxylem, therefore, provides an 

 unusual and instructive example of an undoubted vestigial tissue. Further, it suggests 



