Davallias and the Petiole of Lygodium . 
497 
Lygodium . 
LYGODIUM SCANDENS. The petiolar bundle has the well-known 
peculiar form in transverse section (Text-figure i,b)j the xylem is roughly 
triangular, the adaxial side being slightly concave. The abaxial median 
protoxylem is single in young petioles, but in other cases it is apparently 
double ; the lateral protoxylem groups are some distance in front of the 
lateral prominences* The abaxial protoxylem is apparently exarch, though 
some evidence was obtained of a mesarch position in young petioles. 'I he 
protoxylem elements are annular and spiral, and are associated with 
distinct cavity parenchyma. At the base of the petiole the median proto- 
xylem is not double, and the protoxylem is not crushed and consists of 
Text-fig. i. Lygodium scandens. a. Habit sketch, B. Diagrammatic transverse section 
of petiolar trace. C 1-6 . Primary branching of the petiole. D. Branch trace in secondary or tertiary 
branches. In B and D, and in some of the following text-figures, the protophloem is indicated by 
a line or dotted line, the sieve-tubes of the metaphloem by small circles, and the incipient phloem- 
fibres or small thick-walled sieve-tubes by f. 
small elements which are probably scalariform. It is thus questionable 
whether there is true protoxylem in this region* Protophloem and phloem 
are present all round the xylem. The chief development of large sieve- 
tubes of the metaphloem is in the lateral and adaxial bays. Near the 
dorsal and lateral prominences of the xylem the small sieve-tubes have 
thick, soft-looking walls and may be regarded as incipient phloem fibres, 
the walls of which, however, are never lignified . 1 
In the origin of a primary branch of the petiole one of the lateral proto- 
xylem groups elongates (Text-fig. I, C 1 ) and divides into two (Text- 
fig. i, C 2 ). The outer protoxylem is carried out as the protuberance of the 
1 Cf. Boodle, 1 . c., p. 368. 
