Knowledge of Rachiopteris cylindrica, Will. 545 
formation. In /3 stems the branches are typically equal in size (PI. XXVI, 
Figs. 8 and 9 ; Text-fig. 8). 
2. Petioles. 
i. Development .—The production of petioles has been described by 
Hick as a process of unequal division of the stem. 1 The lowermost stages 
of their development are exactly similar to those seen in the formation of 
stem branches; the division of the single, or one of the several, protoxylem 
Text-fig. 9. Diagrams of leaf-trace production in the smaller branch of the a stem shown 
in Text-fig. 7. In K20A indications of root-production are seen at r; K207 and K2ok indicate 
the appearance of separating cells immediately in front of the protoxylem of the leaf-trace; K 20 m 
shows the position of the fully-formed petiolar bundle as compared with that of the trace in K 20 /. 
Note the distribution of the protoxylem along the anterior surface of the leaf-trace, and the absence 
of adaxial metaxylem. x 35. (From series K 20, University College, London.) 
groups (Text-figs. 8 and 9) provides the initial petiolar strand, which in 
passing upwards becomes separated from the rest by the formation of 
metaxylem elements (Text-figs. 8 and 9). At a slightly higher level in the 
stem, thin-walled parenchymatous cells may be observed in front of the 
petiolar protoxylem (Text-fig. 9 ; PI. XXVI, Fig. 6) ; higher still, the lateral 
extension of these cells completely separates the semi-cylindrical mass of 
foliar xylem from the stem stele. Always in (3 types, and in some a types 
also, the protoxylem occurs on the adaxial margin of the petiolar strand 
(Text-figs. 8 and 9). In other a types the protoxylem, just above the level 
' of trace-separation, is slightly immersed, owing to the occurrence of some 
1 1 -c., pp. 9 " 11 - 
