716 MR W. T. GORDON ON 



H -shaped by the two arms on each side leaving the horizontal bar at 90° (PL IV. fig. 42). 

 At intervals pinnae depart from these petioles, and, except in a few details, the 

 departure of the pinna-traces from the petiole-trace is exactly as Renault has stated. 

 His material was not complete enough, however, to show these details. In the Pettycur 

 specimens I find that, shortly after the departing pinna-trace-bar # (PI. IV. fig. 42 b) 

 has left the petiole-trace (and while still in the cortex of the petiole), it divides up into 

 several traces. The division is probably into two and then into four. Of these four 

 traces, the two extreme are smaller than the two mean. The large median strands 

 soon become C-shaped and pass out into the pinnae. PL IV. fig. 42 shows the xylem- 

 bar departing from the petiole-trace, while PL IV. figs. 43, 44, 45, 47, and 48, pin. tr. 

 show the pinna-trace at different levels. 



The smaller traces on the extremes of the pair of pinna-traces become gutter-shaped 

 or even rounded, and no doubt supplied aphlebiae. They divide into two in passing 

 through the cortex of the petiole, thus producing four traces which leave the petiole in 

 pairs, one of each pair being derived from each of the original aphlebia-traces. These 

 aphlebia-traces do not enter the pinnae, but emerge at their bases. No such aphlebia- 

 traces appear at the base of the petioles, but they are found at every normal pinna 

 departure (PL IV. figs. 43, 44, and 45, aph. tr.). Similar bundles have been recorded by 

 Williamson (15) as departing at a similar level in the petiole of Metaclepsydropsis 

 duplex. He called them "ternary bundles," i.e. secondary pinnae. They are, however, 

 specialised pinnae, and, by their cortex remaining fixed to that of the petiole and of the 

 pinna for a considerable distance, they would serve both to support the heavy pinnae to 

 some extent, and to prevent these latter from snapping away from the petiole if the 

 plant were subjected to any strain. 



As the two pinna-traces pass slowly out through the cortex of the petiole they diverge, 

 and so, when the pinnae themselves are free from the petiole, there is an angle of more 

 than 90° between them. This angle increases as they pass out. At the same time, the 

 angle between the plane containing the two pinnae and the vertical plane t of the petiole 

 is a small one. The open side of the pinna- trace always faces the petiole ; and the 

 whole array of secondary pinnae belonging to each primary pinna, i.e. each " spread " of 

 secondary pinnae, lies in a plane facing obliquely upwards towards the main rachis. 



The pair of pinnae immediately succeeding are on the opposite side of the petiole ; 

 the next pair are inserted vertically above the first pair, and so on. There are thus 

 four orthostichies of primary pinnae, and therefore of secondary pinna-spreads, but the 

 small angle between the plane of the two departing pinnae and the vertical plane of the 

 petiole, coupled with the large angle between the pair of pinnae, causes the whole frond 

 to assume a more or less bilateral character (text-fig. 2). It is not bilateral in the 

 sense that all the primary pinnae face in one direction, for those on one side of the 



* It is convenient to have a special name for the arc of xylem formed by the fusion of the two entering pinna- 

 traces. The pinna-trace-bar (the name explains itself) has been used for this xylem arc. 

 t See A. G. Tansley (14). 



