Xylems in the Petioles of Cycads. 185 
give rise to other bundles which go to take their relative places in the 
general disposition of the foliar strands. Their forms vary considerably: 
they may be simple and collateral (Fig. 2), or concentric 1 with parenchyma 
in the centre and medullary rays running radially all round, thus reproducing 
the structure of the stem or even of some peduncles (Fig. 3). A bundle 
may also be a compound structure, combining more or less completely the 
characters of the two first types mentioned (Fig. 1). Intermediate types, 
as one would expect from such a variety of structures, are not infrequent. 
But in this variety of forms there is a distribution of elements which 
remains constant at the base of the petiole. The xylem is entirely endarch 
or centrifugal (Figs. 1-3). An internal protoxylem (px) is succeeded by 
numerous rows of lignified cells continued by cambium and phloem cells, 
with medullary rays running across the xylem, cambium, and phloem 
layers. This structure persists, say, for a distance of 2 cm. from the base of 
the petiole, and then gradually changes in the course of the next 3-4 cm. 
The cells in connexion with the protoxylem are no longer strictly centri- 
fugal, but seem to run in a lateral direction. Only the central rows remain 
properly centrifugal (normal to the pith), undergoing, however, a process of 
reduction (Fig. 4). 
This reduction is more and more apparent as we proceed up the 
petiole ; the lateral rows pass behind the protoxylem, new independent 
groups of scattered lignified cells appear, and the centripetal xylem is initiated 
(Figs. 4 and 9). 
At a higher level the centripetal xylem is more abundant than the 
centrifugal ; the gap w T hich we observed behind, between the two lateral seg- 
ments, is entirely closed ; a complete ring is formed which soon breaks up on 
both sides (Figs. 7 and 8). There remains only a few centrifugal cells scattered 
in small groups and without any visible connexion with the centripetal xylem, 
except through a parenchymatous tissue of thin-walled cells in which it is 
embedded (Fig. 10). This is the ultimate form of structure met with 
practically throughout the petiole, apart from the 5 cm. or 6 cm. we have 
described. A last stage may be reached in which no trace of centrifugal 
xylem is left. This, however, is rare, and I have met with it only in the 
smaller bundles of the leaflets of Encephalartos horridus. 
The above is a general survey of the natural and normal distribution of 
xylem elements in the petioles of Cycads at an adult stage. 
Wound structures . — Other structures may be artificially induced. A 
young petiole of Cycas circinalis , 15 cm. long, was wounded at the end of 
April ; one of the wounds was transverse and not far from the base, the 
other wound was longitudinal and higher in the petiole. The leaf still grew 
well, though less luxuriantly than its companions of the same age. 
On July 12, when it was about 1 metre long, it was cut and examined. 
1 See also H. Matte (’07), PI. I, Fig. 7 . 
