466 COENOPTEEIDEAE [CH. 



xylem elements occur close to the surface of each of the four 

 arms of the xylem ; the bays between the two lateral and the 

 two lower xylem groups contain large sieve-tubes. Portions of 

 the inner cortex are seen in places abutting on the small-celled 

 pericyclic tissue. 



The right and left halves of the stele are not absolutely 

 identical (fig. 320; fig. 308, E); this is due to the fact that 

 secondary branches are given off in four rows, two alternately 

 from the right and left sides. The preparation for the departure 

 of the lateral strands alters the configuration of the stelar xylem 

 groups. The protoxylem groups are not external but separated 

 from the surface by one or two layers of metaxylem. In fig. 308, E, 

 the occurrence of two protoxylem strands in the right-hand 

 groups of metaxylem marks an early stage in the detachment 

 of branches. These two protoxylems are the result of division 

 of single protoxylem strands like those in the left-hand half of 

 the stele. At a later stage the petiolar stele assumes the form 

 shown in fig. 308 F, and two small bundles are detached to supply 

 aphlebiae : this is followed by the stage shown in fig. G, where 

 two four-armed strands are passing out to a pair of branches of 

 the leaf axis. The separation of these two meristeles leaves 

 the right-hand half of the stele in the condition seen on the 

 left-hand side of fig. E. The diagrammatic sketch represented 

 in fig. 310, C, shows one pair of branches in organic connexion 

 with the rachis, and each of these arms contains an obliquely 

 cut vascular strand like those in fig. 308 G. 



The cortex consists for the most part of fairly thick-walled 

 parenchyma (fig. 321) which in the hypodermal region is replaced 

 by a zone of thin-walled lacunar tissue. A few stomata have 

 been recognised in the epidermis^. The lower left-hand branch 

 seen in fig. 310, C, has been shaved by the cutting wheel so 

 that the aerenchymatous tissue, I, is shown in surface-view : a 

 portion of this tissue is enlarged in fig. C. The same delicate 

 chlorophyllous tissue forms a folded and shrivelled layer with 

 an uneven margin on the surface of the rachis and lateral 

 branchest This hypha-like tissue, which was discovered by 



1 Bertrand, P. (09) PI. vii. fig. 48. 



