THE ANATOMY AND AFFINITY OF PLATYZOMA MICROPHYLLUM. 647 



The typical filiform leaf is now to be described. At its point of insertion upon 

 the axis it is compressed laterally and its epidermis is thickened (fig. 18). The 

 cortex is composed of mucilaginous cells which form a bulky water-storage tissue 

 towards the lower side. The leaf-trace is small and compressed, and has altered 

 very little from the condition which it showed on its way through the cortex of the 

 rhizome. A slight indication is already present of the formation of the flanges which 

 are so conspicuous in the basal region. Fig. 19 represents a section at a slightly 

 higher level. The trace has become slightly more bulky, and the xylem-strap has 

 become thinner and broader. The flanges are clearly established, and the lateral 

 grooves are apparent. In fig. 20 a typical section of the basal region is depicted. 

 The flanges are well developed, and the cortex is lacunar. The trace has grown con- 

 siderably, and, while the xylem-strap has extended, no very marked differences can 

 be recorded between the elements of its middle and marginal portions. The passage 

 from the basal to the middle region is characterised by the possession of two pairs 

 of grooves — one pair on the upper surface, and the other on the lower surface. The 

 grooves on the lower surface of this middle region are the continuations of those 

 grooves which exist in the basal region, and which have passed to the lower surface 

 at the junction of the basal and middle leaf-regions. The grooves on the upper 

 surface are new grooves which have commenced at the base of the middle region. 

 A typical section of the lower half of the middle leaf-region is shown in fig. 21. 

 The epidermis is strongly cuticularised, but not thickened as is the epidermis of the 

 basal region. Stomata are occasionally found in the grooves on the lower surface. 

 The xylem of the trace is of particular interest. It has developed into a broad strap 

 with long, thin, curved margins. The margins are composed of small tracheides, 

 while the middle is a zone of large, thick-walled tracheides. It will be apparent that 

 in this region the trace thus resembles the trace of the simply-pinnate leaf in the 

 rachial portion particularly. But this condition of the xylem-strap is not always 

 found. In a number of filiform leaves the condition represented in fig. 22 is found. 

 It is clear that the margin of the xylem is not characterised by small tracheides, 

 and, in fact, short storage tracheides are irregularly distributed in both the middle 

 and marginal portions. 



Passing forward in this middle region the grooves on the upper surface deepen, 



and the xylem of the trace undergoes remarkable modifications. As is shown in 



fig. 23, the middle zone of the xylem remains typically large-celled, but the margins 



have become extended until they have met to form a practically closed tube of xylem 



around a core of parenchyma. Here again there are variations, for in several leaves 



the condition found at this region is as shown in fig. 24. The xylem is more 



massive than in the former case, the parenchymatous core is almost obliterated, and 



there is not so marked a difference in size between the tracheides of the middle zone 



of xylem and those of the revolute and now fused margins. 



Sections through the tips of the terminal lobes of two filiform leaves are repre- 

 TRAXS. ROY. SOC. EDIN., VOL. LI, PART III (NO. 20). 95 



