640 MR JOHN M'LEAN THOMPSON ON 



the parenchymatous part of the xylem. The outer cylinder is composed of small 

 and narrow tracheides which are more closely packed than are the tracheides of 

 the inner cylinder. Parenchyma does not bulk largely in the organisation of 

 this outer cylinder, but the chains and groups are short and narrow, and the 

 elements composing them are few in number. This outer cylinder is alone 

 involved in the process of supplying the traces of the leaves ^nd roots. 



It has been noted that the xylem is thin on the upper surface of the stele, while 

 on the lower surface it is usually comparatively thick. It may be stated at once 

 that the difference in xylem thickness, between the lower and upper sides of the 

 stele, is not primarily due to a progressive thinning of the xylem on the upper 

 surface as a whole. It is mainly due to an increase in the bulk of the parenchymatous 

 storage xylem towards the lower surface of the stele (figs. 1, 4, 10, 13, 63). The 

 outer xylem-cylinder does not vary markedly as one passes from the upper to the lower 

 surface of the stele. It is true that it is of irregular thickness from point to point, 

 but it is maintained consistently on all sides of the stele, whereas the inner xylem- 

 cylinder may be locally massive towards the under surface, and at the same time 

 reduced almost to the vanishing point on the upper side of the stele. It is 

 therefore evident that it is the parenchymatous inner portion of the xylem- 

 cylinder which is most liable to fluctuations, either in the direction of increase or 

 reduction (figs. 4 and 63). 



Further, the great increase in the parenchymatous xylem just noted is subject 

 to local accentuation ; for, while this type of xylem is certainly conspicuous at all 

 points along the lower surface of the stele, it becomes very massive in those 

 regions where the numerous and closely crowded traces are being liberated (figs. 13 

 and 63). On the other hand, it is usually less dominant in those portions of the 

 stele where the leaf-traces are less numerous and crowded. 



As to the absence of definite protoxylem groups a word may be said. It is 

 generally recognised that the presence or absence of definite protoxylems depends 

 upon the degree and rate of extension which take place in the xylem. There can 

 be no doubt that the axis of Platyzoma is either slow-growing or is subject to little 

 or no extension as maturity is approached. To this the close succession of the 

 leaves and the condensed nature of the rhizome bear ample witness, and accordingly 

 the absence of protoxylems need be cause for no surprise. The absence of protoxylems 

 may then be regarded as indicative of a slow rate of growth or of but slight exten- 

 sion of the stele. It is an index of the peculiar physiological conditions of 

 Platyzoma, but as a systematic character it is unreliable. 



The phloem follows almost immediately upon the outer surface of the xylem. It 

 is separated from the xylem only locally by a single layer of small parenchymatous 

 cells ; it is from three to five layers broad, and is composed entirely of typical fern- 

 sieve tubes. It is only interrupted at points where leaf- and root-traces are departing. 

 The phloem is followed externally by a large-celled pericycle of three to four par'en- 



