﻿9 2 



VEGETATIVE FEATURES. 



we have the rare advantage of so many serial transverse sections with all their 

 bundles present, it is possible to plot whenever desirable the plan of pinnule nerva- 

 tion, which, as in the existing cycads, is of the strictly dichotomous type. 



Pinnule Structure and Comparison with Existing Cycads. 



The structure of the pinnules of C. ingots, as represented in the extensively 

 retouched photographic figure 45, is essentially that of leathery to dense and woody 

 cycadaceous foliage types. Each bundle is surrounded by a prominent more or less 

 continuous sclerenchymatous sheath, an extension of which passes out from the 

 upper or xylem side to the epidermis and spreads out beneath it as a layer of hypo- 

 dermal sclerenchyma several cells thick opposite to the bundles, but diminishing to 

 a single cell in thickness between them. Beneath this upper hypodermal scleren- 

 chyma there is a layer of palisade parenchyma one cell thick, followed by rounded 

 and large parenchyma cells filling all the space between and as far as the lower side 

 of the bundles. Finally, through all the space beneath the bundles the tissues are 

 dense and woody to the nearly complete exclusion of parenchyma, but are for the 

 greater part indistinctly indicated, owing mainly to the lack of natural staining 

 rather than of preservation. Especially just beneath the bundles large areas of 

 sclerenchyma, not very sharply outlined, may be noted. The upper boundary of this 

 lower area or zone of indistinctly stained tissue appears in all the transverse sections 

 as a distinct line evenly curving beneath and hence uniformly waved as many times 

 as there are bundles. 



The bundles are of mesarch collateral structure without radial arrangement of 

 either xylem or phloem, and the prominent surrounding sheath is made up of an 

 inner smaller-celled and an outer larger-celled "peridesmic" (?) layer, confluent, as 

 mentioned, with the hypodermal sclerenchyma. The inner elements of the sheath 

 are reinforced to a thickness of from three to four cells on both the xylem and 

 phloem sides of the bundles, more especially near the base of the pinnules ; and scat- 

 tered among the outer sheath elements one notes occasional very heavy-walled 

 cells, as in Bowenia spectabilis. Also from the lower side of the bundle a layer of 

 transfusion (?) tissue a cell in thickness extends out, ala-like, underneath the round- 

 celled parenchyma of the middle pinnule and above the lower region of mainly 

 sclerenchymatous elements already noted. Evidently these cells are in part respon- 

 sible for the sharpness with which the two zones they separate have been delimited 

 by the natural course of silicification. 



Owing to the absence of radial cell grouping, and the extensive and complex 

 development of sclerenchymatous ensheathing tissue, the exact extent and position 

 of the centripetal as compared with the centrifugal xylem, and the limits of the 

 phloem, are not easy to determine, a difficulty that is obvious enough in the accom- 

 panying diagrammatically-retouched photographs of transverse pinnule sections. 

 But in the absence of better naturally or artificially stained sections, and especially 

 in the absence of a study of the pinnules of all the existing species of cycads, it is 

 sufficiently exact to describe these bundles as of mesarch collateral structure, with 

 extensive secondary sclerenchymatous development and modification, as in Cordailes 

 {cf. 161) and Encephalartos. 



