﻿64 VEGETATIVE FEATURES. 



It is on such grounds that one may account for the characteristic difference 

 in the general contour of pinnule insertion in many of the Mesozoic cycads as 

 compared with existing forms. Thus, for instance, it is quite probable that the 

 leaf-base pattern of C. nigra, as shown in figure 31, persisted in the petiole. In this 

 case, if the two loosely-concentric bundles situated above the U-shaped depression 

 gave origin to the bundles entering the base of the pinnules, by alternately crossing 

 from side to side, it may be readily conceived that there might result a form of 

 petiole and a pinnule attachment like that of Otosamites. It is, indeed, quite likely 

 that transverse pinnular bundle traces were not uncommon among the Cycadeoidese. 



As in the case of the leaf-base spirals, it is of more immediate interest to present 

 in figure ^ a series 01 transverse sections of leaf bases showing their bundle patterns 

 than to atcempt a determination of species. In fact, since the sections are not cut 

 at the same approximate level and from the same trunk regions, precise specific 

 determinations from such data alone would not be practicable or possible. 



Leaf-Base Bundle Structure. 



Passing by the leaf-base parencltyuia and the dermal and hypodermal scleren- 

 chyma, which are often very dense, but of strictly cycadean type, it remains to note 

 briefly the structure of the bundles. These, as already described by Seward (144) in 

 C. [Bennettitei) Gibson ian its and C. gigantea, are mostly as in the existing cycads 

 of mesarch type, and the sections here described and figured show that there is exact 

 further correspondence in the diminution, as distance from the cortex increases, of 

 centrifugal with a corresponding increase of centripetal xylem. 



In the section shown in text-figure 34 c and also in photograph 1, plate xx, 

 cut from the base of a leaf inserted near the crown of trunk 214 (C dacofeiisis), and 

 hence quite young, the large body of radially-arranged centrifugal xylem is the 

 only portion of the bundle preserved and consists almost entirely of scalariform 

 tracheids. The bundles illustrated in figures 34, a and B, are cut relatively 

 farther out at a distance of from 2 to 3 centimeters distal to the cortex, and are from 

 the leaf bases surrounding one of the bisporangiate strobili (No. 1), cut from 

 well down on the side of trunk 214. The proto xylem is usually seen to consist 

 of small, heavy-walled, and flattened cells, accompanied by some parenchvma cells. 

 Interior to these are the cells of the centripetal xylem, varying much in size and 

 including several of large diameter. The area of centripetal xylem is decidedly more 

 pronounced in these sections from C. dacotensis than in C. gigantea, as figured by 

 Seward, while in the finely-preserved bundles of C. micromyela, figured by Lignier, 

 centripetal xylem does not appear. In both these European species and the prer.ent 

 form, however, the remaining elements of the leaf-base bundles agree in ever}- par- 

 ticular. In common, there is a large and broad area of radially-arranged centrifugal 

 xylem, made up of rays from one to several or more cells in breadth, separated by 

 medullary rays one cell in thickness. Following this there is a thin baud of 

 cambium and a rather narrow crescentic area of phloem, usually crushed or but 

 indistinctly preserved, and encroached upon on the outer side of the bundle by a 

 larger area of sclerenchyma cells. The latter are as large and as heavily walled as 

 those of the centrifugal xylem, but are not regularly arranged. They are best 



