﻿FOLIAGE. 87 



gence, but not relatively so much as in Dion. Hence it may be said that Cycadeoi- 

 dea exhibits characteristics of erect prefoliation and frond growth seen in both Dion 

 and Macrozamia. Also, had the young fronds had the prefoliar habit of Zamia 

 (fig. 47) in which petiolar growth takes place early, they would mostly have pro- 

 jected beyond the zone of preservation, and shared the fate of the older foliar organs. 

 In such case, therefore, only very young fronds would have been silicified, and such 

 could not yield the handsome transverse sections shown on plate xix. ( Cf. figs. 46-48, 

 showing the three chief types of prefoliation in the existing cycads.) Scale leaves 

 are not believed to be present in C. ingens, but are also absent in some of the 

 Macrozamias. 



FORM AND STRUCTURE OF FRONDS AND PINNULES. 



The above-given facts concerning prefoliation may in reality be determined 

 without the aid of thin sections ; especially since among the specimens in the several 

 American cycad collections there is at hand much supplementary material showing 

 the emergent or broken rachis or petiole tips and the ranked pinnules, all the 

 evidence going to indicate that, as in the existing cycads, the apically borne and 

 helicoidally arranged young fronds emerge in both crown-forming groups and the 

 successional or nearly continuous growth order. In the former case as many as a 

 score or more of fronds may be present ; in the latter a lesser number. 



When, in addition to the macroscopic study of such specimens, the finer struc- 

 tures are considered, the entire arrangement of parts becomes very clear. For con- 

 venience a hypothetical frond with a reduced number of pinnules, the uppermost of 

 which project beyond the short rachis, as in the fossil forms, is given in figure 49, 1. 

 By reference to this figure the relative position of the thin sections represented in 

 figures 49, 2-4, may at once be determined, as well as the disposition of parts and 

 the orientation of the vascular bundles. The thin section of a folded frond tip, 

 shown in figure 49, 2, passes well above the rachis, while that shown on plate xix, 

 photograph 3, is cut from the same frond perhaps a centimeter lower down, or more 

 proximately to the trunk, though not enough so to pass through the tip of the rachis. 

 There are in the latter section ten more pairs of the transversely cut tips of pinnules 

 than in the preceding one. Again, photographs 1 and 2, plate xix, show sections 

 of two adjacent fronds at as high a level with respect to the crown as either of the 

 foregoing, but yet low enough to strike the rachis well beneath its tip. The latter 

 fronds are hence a little more advanced in growth than the first mentioned, and 

 their position in the helicoid of emerging leaves therefore distal. Finally, photo- 

 graph 6, plate xxxi, shows the transverse section of the most nearly emergent frond 

 of all those in the crown of C. ingots type actually observed to bear pinnules. In 

 this frond but seven pairs of basal pinnules are cut; and from inspection of the 

 specimen itself it may be determined that not more than a few, if any, inner and 

 therefore lower pinnules were present. It is hence clear that of all the pinnules 

 borne by this nearly emergent frond possibly no more than seven and certainly not 

 more than three or four additional basal pairs were still ranked and folded in the 

 ramentum of the crown, although to determine the exact number would require the 

 making of several sections very difficult in this instance to obtain for the sake of 



