G. It. Wieland — American Fossil Cycads. 307 



be cut through their bases, while in a given transverse section 

 those farthest from the rachis must be cut through their tips 

 (as may be understood by referring to figure 1), this transverse 

 section virtually represents a series of sections such as would 

 be obtained if a single pinnule were cut transversely 20 times 

 at regular intervals from base to tip. Nearer the base of the 

 leaf, a transverse section cuts 34 pinnules. As there are about 

 20 insertions of pinnules above this basal section, and appar- 

 ently no pinnules lying wholly beneath it, there are approxi- 

 mately 60 pinnules in all. The rachis and pinnular insertion 

 have not yet been studied. 



The principal fact of interest shown by the longitudinal 

 section represented in figure 3 is the emergence of leaf tips on 

 the side next the axis, or rachis, of the leaf. As this section is 

 wholly above the termination of the rachis, this emergence 

 indicates that the topmost pinnse are relatively short. 

 Necessarily, then, the leaf at this stage of its growth was of 

 a somewhat truncate form. 



This fact, however, does not permit surmise as to the exact 

 leaf habit of Cycadeoidea. for subsequent growth either may 

 have increased the truncation, or, because of great elonga- 

 tion of the rachis, resulted in a lanceolate leaf. Presumably 

 the study of the prefoliation and subsequent development of 

 the rachis and pinnse of living cycads, especially Zamia, will 

 indicate what must have been the appearance of the full-grown 

 leaf of Cycadeoidea. 



The microscopic structure of the pinnules is partially shown 

 in figures 4 and 5. In figure 4 is shown a transverse section 

 through a pinnule cutting a fibro-vascular bundle. The struc- 

 ture of the relatively heavy epidermis of the upper side of the 

 pinnule is not distinct even in this young stage, and forecasts a 

 dense and stiff structure. A single layer of sclerenchyma 

 cells of rounded section forms the hypodermis, from which a 

 double row of similar cells passes in confluently to the bundle, 

 where it divides to form a more or less complete circular row 

 of sclerenchyma cells enclosing the phloem and xylem. The 

 uniformity of this arrangement gives the transverse section of 

 the bundles a characteristic flask-shape. Beneath the hypo- 

 dermis is a distinct layer of palisade parenchyma followed by 

 an indistinct one grading into the rounded parenchyma cells 

 occupying the space between the bundles. The lower portion 

 of the leaf, or that beyond the bundles, is composed of well- 

 defined spongy parenchyma, with distinct intercellular spaces, 

 and is bounded by a thin epidermis, the structure of which is 

 not well marked. The lower surface at this stage is somewhat 

 glabrous, although revolute margins are apparently indicated. 



