FOSSIL gym:nosperms 137 



Cycads, and Scott calls attention to the fact that the structure of 

 the leaf of Cordaites resembles that of a single pinna of the leaf 

 of such a Cycad as Zamia, or one of the larger-leaved Arau- 

 carias. Leaf sections show a very xerophytic structure, hypo- 

 dermal masses of selerenchyma producing great firmness, if not 

 rigidity. In the interior of the leaf, between the bundles, 

 " transfusion tissue " is developed, the cells being elongated par- 

 allel to the epidermal layers, and so loosely organized as to 

 appear like thin diaphragms between very large intercellular 

 spaces, just as in the Cycads and Sciadopitys. Each vascular 

 bundle is surrounded by a strong sheath, which connects with the 

 hypodermal mass of selerenchyma. Each bundle is of the 

 mesarch type — that is, the secondary xylem occurs in two parts, 

 with the primary spiral elements between them. This is exactly 

 the structure of the leaf bundles of Cycads, and of other Paleo- 

 zoic plants. The numerous stomata are depressed and occur in 

 rows, apparently only upon the under surface. It is an interest- 

 ing fact that the form and general structure of the leaf suggest 

 the leaves of Conifers, while the anatomical features are more 

 those of the leaves of Cycads. 



The roots which are regarded as belonging to the Cordaitales 

 show structures which relate them easily to the stem. In the 

 various specimens recorded the vascular cylinder of the root is 

 diarch, triarch, or tetrarch. 



Chiefly through the researches of Renault, some knowledge 

 of the strobili of a few species has been obtained. The strobili 

 occur upon special shoots, which may be small or branched, and 

 bear small catkinlike clusters (Eig. 94), which do not often 

 exceed a centimeter in length. Externally the staminate and 

 ovulate clusters are alike, although in some cases an ovulate 

 strobilus may consist of a solitary ovule with few bracts. Each 

 strobilus usually consists of a series of prominent leaflike bracts, 

 in the axils of which the spore-bearing structures occur. It 

 should be borne in mind that the following account is derived 

 from the examination of a few sections of one or two species of a 

 very large and probably diverse group. 



The staminate strobilus consists of a thick axis covered by 

 long spirally arranged bracts, which resemble reduced leaves. 

 Many of the bracts are sterile ; but especially toward the apex 

 of the strobilus these are replaced by structures which may be 



