THE LEAF 203 



present a strong superficial resemblance, particularly in the con- 

 formation and venation of their leaves, to the monocotyledons. 

 The Cordai tales had leaves varying in width, but always char- 

 acterized by few or numerous parallel veins. The structure of 

 the fibrovascular bundles constituting these veins, however, was 

 entirely different from the features of organization found in the 

 nerves of the leaves of living monocotyledons. Fig. 150 illus- 

 trates the general organization of the leaf in Cordaites princi- 

 palis. The veins are numerous and separated from one another 

 by intervals of the soft mesophyll of the leaf. The epidermis is 



FIG. 150. Transverse section of the leaf of Cordaites principalis 



strengthened, particularly in the region of the fibrovascular 

 bundles, by hypodermal bands of thick-walled cells. In Fig. 151 

 is shown an enlarged transverse section of one of the foliar bundles 

 of the species under consideration. The wood was entirely cen- 

 tripetal in its development and ended on either flank in a band of 

 thick-walled elements with narrow central cavities. The zone of 

 thick-walled elements was in contact externally with a second zone 

 made up of cells with thinner walls and much larger lumina. Both 

 the zones just described inclose the phloem, which lies below the mass 

 of centripetal wood. The longitudinal view (Fig. 152) throws addi- 

 tional light on the organization of the. bundle. The double sheath 

 connected with the flanks of the xylem and forming a complete circle 

 about the phloem is composed of elongated elements with bordered 

 pits on their walls. These are known as transfusion cells and are 



