170 



MORPHOLOGY OF GYMNOSPERMS 



conspicuous parallel vascular bundles is invested by a strong sheath 

 which abuts above and below on a hypodermal strand of fibers. 

 These bundle sheaths connect laterally by transverse "bridles" 



of thickened cells, 

 which may represent 

 a "transfusion tissue." 

 In some cases the 

 hypodermal ribs occur 

 only in connection 

 with the bundles; 

 while in other cases 

 they are developed 

 between the bundles. 

 The mesophyll in some 

 leaves is differentiated 

 into palisade and 

 spongy regions, and 

 in others it is very 

 little differentiated. 



There is really 

 greater variation in 

 the leaves of Cor- 

 daitales than is usually 

 appreciated (16), and, 

 as in the case of the 

 anatomy of the stem, 

 they suggest possible 

 connections in various 

 directions, namely, 

 with Cycadofilicales 

 on the one hand, and 

 Coniferales and Gink- 



FlGS. 200-202. — Cordaites: transverse sections of 

 leaves; fig. 200, C. angulostriaius ( ?) ; r, ribs of 

 sclerenchyma; m, mesophyll; 0, centrifugal xylem; 

 i, centripetal xylem; x, protoxj'lem; p, pliloem; 6, 

 bundle sheath; c, elongated cells connecting bundles; 

 X6o; fig. 201, C rhonibinervis, showing sclerenchyma 

 ribs only in connection with bundles; XSo; fig. 202, 

 C. lingulatus, showing well-developed palisade tissue; 

 X50. — ^After Renault (3). 



goales on the other. 

 The leaves of Poroxylon are of the same gerferal type. They are 

 spirally borne on rather widely separated r^des of a slender stem, 

 and are simple, broad, thick, and petioled, with parallel veins repeat- 

 edly dichotomous. The blade is stiffened by hypodermal strands 



