SPERMATOPHYTES 



203 



ent ; in the form and general character of the leaves ; in the microsporo- 

 phylls with their abaxial sporangia in sori ; and in the swimming sperms. 

 These characters are shared with Cycadofilicales and Bennettitales, the 

 ovules of which groups those of cycads further resemble in the three- 

 layered testa (the outer and inner layers fleshy and the middle one 

 stony), the two sets of vascular strands, and the nucellar beak with its 

 pollen chamber. The general habit of the sporophyte body further 

 resembles that of the Bennettitales ; but the relatively terminal and 

 monosporangiate strobili are peculiar to cycads. 



The cycads evidently represent the modern end of one seed, plant 

 line, which has come from the fernlike Cycadofilicales of the Paleozoic, 

 and which gave rise to the Bennettitales during the Mesozoic. 



(4) Cord AIT ALES 



General character. — This is an extinct paleozoic group of gymno- 

 sperms that was contemporary with the Cycadofilicales, and these two 

 groups made up most of the seed plant vegetation of the Paleozoic, 

 the Cordaitales being the dominant gymnosperm forest type. The two 

 groups appear side by side as far back as the records go, but intergrad- 

 ing forms indicate that the Cordaitales probably arose from the 

 Cycadofilicales at a very early period. 



Sporophyte. — The Cordaitales were tall and slender trees, often 

 10 to 30 m. high before branching, with a dense crown of branches, 

 and a great abundance of simple and large leaves (fig. 461). The gen- 

 eral appearance of these trees differs from that of any living gymno- 

 sperm. 



Vascular anatomy. — The structure of the stem combines the char- 

 acters of other groups and suggests its relationships. The siphonostele 

 composed of collateral endarch vascular bundles is common to all 

 gymnosperms ; the mesarch bundles of the leaves, a character shared 

 with the gymnosperm line beginning with Cycadofilicales and ending 

 with Cycadales, testifies to the connection with ferns ; the large pith is 

 shared with the same gymnosperm line ; the double leaf trace is also 

 a feature of the Cycadofilicales; but the branching habit, the simple 

 leaves, and especially the thick cylinder of secondary wood are char- 

 acters of the Coniferales. These characters indicate a connection with 

 the Cycadofilicales, and a development towards the Coniferales instead 

 of towards the Cycadales. 



Leaves. — The leaves are simple, elongated (fig. 461), with parallel 



