SPERM ATOPHYTA 311 



but freely exposed. The class includes seven orders: the Cycadofilicales, 

 Bennettitales, Cycadales, Cordaitales, Ginkgoales, Coniferales, and 

 Gnetales. Of these, the first, second, and fourth are entirely extinct. 



1. Cycadofilicales 



The Cycadofilicales' are the oldest and most primitive group of seed 

 plants. Although some fossil remains have come from late Devonian 

 deposits, the group did not become abundant and widespread until the 

 Upper Carboniferous (Fig. 258). It declined greatly during the Permian, 

 but persisted into the Triassic, when it soon became extinct. The 

 Cycadofilicales are of great interest because, as their name implies, they 

 are transitional between the ferns and the cycads. 



Sporophyte. The general aspect of the Cycadofilicales was distinctly 

 fern-like (Fig. 259). Some forms resembled modern tree ferns, but most 

 of them were smaller. Some appear to have been climbers. The leaves, 

 when found as impressions in the rocks, are so fern-like that they cannot 

 be distinguished from the leaves of true ferns except when found in associ- 

 ation with stems, sporangia, or seeds. The stem anatomy is fern-like 

 also, but the development of secondary wood is characteristic. This 

 wood consisted of pitted rather than scalariform tracheids. Three stelar 

 types were represented among the Cycadofilicales, each constituting a 

 "stem genus." Heterangium was a protostele, Medullosa a polystele 

 (with three separate steles), and Lyginopteris an ectophloic siphonostele 

 (Fig. 260). In each case the primary xylem was mesarch, a fern char- 

 acter. The leaf traces were double and direct; they were mesarch 

 throughout. 



Microsporangium. The microsporangia of the Cycadofilicales were at 

 one time regarded as the sporangia of ferns. The microsporophyll 

 (stamen) resembled an ordinary fern frond having fertile and sterile 

 pinnae. In the ''stamen genus" Crossotheca each fertile pinnule was 

 more or less peltate and bore six to eight bilocular microsporangia on its 

 lower side. This is designated as the "epaulet" type of stamen (Fig. 

 261^4, B). Another, characteristic of the stamen genus Cahjmmatotheca, 

 is known as the "cupule" type because the microsporangia were borne 

 within a cup-like structure formed at the end of a naked branch (Fig. 

 261C). In some of the Cycadofilicales the microsporangia occurred as 

 synangia on the abaxial surface of fern-like leaves. 



Megasporangium. In none of the Cycadofilicales was a strobilus 

 organized. Commonly the fern-like leaves were dimorphic, some being 

 fertile and others sterile. The fertile leaves bore terminal ovules on their 

 ultimate divisions (Figs. 196 and 259). The seeds were usually enclosed 

 in a cupule (Fig. 262). As in other seed plants, the ovule consisted of a 



' Often called Pteridospermae. 



