16 PENNSYLVANIAN FLORA OF NORTHERN ILLINOIS 



Sphenophyllales 



This order contains only one genus, Sphenophyllum, and that is now 

 extinct. We know nothing about any relation of the Paleozoic Sphenophyl- 

 lales with living plants. 



Sphenophyllum was an aquatic and herbaceous plant, had jointed stems 

 with whirls of triangular leaves that always appeared in multiples of three. 

 At the end of the branches were the long and narrow strobili. 



Lycopodiales 



Probably most of the known genera and species of fossil Lycopodiales 

 were arborescent, with the exception of the genus Lycopodites, which was 

 herbaceous. There must have been numerous herbaceous Lycopodiales in 

 the Paleozoic era from which our living Lycopodiales descended. The large 

 tree forms died out almost entirely at the end of the Paleozoic era, but a 

 few medium-sized Mesozoic tree types persisted for a short time. 



The two principal genera of the Paleozoic era are Lepidodendron and 

 Sigillaria, both of which reached a height of about one hundred feet. 



Lepidodendron is the generic name for a stem type that had spirally 

 arranged rhomboidal leaf scars, whereas Sigillaria had its leaf scars ver- 

 tically arranged. Syringodendron was a Sigillaria whose cortex had been 

 removed before fossilization. Lepidostrobus is the generic name for the 

 strobilus or cone of Lepidodendron. Lepidophyllum is the scale of Lepido- 

 strobus, and Stigmaria designates a root form that might have belonged to 

 a Lepidodendron or a Sigillaria stem. The different species of Lepidoden- 

 dron and Sigillaria are distinguished by the shape of their leaf cushions. 

 Leaves seem to have been born only on the youngest branches of these 

 trees, but the leaf cushions continued to grow on the stem and on the 

 older branches after the leaves had fallen off and formed a sort of armor 

 around the tree similar in function to the armor of some living Cycads 

 but differing in the shape of their leaf scars. The leaf scars of Lepidophloios 

 were more broad than long, whereas those of Lepidodendron were more 

 long than broad. Lycopodites was a herbaceous type, similar to the modern 

 genera Selaginella and Lycopodium. 



The root form of Lepidodendron and Sigillaria, Stigmaria ficoides, 

 was covered with small rootlets of which the circular scars are still visible. 



Filicales 



The mo^t numerous and stratigraphically most useful plant fossils 

 of the Paleozoic sediments are impressions of fern-like leaves. A large 

 number, perhaps the majority of them, did not belong to true ferns but 

 were parts of trees that had the habitus of tree ferns and the fructifications 

 of Gymnosperms. This extinct group which seemed to have sprung from 

 the same origin as the true ferns has been discovered only during the early 

 twentieth century and in America is called Cycadofilicales. English 

 paleobotanists use the name Pteridosperms, a Greek translation for seed- 

 ferns. The fructifications of these Cvcadofilicales will be discussed later. 



