158 MORPHOLOGY OF GYMNOSPERMS 



an offshoot from the Bennettitales. And yet, the striking differences 

 in the strobiH of the two groups seem to contradict such an easy 

 disposition of the question, and suggest independent origin from the 

 CycadofiHcales. 



The bisporangiate strobilus of Bennettitales, with its pinnate 

 stamens and stalked ovules, suggests an origin from the groups of 

 Cycadofilicales with stalked ovules and marattiaceous microsporo- 

 phylls. The Cycadales, on the other hand, taking Cycas and Pseudo- 

 cycas as its oldest known representatives, suggest an origin from groups 

 of Cycadofilicales bearing ovules as did Pecopteris Pluckenetii. This 

 separate origin is further emphasized by the fact that in the one 

 case bisporangiate strobili are developed, with a characteristic rela- 

 tion of microsporophylls and megasporophylls; and in the other 

 monosporangiate strobili are produced, with the dioecious habit; 

 and still further by the fact that in the one case a monopodial stem 

 with direct leaf traces is maintained; and in the other a sympodial 

 stem with its characteristic "girdles" is established. It is recognized 

 that such differentiations from Bennettitales are conceivable, but it 

 is easier to conceive of them as having arisen from such a synthetic 

 group as the Cycadofilicales than from such an extremely specialized 

 group as the Bennettitales. The historical testimony that will answer 

 this question will be forthcoming only when the actual connections 

 with Cycadofilicales are discovered. 



The Cycadales are remarkable in the retention of more primitive 

 characters than are possessed by any living group of gymnosperms. 

 The swimming sperms are shared by Ginkgoales, and the structure 

 of the ovule that goes with them, but the vegetative structures are more 

 fernlike than in any other living group. In this sense it is proper to 

 speak of Cycadales as the most primitive of living gymnosperms; 

 but it must be remembered that they are probably not so old as either 

 the Ginkgoales or the Coniferales. 



LITERATURE CITED 



1. Brongniart, a., Recherches sur I'organisation des tiges des Cycadees. 

 Ann. Sci. Nat. I. 16:389-402. ph. 20-22. 1829. 



2. Von Mohl, Hugo, Ueber den Bau des Cycadeen-Stammes. Abh. K. Acad. 

 Munchen 1:397-442. 1832; republished and revised in Vermischte Schriften, 

 pp. 195-21 1. 1845. 



