330 



PLANT MORPHOLOGY 



Cycadales display the following distinctive features: infrecjuent branching 

 of the stem, indirect and double leaf traces, monosporangiate and appar- 

 ently terminal strobili, reduced microsporophylls, and, except in Cycas, 

 reduced peltate megasporophylls with only two ovules. 



Fig. 279. Early stages in embryogeny of Zamia floridana, X25. A, proerabryo with free 

 nuclei; B, wall formation at base of proembryo; C, differentiation into suspensor and 

 embryo. 



4. Cordaitales 



The Cordaitales comprise an extinct group of Paleozoic gymnosperms, 

 contemporaneous with the Cycadofilicales, but plants of a very different 

 aspect. They appeared late in the Devonian, made their greatest display 

 during the Upper Carboniferous, and almost disappeared before the end 

 of the Permian (Fig. 258). Although the Cordaitales differ in many ways 

 from the Cycadofilicales, their resemblances are such as to indicate that 

 both groups may have had a common origin. 



Sporophyte. The Cordaitales were tall trees with slender, branched 

 stems often reaching a height of 30 m. (Fig. 280). The branches were 

 covered with simple leaves that were generally long and narrow and borne 

 in spiral arrangement. A cross section of the stem, which was an ecto- 



