318 



PLANT MORPHOLOGY 



long, and a prominent nucellar beak and pollen chamber were developed 

 (Fig. 2665). Thus it seems certain that the Bennettitales had swimming 

 sperms. Nothing is known of the gametophytes. The embryo was 

 dicotyledonous and completely filled the seed, no endosperm having been 

 present at maturity. 



A B 



Fig. 266. Strobilus and seed of Bennettites (Cycadeoidea). A, diagram of seed-bearing 

 strobilus, showing sheathing bracts, long-stalked seeds, and interseminal scales; B, longi- 

 tudinal section of seed; m, micropylar tube; «, nucellar beak; p, pollen chamber; s, inter- 

 seminal scale; e, embryo space. (A, after Scott and others; B, after Wieland andLignier.) 



Summary. The Bennettitales are a group dominant in the Mesozoic 

 and intermediate in some respects between the Cycadofilicales and the 

 Cycadales. Characters common to the Cycadofilicales include branching 

 of the stem, a ramentum, direct leaf traces, leaf-like microsporophylls with 

 synangia, and the ovule structure. An advance is seen in the organiza- 

 tion of a strobilus. Characters common to the Cycadales are the general 

 habit of some of the genera, the leaves, and the vascular anatomy, the 

 stem being an endarch siphonostele with relatively little secondary wood. 

 Distinctive features of the group are the bisporangiate strobili and the 

 occurrence of both fertile and sterile megasporophylls, the latter bearing 

 solitary terminal ovules. The Bennettitales are considered too special- 

 ized in their spore-bearing structures to have given rise to the Cycadales. 

 Both groups seem to have had an independent origin from the Cycado- 



