iS8 



MORPHOLOGY 



that the microsporangia have advanced very little beyond the fern 



level. 

 Ovulate structures. The megasporangiate structures, however, 



have advanced very far beyond the fern level, and are very peculiar 



(fig. 431). The seeds terminate 

 long and slender stalks, which are 

 packed among interseminal scales 

 that are also stalked structures. 

 The stalked seeds and interseminal 

 scales are arranged so as to form 

 an ovoid, fruitlike body, with a 

 mosaic surface composed of the 

 flaring tops of the interseminal 

 scales, wedged between which 

 the micropylar tubes of the sccc"';: 

 protrude. If this structure be 

 compared with the seed-bearing 

 structures of the Cycadofilicales, 

 especially those in which the seeds 

 terminate the naked branches of 

 a pinna, it will be observed that if 

 these branches be reduced to a 

 single axis, the condition in Ben- 

 nettitales is obtained. The inter- 

 seminal scales are probably sterile 

 megasporophylls ; and all the 

 megasporophylls, leaflike and x 

 spreading in Cycadofilicales, are 

 compacted into a strobilus in 

 Bennettitales. 

 FIG. 429. Strobilus of a species of Seeds. The structure of the, 



Cycadeoidea, in which the seeds are mature, seeds has been obtained from SCO 



:et g twT^ WhiChb0rethe tibns, which sh * basal cupule, 



suggesting a rudiment of the in- 

 vesting and husklike cupule of some of the Cycadofilicales; a two or 

 three layered testa ; and a large dicotyledonous embryo completely 

 filling the seed (fig. 432). This embryo is unlike that of any living 

 gymnosperm, in that in developing it destroys all of the endosperm 

 (see p. 202). 



