15^ MORPHOLOGY OF GYMNOSPERMS 



begun to develop. Suspecting that gravity might be concerned 

 in the suppression of one of the cotyledons, she placed seeds upon 

 a clinostat during the entire period of cotyledon development, and 

 these seeds developed regularly two cotyledons (figs. 187-194). 



The mature seed has a fleshy and variously colored outer coat, 

 developed from the outer portion of the integument, and an inner 

 stony coat, extremely hard and tough, derived mostly from the middle 

 region of the integument. Within the stony layer is a thin, dry 

 membranous layer derived partly from the nucellus and partly from 

 the inner fleshy layer of the integument and containing the inner 

 vascular system of the ovule. In Encephalartos, Cycas, Ceratozamia, 

 and perhaps other genera, the ovule often reaches its full size and see^- 

 like appearance whether pollination has taken place or not. In Dioon, 

 ovules which have not been pollinated usually abort, and consequently 

 grf^enhouse material, and even cones in the field, if widely separated 

 from staminate plants, are likely to have only abortive ovules. 



Seeds germinate readily if simply placed on the surface of moist 

 soil. If covered by the soil, they are raised to the surface, but not 

 much above it, by the cotyledons, which always remain within the 

 seed coats. 



In Dioon, Zamia, Ceratozamia, Microcycas, and probably in all 

 the other genera, the intraseminal development may be followed 

 immediately by the germination of the seed, no resting period being 

 necessary. The coleorhiza, which in early stages constitutes a con- 

 siderable portion of the embryo, becomes reduced to a thin, hard, 

 brown cap which protects the more delicate structures while the micro- 

 pylar portion of the stony coat is being ruptured and the young seed- 

 ling is emerging. After the coleorhiza has broken through the seed 

 coat, the tardy root pierces it and soon thickens into a stout tap root. 



5. History and distribution 



That representatives of the Cycadales existed during the Mesozoic, 

 side by side with the predominant Bennettitales, is a safe inference, 

 but the evidence is extremely meager. Strobili thought to be stami- 

 nate cones of cycads have been used as the basis of the form genus 

 Androstrobus. The structure of a few of these species suggests a 

 possible connection with cycads, but Nathorst (60) has shown that 



