SPERM ATOPHYTA 



329 



Fig. 278. Reconstruction of the ovule of Dioon edule at the time of fertilization. Pollen 

 tube on the left shows undivided body cell; the one in the middle shows two sperms and 

 remains of prothallial and stalk cells; the one on the right shows two sperm mother cells. 

 Two pollen tubes have discharged their sperms. A sperm has entered the egg on the left; 

 the one on the right still shows tlie ventral canal nucleus. Two sperms, in the thick liquid 

 discharged from the pollen tube just above them, are ready to enter the egg. The dark 

 line below the nucellus is the megaspore membrane. {From Chamberlain.) 



are common to the Cycadofilicales, Bennettitales, and Cycadales. These 

 three orders, considered together, are called cycadophytes. They re]!)re- 

 sent a distinct line of evolution reaching far back into the Paleozoic. The 

 stem of the Cycadales, like that of the Bennettitales, is an endarchsiphono- 

 stele with a narrow zone of wood, the xylem in other aerial parts of the 

 plant being mesarch. The dicotyledonous embryo is also suggestive of 

 that of the Bennettitales. As compared with the other cycadophytes, the 



