CONIFERALES 277 



dioecious species are later developments than the monoecious; or, 

 rather, they have progressed farther along this line of evolution ; but 

 dioecism may be associated with very primitive features. Conse- 

 quently, one could hardly claim that a monoecious family, as a 

 whole, is more primitive than a dioecious family. It is simply more 

 primitive in this one respect. 



Monoecism, the prevalent condition in the order, is well illustrated 

 in Pinus, the dominant genus of the Northern Hemisphere (fig. 280). 

 This figure shows staminate cones a day or two before shedding their 

 pollen, and ovulate cones of three successive seasons; at the top, a 

 couple of cones just ready for pollination; lower down, an ovulate 

 cone which was pollinated the previous year and which would show 

 young embryos; and near the bottom, an ovulate cone which was 

 polKnated two years earlier than the one at the top, and has shed its 

 seeds. The ovules in Pinus contorta are pollinated in June, the eggs 

 are fertilized the next June, the embryos complete their develop- 

 ment during the summer, and the ripe seeds are shed the following 

 summer. In some pines, like Pinus radiata, the cones remain tightly 

 closed for years and shed the seeds only when there is a fire or an 

 unusually hot, dry season. Pines with hard cones usually shed their 

 seeds the third season, like Pinus contorta, P. banksiana, and many 

 others. 



BISPORANGIATE STROBILI 



Bisporangiate strobili are always described as teratological. In 

 the evolution of sex there is a constant tendency to wider and wider 

 separation, so that the theoretical series would be bisporangiate stro- 

 bili, monosporangiate strobili with both sexes on the same plant 

 (monoecism), and, finally, monosporangiate strobili on different 

 plants (dioecism). 



In the Cycadophytes bisporangiate strobili are common and nor- 

 mal in the Bennettitales, but are unknown in Cycadales, all of which 

 are not only monosporangiate, but are strictly dioecious. In the Co- 

 niferophytes nothing is known of conditions which preceded the 

 Cordaitales, and even in the Cordaitales themselves it can only be 

 said that there no bisporangiate strobili have been found. Gink- 

 goales are monosporangiate and dioecious. 



