Dec, 1919] Dieciousness in Thalictrum Dasycarpum 33 



The Tubiflorse present the same remarkable conditions as 

 other groups, having closel}^ related bisporangiate and diecious 

 species. In the Olive family, many of the genera are bisporan- 

 giate while Fraxinus has all gradations from bisporangiate to 

 completely diecious species. In the Tubiflorce, just as in the 

 higher Monocotyls, the zygomorphic condition seems to inter- 

 fere with the development of monosporangiate flowers. Perhaps 

 the reduction of stamens in the extreme zygomorphic flowers is 

 the direct cause ; for a further mutation to the monosporangiate 

 condition would probably subject the species to too severe a 

 struggle for life. In such forms cleistogamy and self-pollination 

 are more apt to arise, although only sporadically. The extreme 

 Plantaginace^ are usually considered to be related to the 

 Tubiflorae, but if so they are an isolated group long separated 

 from' the main branch. They have bisporangiate, imperfectly 

 bisporangiate, or monecious flowers with various intergradations. 



Finally, the Infers are no exception and show the same evo- 

 lutionary gradations from the bisporangiate to the monospo- 

 rangiate condition as the lower subclasses. Among the Com- 

 positales, the lower species are bisporangiate or imperfectly 

 bisporangiate while various, related specialized species are 

 monecious or diecious. For example, the Solidagos and Asters 

 have bisporangiate disk flowers while the related Baccharis is a 

 genus of diecious plants. Gnaphalium has at least part of the 

 flowers bisporangiate while its near relatives, Anaphalis and 

 Antennaria are diecious, some species of Antennaria showing 

 considerable sexual dimorphism. In general, the distribution of 

 bisporangiate and monosporangiate flowers on the heads of the 

 Composites is exceedingly interesting and instructive and recalls 

 similar distribution of spikelets in some of the higher grass 

 inflorescences. Such distributions show that the various sex 

 conditions are not at all due to Mendelian segregations but to 

 differentiations arising in tissue systems having a common 

 origin and presumably a similar hereditary nature. The prob- 

 lem is much more complicated than the simple shifting of an 

 hermaphrodite condition in the individual to a unisexual one. 

 For instance, in the genus Artemisia, some species have all the 

 flowers of a head bisporangiate, some species have the central 

 flowers of the head bisporangiate and seed bearing while the 

 marginal flowers are carpellate, some species have the central 



