402 MORPHOLOGY OF GYMNOSPERMS 



LoTSY has described (17) the occurrence of parthenogenesis in 

 Gnetum Via. When the embryo sac was full of loose tissue, no pollen 

 tubes were found, but many of the loose cells behaved like fertilized 

 eggs in sending out tubes. 



S. History and distribution 



At present Gnetales are unknown as fossils, but they give, evidence 

 of a considerable history (24). Their great dissimilarity in habit, 

 structure, and habitat, associated with their widely scattered distri- 

 bution, indicates a relatively large group of ancestors of somewhat 

 general distribution. Even if this should prove to be the case, the 

 group is doubtless a modern one among gymnosperms. 



The present geographic distribution of Gnetales has been indicated 

 already (p. 363). Ephedra is distinctly a genus of arid regions, its 

 species ranging throughout the dry borders of the Mediterraneasn 

 and extending eastward into the Caspian region of Asia, and in 

 similar habitats in both North and South America. Tlie species of 

 Gnetum, on the contrary, belong to the tropical forests of both hemi- 

 spheres; while Welwitschia is isolated from its congeners in a desert 

 waste of western South Africa. The facts of this distribution can 

 be stated, but their meaning is entirely obscure. 



6. Relationships 



The origin of Gnetales is very obscure, and in the absence of his - 

 tory any statement in reference to it must be regarded as a guess. 

 It is clear that Ephedra is more closely related to the other gymno- 

 sperms than are Welwitschia and Gnetum; and that if it is to be con- 

 nected with them at all, it is most reasonably connected with the 

 Coniferales. This general connection is so uncertain that any 

 attempt to select for it a particular family or tribe of conifers is pecul- 

 iarly unprofitable. It must be understood that the connection 

 referred to may mean only the parallel development of the two groups. 



It is equally clear, whatever may be the connections of Ephedra 

 with other gymnosperms, that it cannot be separated from Welwitschia 

 and Gnetum. These two genera have many things in common with 

 Ephedra, but they have more things in common with one another, and 

 have advanced much farther in the development of angiospermous 



