GNETALES 403 



features. It is not worth while to arrange the three genera in a series, 

 for this would only involve the- impossible determination as to whether 

 Welwitschia or Gnetum is the more advanced in some selected direc- 

 tion. Certainly they can hold no such serial relation to one another, 

 for they must represent divergent lines from some general common 

 stock. So far as the features of the embryo sac can determine advance- 

 ment, as measured by the angiosperms, Gnetum is more advanced than 

 any other gymnosperm. All that is necessary, however, is to recognize 

 the facts that the group as a whole, while clearly related to other 

 gymnosperms, is remarkable for its angiospermous features, and that 

 Welwitschia and Gnetum are farther removed from the other gymno- 

 sperms in this regard than is Ephedra. 



The most interesting question in connection with the relationships 

 of Gnetales is their possible relationship to angiosperms. It was 

 natural at first to regard them as the immediate progenitors of angio- 

 sperms, for their structures seemed to point irresistibly to that con- 

 clusion. When transition groups were in vogue, there was no better 

 illustration of one than the Gnetales. But those who are now claim- 

 ing a phylogenetic connection between Gnetales and angiosperms 

 do not claim it in the sense that the former is a transition group. To 

 many it seems very improbable that there is any cormection between 

 the two groups, except one so distant that it does not fall within the 

 claim of connection. After a critical cytological investigation of the 

 female gametophyte and of fertilization in Gnetum, Strasburger 

 has concluded (19) that it is impossible to derive the angiospermous 

 embryo sac from that of Gnetum, and that such resemblances as do 

 exist are illustrations of phylogenetic parallels. 



Arber and Parkin (24) have applied their strobilus theory of 

 angiospermous descent to an interpretation of the origin of Gnetales, 

 and the result may be regarded as combining the two views mentioned 

 above. This theory was referred to in cormection with the Bennet- 

 titales (p. 74). These authors do not regard the Gnetales as a very 

 modern group, in spite of the fact that they are unknown as fossils. 

 It is claimed that the three survivors of this ancient group have " pro- 

 anthostrobili" (an "anthostrobilus" is an axis bearing both micro- 

 sporophylls and megasporophylls, with the latter above the former; 

 and a "proanthostrobilus" is one in which the ovules are gymno- 



