370 THE ANATOMY OF WOODY PLANTS 



while in the other two genera they are invariably so characterized. 

 The phenomenon of fusion of the pits to form the scalariform 

 perforations present in the lowest type of vessel of the dicoty- 

 ledons is rare in the woods of the Gnetales. The general 

 anatomical organization of the fibrovascular tissues in the Gnetales, 

 however, may be said distinctly to favor the hypothesis of dicoty- 

 ledonous affinities, or, at any rate, to be more in harmony with 

 such relationship than with that of any other group of seed plants. 

 The affinities of the Gnetales on the gymnospermous side are 

 much more difficult to discover. This situation is due no doubt 

 in large part to the fact that as yet no properly authenticated 

 fossil representatives of the group have been found. In recent 

 years there has been a tendency to associate this group with the 

 Cycadales, but it is extremely difficult to discover any valid 

 reasons from the anatomical side to justify such a relationship. 

 There seem, in fact, to be no anatomical features of wide validity 

 which can be invoked in favor of an affinity between the highest 

 living gymnosperms and the lowest. It seems on the whole 

 more probable that the Gnetales are cognate with, or derived 

 from, the Coniferales, since there are a number of features which 

 make such a connection likely. The transfusion tissue of the 

 highest gymnosperms is clearly of the coniferous type, and this 

 feature must count strongly against any close relationship with 

 the Cycadales, in which, owing to the strong persistence of the 

 centripetal wood, true transfusion tissues have not yet made their 

 appearance. The uniseriate rays of Ephedra and Gnetum also 

 point to coniferous affinities and not toward a connection with 

 the Cycadales, in which the rays are universally multiseriate. 

 The presence of bars of Sanio in the vessels of Ephedra supplies 

 likewise an argument for the coniferous rather than the cycadalian 

 relationship. 



The main support for the cycadean origin of the gymnospermous 

 group which forms the subject of the present chapter has been 

 derived from the reproductive structures of the living and extinct 

 representatives of the cycadean stock. The investigations of 

 Wieland have brought to our knowledge the complete organization 

 of the reproductive parts in the Bennettitales. These consist 



