368 THE ANATOMY OF WOODY PLANTS 



which are of a primitive nature, we are unfortunately even less 

 well informed as to their fossil representatives than in the case 

 of the angiosperms. There are, in fact, no well-authenticated 

 gnetalian remains, even from the later period of the Mesozoic, 

 in which the dicotyledons had become well established as an 

 important component of the plant population of the earth. The 

 wide geographical distribution of the three living genera may 

 perhaps be regarded, in conjunction with their anatomical organ- 

 ization, as a definite indication of their earlier more abundant 

 occurrence. 



The root in the Gnetales needs no special consideration in the 

 present connection. In the genus Gnetum it furnishes some 

 evidence as to the original organization of the large rays, but in 

 Welwitschia it is polydesmic like the stem and is of little value from 

 the standpoint of evolutionary anatomy. The structure of the 

 root in Ephedra closely resembles that in the stem, except as 

 regards those general features which distinguish root from axis. 



The foliar organs of the group which forms the subject of the 

 present chapter are distinctly gymnospermous in their anatomy 

 and frequently exhibit the copious development of transfusion 

 tissues of the type characteristic of the leaves of the Ginkgoales 

 and Coniferales. Centripetal wood is conspicuously absent in the 

 leaves of the Gnetales, unless it be assumed that the transfusion 

 elements are actually representatives and not merely derivatives 

 of the old centripetal or cryptogamic wood. On account of the 

 small size of the leaves in Ephedra the transfusion tissues are 

 relatively poorly developed. In Gnetum a higher systematic 

 position is strongly vouched for by the organization of the leaf, 

 in which transfusion elements are not particularly well developed. 

 The foliar organ of Welwitschia supplies us with transfusionary 

 features most strongly manifested. The truth of this statement 

 may be verified by reference to Fig. 265, which illustrates a part 

 of a transverse section of the leaf of the South African genus. 

 Transfusion elements originating on the flanks of the xylem extend, 

 as in certain Coniferales, above and below outside the sclerenchyma- 

 tous sheath which surrounds the fibrovascular bundles of the 

 leaves. 



