[sifton] bar of SANIO IN GYMNOSPERMS 93 



seedlings the rims are closely approximated to the bordered pits in 

 true Araucarian fashion while in the adult stem they have drawn away 

 and are often much elongated. The same adult structure is found in 

 Podocarpus, Saxagothaea, Phyllocladus, Torreya and Taxus. A 

 further tendency in the evolution of the primordial pit is illustrated in 

 the family, however, and it is important because it apparently leads 

 up to the Gnetalian and lower Dicotyledonous types. The vertical 

 height of the enlarged primordial pit lessens so that while it still 

 reaches from side to side of the radial wall of the tracheid, its edges 

 approach those of the bordered pit above and below. Fig. 15, 

 Plate IV, is a radial section of the root of Torreya micifera where this 

 development has not yet been consummated. Tertiary thickenings, 

 characteristic of the Taxineae, are present, but the primordial pits 

 and rims of Sanio are of the Abietinean type. Fig. 19, Plate IV, is 

 a radial section of the stem of Taxus cuspidata, where the tertiary 

 thickenings are still more in evidence. The rims and bars of Sanio 

 are faint but may be made out quite clearly in several of the tracheids. 

 In the Pinaceae such primordial pitting has been noted in regions 

 where the elimination of bordered pits has not gone far enough to 

 permit of vertical expansion, but here it is seen even where the pits 

 are separated, for example, near the centre of the third tracheid from 

 the left, and the condition in primitive Taxaceae, as well as in the 

 more primitive parts of the Taxineae themselves (e.g., the root of 

 Torreya, Fig. 15), suggests that it is a further modification of the 

 specialized compound primordial pit. 



Fig. 17, Plate IV, is a radial section of the stem of Gnekim scandens 

 showing a part of the wall of one of the enormous vessels characteristic 

 of this vine-like form. Between the rows of pits are particularly 

 heavy bars of Sanio outlining long, primordial pits which suggest 

 those of the lower Angiosperms. Their vertical height is very small 

 when compared with the width of a complete vessel, so that, as in 

 the Dicotyledonous form, they at once suggest the primordial scalari- 

 forms of Fig. 1, Plate I. A consideration of the wood of other members 

 of the Gentales, and of conservative regions of Gnetiim scandens itself 

 suggests a different origin. 



Fig. 18, Plate IV, is a radial section of the stem of Ephedra 

 gerardiana, where the vessels are not so large. Rims of Sanio are 

 not strongly marked but are quite evident, especially in the element 

 to the right. They and the primordial pits enclosed are similar to 

 those of Taxus, and there is nothing to suggest a different origin. 



Were no further evidence at hand it could scarcely be considered 

 probable that the rims of Sanio in Gnetum are simply relics of the 



