94 THE ROYAL SOCIETY OF CANADA 



ancestral scalariform while those of the closely related Ephedra are 

 the outcome of the series of developments outlined in this paper. 

 Fig. 16, Plate IV, however, shows the Taxus type of rim in Gnetum 

 scandens itself, in the primary wood. At the left of the figure is a 

 scalariform tracheid, indicating proximity to the protoxylem. The 

 right-hand tracheid belongs to the metaxylem and in it the structures 

 referred to come out clearly between the bordered pits. In general, 

 the phylogenetic development of the metaxylem in a group of plants 

 follows along the same lines as that of the secondary wood, but is 

 less advanced, and in other families of the Gymnosperms where a rim 

 of Sanio has been observed in this region, it is of the Araucarian 

 type. The presence in the primary wood of Gnetum of bars such as 

 those illustrated is, therefore, further ground for assuming that the 

 long, well marked bars on the vessels of the secondary wood are a 

 culmination of the series of developments outlined. 



A study of various regions in the Dicotyldeons with reference to 

 the bar of Sanio is beyond the scope of this paper, but from the 

 appearance of the structures and the description of them in anatomical 

 literature there is no reason for assuming their origin to be different 

 from that of those found in the Gnetales. 



The Ginkgoaceae, of which Ginkgo biloba is the only living 

 species, will now be considered. In this form the pitting is generally 

 described as opposite, with bars of Sanio passing from side to side of 

 the tracheid. It is found on careful examination, however, that the 

 elimination of pitting has not gone so far in this as in higher forms, 

 and so the bordered pits and the primordial ones on which they are 

 formed have enlarged to a less extent than those of such forms as the 

 Abietineae. This can easily be seen by comparing the pitting of 

 Ginkgo as shown in Plate IV with that of Abies amabalis, which is 

 shown at less than half the magnification in Fig. 11, Plate III. 

 The compound primordial pit and bar of Sanio is thus arrested in this 

 form, in the midst of its development and, as would naturally be 

 expected, incomplete stages are very plentiful. 



Fig. 20, Plate V, is from a young stem of Ginkgo, near the pith, 

 and typical Araucarian pitting is shown. In the left hand tracheid 

 the pits are alternate and flattened to hexagonals by mutual contact — 

 a condition typical of the stem in the Araucarineae. This flattening 

 of the borders is apparently not a primitive condition for the forms in 

 which it is plentiful for it is not characteristic of the most primitive 

 regions of such forms as the Araucarians. In such places the pits are 

 rounded, and separated by appreciable bands of the cell wall (see 

 Fig. 4, Plate I). The crowding is due, apparently, to the general 



