96 THE ROYAL SOCIETY OF CANADA 



in comparison to the size* of the tracheid is small. Under these 

 conditions the bars naturally present a condition closely resembling 

 that found in the stem of Dloon spinulosum. The condition shown 

 in ¥\g. 26 is very common in adult wood — the pits uniseriate and rather 

 close to each other but with clear-cut bars between. In Fig. 27 the 

 secondary pitting has been almost completely eliminated, but the 

 bars are left. 



It is thus quite apparent that, owing to the partially arrested 

 development of the primary wall at a stage midway between the 

 Cycadean-Araucarian structure and that found in higher forms, the 

 species is peculiarly rich in evidence bearing on the development of 

 the compound rim and bar of Sanio. A complete series of develop- 

 mental stages is easily obtained from the one plant. The Araucarian 

 bar is seen in very conservative regions from which Jeffrey believed 

 that all bars were absent, and stages from this to the advanced 

 Abietinean type are scattered freely through the secondary wood of 

 root and stem. Such a condition is in complete agreement with the 

 line of development outlined in the present article. 



Summary 



1. A study of primitive forms of pitting indicates that soon after 

 short, bordered pits formed on the ancestral scalariforms, the scalari- 

 form primordial pits also gave way to rows of shorter ones. 



2. Rims of Sanio are probably not relics of the old, primary 

 scalariforms, but new structures resulting from an extra thickening 

 of the primary wall at the edges of primordial pits. 



3. The bars or " Querleisten " of Sanio are formed by vertical 

 fusion between the rims of closely approximated primordial pits. 



4. Numbers of the bordered pits which covered the radial walls 

 of tracheids decreased in size and were finally eliminated, after which 

 the remaining pits, both primary and secondary, enlarged. 



5. The growth of the secondary bordered pit failed to keep pace 

 with that of the primordial pit, which gave rise to rims of Sanio 

 at some distance from the edges of the borders. 



6. In all the higher Gymosperms the enlargement of primordial 

 pits caused an overlapping and fusion of horizontal rows into single 

 broad ones reaching across the tracheid. 



*It should be remembered that, as noted by Sanio, the primordial pit increases 

 in size with the growth of the tracheid. This being so the absolute size of the pri- 

 mordial pits is not of great phylogenetic interest, through their enlargement as 

 compared with the size of the tracheid on which they are placed, is significant. 



