88 THE ROYAL SOCIETY OF CANADA 



upper part of the right-hand tracheid are seen bars which, unlike 

 those in Figs. 3 and 4, pass beyond the area covered by the bordered 

 pit with which they are in contact. Their mode of origin is indicated 

 by the bar just above the three grouped pits at the bottom of the 

 figure on the same tracheid. The constriction in this bar shows- it to 

 be formed by an imperfect fusion end to end between two short bars, 

 one connected with the bordered pit and the other with an adjacent 

 primoridal pit on which no secondary pit has formed, and which 

 shows a tendency to disappear. On the tracheid to the left are shown 

 several bars, some of which strike the sides of bordered pits, indicating 

 that the primoridal pitting is alternate. These are interesting from 

 their pronounced thickness as compared with that of the normal 

 primary wall at the sides of the bordered pits, thus bringing out their 

 nature as superimposed structures. 



In many cases bars of exactly the appearance of those noted by 

 Thomson in the stem of Araucaria are seen in Dioon. Such structures 

 are plainly indicated in Fig. 6, PI. II, separating the six closely approxi- 

 mated pits in the upper half of the right-hand tracheid. Above this 

 group, where the pitting is more scattered, separate rims are in 

 evidence, closely approximated to the bordered pits. The structures 

 are plentiful in the remaining tracheids of the figure, of which the 

 one to the left is especially to be noted, the upper portion showing 

 bars of Sanio in a region from which secondary pits have been entirely 

 eliminated. 



The method of elimination of bordered pits to produce the 

 "scattered" condition is illustrated in the central portion of Fig. 6, 

 particularly in the second tracheid from the left. In regions of 

 elimination certain of the pits have become smaller than normal. 

 In carefully stained sections the primordial pits of normal size are 

 to be seen with these small bordered pits in the centre, but their 

 outline is so delicate as contrasted with that of the bordered pit that 

 it is difficult to reproduce in a photograph. The condition has been 

 caught by the camera in one case near the centre of the tracheid 

 under consideration. This is the most primitive living plant in which 

 bordered pits which do not cover the whole area of the corresponding 

 primordial pit have been recorded. Elimination pitting also occurs 

 in the Cordaitales and the Araucarineae, and is important in that 

 it makes way for the larger pits found in the wood of more specialized 

 forms. 



For convenience of presentation the remaining groups are con- 

 sidered in the following order: Cupressineae, Taxodineae, Abietineae, 

 Taxaceae, Gnetaceae and Ginkgoaceae. Needless to say, the arrange- 



