[sifton] bar of SANIO IN GYMNOSPERMS 87 



Fig. 3, Plate I, also from a Cycas petiole, shows tracheids repre- 

 senting a stage of development beyond that shown in Fig. 2. It 

 will be noticed that the bordered pits are not crowded over the entire 

 surface of the tracheids as in the former figure. In the upper part 

 of the tracheid at the right several bordered pits are plainly missing. 

 The primordial pits remain and are shown as lighter areas surrounded 

 on all sides by dark lines where the middle lamella has not become 

 thinned. Primordial pits on which bordered pits are not super- 

 imposed appear darker in the photograph than the others, owing to 

 the fact that the total thickness of the secondary wall covers them. 

 Similar areas, though with the primordial pits less sharply outlined, 

 are present in other parts of the section. Another important develop- 

 ment is also present. Above and below certain of the pits the middle 

 lamella is thicker than at their sides, a fact indicated by the deeper 

 staining. This is true not only where opposite pitting is in evidence, 

 but in cases where the pits are alternate and where, if Bailey's theory 

 is correct, the thick part should form a reticulum enmeshing the pits. 

 Several points suggest very strongly that this is an extra thickening 

 of the membrane at the edge of the primordial pit and not merely 

 the normal, unthinned middle lamella. Thus in the left hand tracheid 

 of the figure there are to be seen thickenings in the form of arcs closely 

 applied to the edges of the bordered pits. I have not applied micro- 

 chemical tests to ascertain whether these arcs are of pure cellulose 

 or of a pectic nature as Groom and Rushton found them to be in 

 Pinus Merkusii, but their form and position leave no room for doubt 

 that they are true rims of Sanio. Where the pits are closely approxi- 

 mated the two adjacent rims fuse forming Sanio's "Querleisten." 

 These structures, as in a former paper (10), will be referred to as 

 "bars " of Sanio. In one or two cases, where the pits are not crowded, 

 the rims are separate from each other, e.g., between the second and 

 third pits above the pair of opposite ones. 



Fig. 4, Plate I, shows the same structures in the cone axis of 

 Araucaria Bidwilli, where they were discovered independently by 

 Jeffrey and by Thomson. In two tracheids of this figure the pitting 

 is opposite, and if Bailey's theory of the bar were correct the structures 

 should extend in unbroken lines from side to side of the tracheid. 

 This, however, is not the case. Each pit has a rim clinging to its 

 edge with the result that small diamond-shaped areas of a lighter 

 colour can be seen between the pits. 



Fig. 5, Plate I, illustrates bars of Sanio as seen in a radial section 

 of the stem of Dioon spinulosum. The pitting is more or less scattered 

 owing to the elimination of certain of the bordered pits. On the 



