Relationships of the Gnetales. 1085 
similar studies on the wood of Conifers and Dicotyledones. A transverse 
view in which the general characteristics and arrangement of all the elements 
may be distinguished is shown in Fig. 10. 
1. Tracheides. 
The pitting of the tracheides is usually uniseriate with the individual 
pits well separated (see Fig. 11). In this respect it agrees with that of all 
the Conifers except the Araucarians, and is sharply distinguished from that 
of the Cycads and Bennettitales. In the latter group the pits were almost 
always of the very primitive scalariform type, 1 while in the Cycads the 
multiseriate pitted condition is the dominant one, although the scalariform 
condition is not uncommon. 
The structure of the individual pits presents additional points of differ- 
ence. The pit mouths in Ephedra are circular, as in the majority of the 
Conifers, whereas in the Cycads they are more or less slit-shaped. The 
torus is also very well developed (Fig. 11), while in the Cycads it is absent. 
Although the pits are usually well separated, conforming thus to the 
4 Abietinean ’ type, one often finds the mutually compressed type or 
‘Araucarian’ condition. This is represented in Fig. 12. It is most 
frequent at the ends of the tracheides. Often also the more typical 
Araucarian condition with two alternating rows of compressed pits is to 
be found. Rarely are more than two rows present. 
In association with the Abietinean pitting the tracheides possess a 
feature which has been shown recently to be of primary importance, 
namely, the so-called bars of Sanio. These structures, named in honour 
of their discoverer, are horizontal bars or folds of cellulose crossing the 
tracheide wall between the pits. Stained in haematoxylin they appear 
as dark bands. They may be distinguished in the central tracheide of 
Fig. 11. Miss Gerry 2 has shown that they are present in all Conifers 
except the Araucarians, i. e. wherever the Abietinean type of pitting occurs. 
Outside the Conifers they are to be found only in Ginkgo , which also has 
uniseriate separated pits. Naturally they cannot exist where the pits 
are closely compressed. Their presence in Ephedra serves to distinguish 
this genus sharply from the Cycadales, and associate it rather with the 
Coniferales and Ginkgoales. 
The bordered pits and bars of Sanio are not confined to the radial 
walls of the tracheides as in most woods, but are found also on the tan- 
gential walls, as may be seen in PI. XCV, Fig. 13. In the Conifers tan- 
gential pits, when present at all, occur only in the summer wood, but in 
Ephedra they are to be found throughout the year’s growth. They are 
usually somewhat smaller than those of the radial wall. 
1 Wieland : loc. cit. 
2 Distribution of the ‘ Bars of Sanio * in the Coniferae. Annals of Botany, Jan., 1910. 
