igig] 



BAILEY— BARS OF SANIO 463 



or the relatively small tracheids of the first formed portion of the 

 secondary xylem. It is in these larger thin-walled tracheids that 

 the most typical scalariform primary pit areas tend to occur. 

 Occasionally, where the tracheary pitting is very strongly developed, 

 scalariform bordering areas of the secondary wall are superimposed 

 over portions of these elongated primary pit areas (fig. 23). This 

 is likely to occur in Taxodium and Ginkgo and roots of certain of the 

 Abieteae. As has previously been shown, the scalariform structure 

 of the middle lamella and narrow straight Querleisten become gradu- 

 ally modified with increasing reduction in the number of bordered 

 pits. Furthermore, it has been shown that a similar widening of 

 the bandlike thickenings of the middle lamella may occur in certain 

 of the Pteridophyta, as well as the Angiospermae, when the bordered 

 pits tend to become more or less isolated. 



The occurrence of these interesting structures in the Abieteae, 

 Taxodieae, Cupresseae, Taxaceae, and Ginkgo, and their absence 

 in the secondary wood of Araucarieae, are difficult to explain upon 

 the assumption that the former groups are descended from ancestors 

 having "alternate multiseriate" pitting. On the other hand, from 

 analogy with similar phenomena in other groups of vascular plants, 

 their occurrence is easily accounted for if the microphyllous and 

 relatively xerophytic Coniferae are descended from forms having 

 scalariform tracheary pitting. 



Primary pit areas of cambium and their relation to the pitting of 



xylem and phloem 



The important observations of De Bary, Janczewski, Russow, 

 Strasburger, Kienitz-Gerloff, KRUGER,-and others upon the 

 occurrence of bandlike thickenings of the middle lamella in the cells 

 of the cambium, and their relation to similar structures in the ele- 

 ments of the xylem and phloem, have been overlooked entirely in 

 discussions concerning the phylogenetic significance of the so-called 

 rims or bars of Sanio. Kruger (8) found that, in the cambia of all 

 plants (stems and roots of gymnosperms, dicotyledons, monocoty- 

 ledons, including trees, shrubs, herbs, and succulents) investigated 

 by him, there were leistenjormige Verdickungen in the radial parti- 

 tions (figs. 17, 18). These ridgelike thickenings separated roundish 



