iqiq] bailey— bars OF SANIO 455 



somewhat more conservative or slower to change than others, 

 considerable difficulty must inevitably be encountered in formulat- 

 ing such facts as these into laws for use as "short cuts" in the study 

 of phylogeny. 



That this is likely to be the case in dealing with the Ginkgoales 

 and Coniferae is indicated by a number of facts in the comparative 

 anatomy and ecology of the Pteridophyta and Gymnospermae. In 

 the evolution of these groups the primary, as well as the secondary, 

 tissues appear to have been considerably modified. For example, 

 the more primitive vascular plants were characterized by having 

 relatively wide zones of primary xylem; whereas the Coniferae 

 have usually only a relatively limited amount of this tissue, which 

 is correspondingly specialized in structure. 



Structure and distribution of bandlike thickenings of middle 

 lamella in Pteridophyta, Gymnospermae, and Angiospermae 



In view of the fact that much emphasis has been placed upon 

 bars of Sanio in the identification of fossil woods of the Mesozoic, 

 and that these structures have been used as the basis for important 

 but conflicting generalizations in regard to the phylogeny of the 

 Coniferae and the relative conservatism of different organs or regions 

 of plants, the structure and distribution of bandlike thickenings 

 of the middle lamella in the Pteridophyta, Gymnospermae, and 

 Angiospermae deserve more careful consideration than they have 

 received heretofore. 



» 



As is well known, the metaxylem of most Filicales is composed 

 largely of scalarifomi tracheids. The bordered pits in these 

 tracheary elements are much elongated horizontally, at right angles 

 to the long axis of the tracheids, and are closely approximated in 

 vertical series (fig. 2) . The elongated bordering areas of the second- 

 ary walls are exactly superimposed over attenuated areas of the 

 middle lamella; and the outlines of these areas are more or 

 less effectively concealed by the margins of the bordering areas. 

 The primary pit areas are separated by narrow, bandlike, thicker 

 portions of the middle lamella, which, in carefully stained^ longi- 

 tudinal sections of the xylem, appear as fine dark lines between the 



^ Haidenhain's iron-haematoxylin and safranin. 



