62 ANATOMY OF THE GYMNOSPERMS 



Such a comparison brings into strong relief the fact that the 

 Araucarias, both past and present, constitute a transitional group 

 with a somewhat wide range of variations, and in this respect 

 they may be said to stand between the more stable Cordaites 

 and Dammara on the one hand, and the far more variable Coni- 

 ferae on the other, since in Dammara australis we find essentially 

 the same features of structure and distribution as in Cordaites, 

 the pits being one- to three-seriate. The sequence presented 

 above may be held to be in the inverse order of development, 

 and A. Robertianum must therefore be held to represent the 

 most primitive form. 



It is apparent that in Cordaites, Araucaria (including Arauca- 

 rioxylon), and Dammara the pits are invariably distinguished by 

 two constant features, -- (i) their hexagonal form, and (2) their 

 very compact disposition throughout the entire extent of the 

 tracheid. They often deviate from the multiseriate arrangement 

 typical of the group as a whole in that in individual cases they 

 are reduced to a one-seriate arrangement. They thus tend to 

 overlap the next group, which is distinguished by a two-seriate 

 disposition, but any confusion which might arise from this cause 

 may be overcome by reference to the special form and disposi- 

 tion of the pits, as will more fully appear in the following lines. 



Among the remaining Coniferales twenty species of various 

 genera, or 17.2 per cent in all, show a two-seriate arrangement, and 

 to this group we must also add the Gingkoales and various fossil 

 species. Here the multiseriate disposition of the pits involves 

 features which at once distinguish the group as a whole from the 

 preceding, clearly placing it upon a higher plane of development. 

 The pits are never hexagonal but are generally elliptical or round, 

 while they also show a strong tendency to extreme segregation. 

 When brought into a compact arrangement, as in Cupressoxylon, 

 Sequoia, or various species of Finns, they are flattened only along 

 the lines of limited contact, which are usually confined to one 

 end of the pit (fig. 8). A very characteristic feature of this group is 

 the further fact that the two-seriate arrangement is not constant, 

 either in the same section or in the same tracheid. Both Pinus 



