58 ROYAL SOCIETY OF CANADA 
end and bordered pits at the other.! More recently Williamson has 
shown that a regular transition from one to the other form, occurs in 
the wood of Dadoxylon and Dictyoxylon, and he rightly attaches im- 
portance to these facts as showing the derivation of pitted structure 
from scalariform.? At the same time, however, he falls into the error 
of misinterpreting certain structural features, and is thus led to wrong 
conclusions upon which he erects a new genus distinguished from 
Dadoxylon by details of structure in the bordered pit. It is not my 
purpose to discuss this aspect of the question at length on the present 
occasion, but it must be pointed out that the simple pits “ without a 
central pore,’ which he regards as the essential characteristic of 
Dadoxylon, have without much doubt originated either in conditions of 
extreme decay, or in mechanical treatment incident to preparation of 
the sections, one or both. My own studies have shown several instances 
where a similar interpretation of appearance could be made, but in every 
case a more searching examination has always disclosed the true nature 
of the appearance thus presented. Such structural transitions as have 
just been shown to occur, are of the greatest interest and importance 
from a phylogenetic point of view. They are what one might reasonably 
look for upon the hypothesis that these woods are the most primitive 
Gymnosperms, and they serve to throw much light upon the origin and 
relationships of the various vascular elements in the higher plants, both 
Angiosperms and Gymnosperms. 
The typical structure of the bordered pit in these plants is too well 
known to require explanation or amplification at this time, since it is 
correctly given in all the diagnoses by leading authorities, but by way 
of recapitulation, it may be well to state that the pits are generally 
hexagonal and provided with an oblong or narrowly lenticular, diagonal 
pore which is three-fourths the diameter of the pit, or more (fig. 11 and 
18). When the number of rows. diminishes to one, the pits commonly 
assume a more oval form but remain more or less compressed; and a 
still further modification appears in the separation of the pits to such 
an extent that they become round, as may be seen in Araucaria where 
such transitional forms occur. Kraus has employed these facts as the 
basis of classification, distinguishing between certain genera which 
show on the one hand “Pori rotundi, vel contiguitate polygoni,” and 
on the other hand “Pori compressi, oblongt.” 

1 Jn’1 Lin. Soc., 1840. 
2 M. Mic. Jn’l, 1869, 67-70. 
8 Schimper, Pal. Veg., 370. 

