1915] BURLINGAME—ARAUCARIANS 15 
leaf gaps, there has been a very general disposition to accept this 
distinction as entirely valid so far as it concerns the Pteropsida. 
Striking as is this fact in the other groups of the Pteropsida, it is 
preeminently so among the conifers, a small-leaved group where it 
is not only present in the mature stem but also in the seedling and 
reproductive axes (31). It is true (61) that the cladosiphonic ° 
exarch stems of the ancient lycopods did occasionally become 
medullated, and it is possible that in the course of evolution some 
member might have lost all of its centripetal wood, and have 
developed centrifugal wood and leaf gaps, but there is no evidence 
as yet that any of them ever actually did either. 
The histological structure of the stem is only less strikingly 
uniform among gymnosperms than the general organization. In 
fact, the wood of araucarians and cordaiteans is so nearly identical 
that no absolutely trustworthy tests have yet been discovered 
for distinguishing them. Although the other gymnosperms do not 
all have exactly the same arrangement of the bordered pits, they 
do all have such pits on the radial walls of the tracheids, and they 
are, on the contrary, with a single exception (48, 53), unknown 
among the lycopods. While there is greater diversity in the phloem, 
perhaps that of lycopods differs still more widely. 
Aside from the Araucarineae, the structure of the ovulate cones 
is more readily brought in line with a filicinean than a lycopodinean 
ancestry. Though the ovulate cone readily lends itself to the 
derivation of araucarians from lycopods, it can nevertheless be 
explained in terms of the Cordaitales; while, on the contrary, it 
is very difficult to explain the cone of a pine in terms of a lycopod 
ancestry, and next to impossible to so explain those of cycads. 
The structure of the seed is remarkably uniform through the 
entire phylum, from the oldest to the living representatives. Very 
few lycopods (3, 51, 53) are known to have borne seeds of any 
kind and even those are much simpler than those of any gymno- 
sperms. I have elsewhere (7) pointed out that these seeds do offer 
us an analogy of the way in which the peculiar pollination processes 
of the araucarians may have originated. It may be objected, 
however, and I think rightly, that if lycopods had developed high 
grade seeds, they would have been likely to parallel the structures 
