168 
probably stands as the most ancient repre- 
sentative of the heterosporous filicineze we 
know. Other less conspicuous cases might 
also be cited, but the general fact that it is 
only the more resisting structures, such as 
those of the stem and leaves, which are 
recognizable, has led to attaching great im- 
portance to the anatomy of the wood. This 
was first pointed out by Witham, who has 
since been followed by many of the leading 
paleobotanists of Europe, and to some ex- 
tent by those of America. On this side of 
the Atlantic, the idea was first taken up by 
the late Sir William Dawson in 1845, and 
since that time its application has been 
made more prominent through the work of 
Knowlton and others. 
As a special indication of climate, the 
presence of growth rings—from the time of 
Witham who considered the absence of 
such structural features to be evidence of 
a tropical climate—has been regarded as 
one of the important factors in this respect. 
The history of the subject shows a great 
divergence of opinion as to the origin of 
these structures, but the general position 
may no doubt be correctly taken, that 
growth rings are largely dependent upon, 
and are therefore expressive of alternating 
periods of growth and rest. Such perio- 
dicity in functional activity bears amore or 
less close relation, among other things, to 
seasonal change in such a way that, other 
things being equal, the more definite the 
changes from summer to winter, the more 
pronounced will the growth rings be- 
come, and the more definite will be their 
correspondence with age. That this holds 
true only within certain limits is well 
known. Thus many Chenopodiaceous plants 
make several growth rings within one sea- 
son independently of latitude, and the red 
maple, within the latitude of New York 
State, has been known to make twelve 
growth rings within a period of eight years. 
Fernow has recently drawn attention to 
SCIENCE. 
[N. S. Von. XIII. No. 318. 
the fallacy of many of the opinions respect- 
ing the value of growth rings as an index 
of age, and the correctness of the views 
which hold them to be of importance in 
this respect, and he points out that under 
ordinary circumstances in temperate lati- 
tudes the ages of trees may be correctly 
ascertained within a limit of one-half to 
one year of error. Tropical trees generally 
exhibit an absence of growth rings, but ex- 
ceptions to this rule are not unknown. It 
would therefore appear that while much 
caution must be exercised in applying such 
a test to the case of fossil plants, when 
used with a due recognition of its limita- 
tions, the test of growth rings as evidence 
of climate becomes a valuable aid. 
One of the most common methods of 
identifying fossil plants has been through 
their leaves. This has of necessity resulted 
from the fact that these organs are much 
better adapted to preservation than most 
other parts of the plant, and that they 
therefore constitute the most accessible of 
all plant remains. The work of Lesquereux 
and Newberry rests very largely, indeed al- 
most exclusively, upon evidence of this sort, 
and the venation has been made the basis 
of both generic and specific distinctions. 
Leaves, however, offer an unreliable basis 
when taken alone, and this results from the 
fact that the venation is not of specific 
rank, and often fails also in generic signifi- 
cance, while the form of the Jeaf may pre- 
sent so many changes within the limits of 
the species that no great dependence can 
be placed upon it. This is notably the case 
in such genera as Liriodendron, Sassafras 
and Platanus, and it is only necessary to 
refer to the comparative studies by Ward, 
on the ‘ Paleontologic History of the Genus 
Platanus,’ and to the observations of Holm 
on Liriodendron, to give emphasis to facts 
long known to botanists, and show what 
erroneous and extremely misleading results 
may readily follow from the study of fossil 
