PLANTS OF PAST AGES 29 
their fortunes of still living members of the plant 
kingdom. These fragmentary relics, ‘the ghostly 
language of the ancient earth,’ suggest problems 
that are more easily stated than solved. 
Many records of ancient floras are readily de- 
cipherable, foliage shoots and clearly outlined 
leaves showing the finest veins, the plant substance 
changed into a thin film of coaly substance which 
on treatment with certain chemicals reveals under 
the microscope details of the surface cells and 
throws light both on the affinities of the plants 
and on their relation to the world in which they 
lived. The minute structural details of petrified 
wood after it has been cut into transparent sections 
can be examined with as much thoroughness as 
those of a living stem; the living substance has 
gone, but the framework remains and through it 
we obtain an insight into the mechanism of the 
plant which was alive some millions of years ago. 
Other fossils are but ‘age-dimmed tablets traced 
in doubtful writ,’ and these add zest to the task of 
interpretation. 
Two among the many problems which exercise 
the ingenuity of geologists and botanists may be 
mentioned: if, as seems certain, the climate of 
Greenland was warm enough to support a vegeta- 
tion including forest trees and other plants closely 
related to species now growing in warm temperate 
and sub-tropical districts in North America, south- 
ern Europe, China and elsewhere, what causes can 
be invoked to produce the necessary change? Of 
the plants that exist in Greenland some occur also 
