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 substancfc 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 



