FOSSIL PLANTS AS RECORDS 171 



graphy of the Tertiary period. The causes for such 

 marked changes of climate must be left for the con- 

 sideration of geologists and astronomers. Plants are 

 passive, driven before great climatic changes, though 

 they have a considerable influence on rainfall, as has 

 been proved repeatedly in India in recent times. 



From the more distant periods it is the plants of the 

 Carboniferous, whose structure we know so well, that 

 teach us most. Although there is still very much to 

 be done before knowledge is as complete as we should 

 wish, there are sufficient facts now discovered to correct 

 several popular illusions concerning the Palaeozoic period. 

 The "deep, all-enveloping mists, through which the sun's 

 rays could scarcely penetrate ", which have taken the 

 popular imagination, appear to have no foundation in 

 fact. There is nothing in the actual structure of the 

 plants to indicate that the light intensity of the climate 

 in which they grew was any less than it is in a smoke- 

 free atmosphere to-day. 



Look at the "shade leaves" of any ordinary tree, 

 such as a Lime or Maple, and compare them with those 

 growing in the sunlight, even on the same tree. They 

 are largfer and softer and thinner. To absorb the same 

 amount of energy as the more brilliantly lighted leaves, 

 they must expose a larger surface to the light. Hence 

 if the Coal Measure plants grew in very great shade, 

 to supply their large growth with the necessary sun 

 energy we should expect to find enormous spreading 

 leaves. But what is the fact? No such large leaves are 

 known. Calamites and Lepidodendron, the commonest 

 and most successful plants of the period, had narrow 

 simple leaves with but a small area of surface. They 

 were, in fact, leaves of the type we now find growing in 

 exposed places. The ferns had large divided leaves, 

 but they were finely lobed and did not expose a large 

 continuous area as a true "shade leaf" does; while 

 the height of their stems indicates that they were grow- 

 ing in partial shade — at least, the shade cast by the 



