384 GENERAL MORPHOLOGY OF PLANTS 



plant material 280-450 meters thick (900-1500 feet in thick- 

 ness). In the United States there are several hundred thousand 

 square miles of coal-bearing areas, of which about fifty thousand 

 are worked. This gives an opportunity to see what an enormous 

 amount of vegetation must have existed during that age, con- 

 sidering the fact also that much of it must have decayed before 

 the geological changes occurred which submerged the material 

 converted into coal. In the coal beds very little of the plant 

 remains is preserved because the great heat consumed them and 

 changed them largely to carbon. But numerous impressions 

 remain which enable the paleobotanist to determine the nature 

 of the plants which flourished on the earth at that time. These 

 impressions and the carbonized remains of stems are more evi- 

 dent in the soft coal, but evidences are also found in the hard 

 coal beds to indicate that they also are the remains of plants. 



537. The remains of the fern plants found in the coal measures 

 of the Carboniferous Age are sufficient to show that the number 

 of genera and species was far greater at that time than at the 

 present day. The evidence also shows that they were much 

 larger in size. Besides the tree ferns, the lycopods were of tree 

 size as were the closely related plants Lepidodendron and Sigil- 

 laria. Equisetum-\ike plants called Catamites were also of tree 

 size. There were also many other tree forms which are extinct 

 to-day. There was much more moisture and carbonic acid in 

 the air at that age. This probably accounts to some extent for 

 the luxuriance of the vegetation. It is possible that the greater 

 amount of moisture occasioned large areas of swampy and wet 

 ground, for these tree forms of the lycopods, horsetails, etc., flour- 

 ished in the wet, marshy places. This may have led to a condi- 

 tion of things similar to the peat moors of the present day, where 

 decay of plant parts is only partial and the firmer parts are 

 even preserved from decay. In this way the deep layers of plant 

 material may have accumulated, and later, by the subsidence of 

 the earth's crust at these points, they may have been covered with 

 water for long periods, during which deposits of another char- 

 acter covered them which later formed the rock stratum overlying 



