The Evolution of Plants 



319 



mud may be covered by other layers of sediment. When the 

 mud dries, a perfect imprint of the outline and veins may be 

 left. As time goes on and 

 the mud becomes more 

 deeply buried, it may harden 

 into rock and retain the im- 

 print of the leaf as a record 

 of a plant that lived when 

 the rock was merely soft [^ f^ 

 mud. In this way leaves, 

 fruits, seeds, stems, and 

 roots have left their im- 

 prints to testify, thousands 

 and millions of years after- 

 ward, to their former ex- 

 istence. 



Plant remains also ac- 

 cumulate in deep water or 

 in water containing large 

 amounts of mineral matter in solution. In such places they 

 may not decay, but the material of which they are composed 

 may gradually be oxidized and replaced by the mineral 

 substances in the water. Under the most favorable condi- 

 tions the internal structures of the plant are preserved. As 

 animal remains are preserved in the same way, we ha\e m the 

 rocks a record of the plants and animals of the past. These 

 petrified plant and animal remains and the plant and animal 

 imprints from former geological ages are called fossils. 



WTien large collections of fossils are studied, we find in the 

 oldest fossil-bearing rocks few, if any, plant remains. This 

 is as we should expect, for plants like algai and Uverworts are 



Fig. 192. Fossil imi»rints of fern leaves in 

 a rock of the Carboniferous period. 



