294 ESSENTIALS OF BOTANY 



our coal is made up of the trunks and branches of these 

 great plants and even of their enormously abundant spores. 



The leaves and stems of extinct species of ferns and 

 of primitive gymnospermous trees are also important con- 

 stituents of coal. Tree-like plants allied to the horsetails 

 were conspicuous features of the Carboniferous landscape, 

 and their remains are characteristic fossils in many coal 

 deposits, but they did not contribute much to the fuel 

 value of the deposits. Our knowledge of the luxuriant 

 vegetation from which coal was made has been gained from 

 the study of the fossil plants and parts of plants recogniz- 

 able in coal itself and in the rocks which lie beneath or 

 above it. In some instances the roots of trees are found 

 well preserved in underlying clays, while their trunks and 

 branches rise through the coal seams above. It is probable 

 that some coal beds have been derived from immense peat 

 bogs (Sect. 359). 



Living pteridophytes are of little economic importance. 

 Many species of ferns and some of Selaginella are cultivated 

 for ornament. The rootstocks of one species of fern are 

 somewhat used in medicine, young shoots of the bracken 

 fern are edible, and the spores of various species of Lyco- 

 podium are still occasionally used by apothecaries to dust 

 over the surfaces of pills and for other purposes. 



