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. 859). 
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. 
