STUDIES OF SPORE-PLANTS 241 



VII). The group was much more important in the early 

 periods of the earth's history than at present. During the 

 Carboniferous period, pteridophytes formed the main mass 

 of the land vegetation ; but they decreased in prominence 

 in the later period when the gymnosperms and still later 

 the angiosperms appeared. The horse-tails and club-mosses 

 which exist to-day are rather small plants, but some of the 

 ancient species which lived before the end of the Carbon- 

 iferous period were stately trees. 



229. Economic Relations of Pteridophytes. The ferns 

 and their allies furnished much of the organic material which 

 was transformed into coal. Many leaves and stems of ferns 

 and allied plants are found as coal fossils, splendid specimens 

 of which are on exhibition at natural history museums. 



At the present time little use is made of these plants except 

 for ornamental purposes, for which their foliage is unsur- 

 passed. Many tropical species of ferns are kept in green- 

 houses, and several species have long been favorite house- 

 plants. Various kinds of club-mosses are also used in the 

 same way. 



One fern contains in its stem a powerful drug, known in 

 pharmacy as " extract of male fern," and often used for ex- 

 pelling tape-worms and round-worms from the intestines. 



The horse-tails were known as " scouring rushes " in the 

 pioneer days in America, because their stems were used to 

 scour pewter and brass utensils. The scouring property de- 

 pends upon the silicon particles in the stems (easily dem- 

 onstrated by burning). 



MOSSES 



230. Distribution of Mosses. One of the most interesting 

 points concerning mosses is their wide distribution, which is 

 made possible by adaptations to various conditions of life. 

 They are common everywhere. We may find them on ex- 



