280 INTRODUCTION TO BOTANY 
263. Club-mosses, or ground pines. These plants constitute 
another class of the pteridophytes. The most common ones 
belong to the genus Lycopodium (fig. 218). Lycopodiun 
plants are found mostly in the cooler temperate regions. At 
Christmas time they are shipped and used extensively in deco- 
ration throughout the United States. Their stems, which are 
underground or under leaves, etc., their upright branches and 
symmetrically arranged leaves, and their prominent spore- 
bearing cones are striking features of these plants. The col- 
lections of sporophylls in these plants and in Hguzsetum are 
sometimes spoken of as flowers but are more properly called 
strobili, or cones. 
264. Pteridophytes of past ages. The surface of the earth 
has undergone many sweeping changes since plants first began 
to live upon it. In some periods of the earth’s history condi- 
tions favored certain kinds of plants, and these flourished. 
When less favorable periods came, these once-successful plants 
were greatly reduced in number, or possibly were eliminated. 
We have records of what some of these former plants were. 
These records were made by the plants themselves, for when 
they died they sometimes became fossilized, or made prints in 
soft mud or other substances which afterwards hardened. By 
means of fossils much is being learned about the kinds of 
plants that used to exist. The ancient fern-like plants were 
widely distributed over the earth. Certain periods (as the so- 
called carboniferous period) were peculiarly favorable in tem- 
perature and moisture to the growth of pteridophyte types of 
plants, and they grew to much greater size and in much greater 
profusion than even our present tree-ferns of the moist tropics. 
After a long period of growth, when multitudes of the plants 
had died and fallen, conditions changed and these masses of 
plant material became submerged and then buried beneath lay- 
ers of rock and earth, and at length coal was formed from them. 
265. Classification. The following classification may assist in 
keeping leading facts in mind, but it is presented as a means 
of review rather than as an outline to be committed to memory. 
