296 PRACTICAL BOTANY 



fossil plants, or paleobotany as it is called, offers a field of great 

 interest to the specialist. 



Ferns and their relatives are among the principal fossil 

 plants concerning which paleobotany has given us informa- 

 tion. The introduction of vascular tissue furnished structures 

 better fitted for preservation than there were in the less rigid 

 bryophytes and thallophytes. It has been definitely deter- 

 mined that pteridophytes and other plants resembling them 

 were once more abundant and larger than those now living. 

 These ancient ferns were widely distributed over the earth. 

 Certain periods (as the so-called Carboniferous Period) were 

 peculiarly favorable in temperature and abundant moisture to 

 the growth of pteridophyte types of plants, and they grew in 

 size and profusion much greater than even our present tree fern 

 forests of the moist tropics. Their range extended much farther 

 than at present. Many of these giant forms were very unlike 

 our existing ferns, horsetails, and club mosses, but they are 

 the ancestors from which our present forms have descended. 



During these periods there also lived plants which are 

 closely related to some of the seed plants. 



273. Coal formation. There have been several coal-forming 

 periods during the history of the earth. The Carboniferous 

 age, however, is the period of chief coal formation. During 

 this time almost unimaginably large quantities of plant life 

 matured and fell in the immense swampy areas in which they 

 grew. As is true when Sphagnum and other plants in peat 

 bogs decay but partially, and under moderate pressure become 

 peat, it is thought that in the same way these much larger 

 masses of plant life formed immense beds of partially decayed 

 plant material. After a long period of such growth and dep- 

 osition of plants conditions changed, and these masses of 

 plant material became submerged and then buried beneath 

 layers of rock and earth. The surplus water in the beds of 

 plants was driven away, the carbonaceous material was pressed 

 into solid masses, the oily materials were pressed out, or, as 

 has been said, under the pressure the coal " wept bitter tears 



