FERN-LIKE PLANTS OF CARBONIFEROUS TIMES 353 



ennial can live through seasons when the gametophytes would 

 perish. Each season it sheds its spores, so that when a favorable 

 period arrives the gametophytes develop and produce new sporo- 

 phytes. 



Sixth. The gametophyte decreases in size and importancs as 

 the sporophyte increases, until in Selaginella and Isoetes it 

 becomes entirely dependent on the sporophyte for its nourish- 

 ment. The gametophytes, it is true, generally become free from 

 the sporophyte, but not until sufficient food is stored up in the 

 spore. In Selaginella the female gametophyte nearly completes 

 its development before escape from the sporangium, and in some 

 cases actually remains in the sporangium until after fertilization 

 and development of the embryo, thus really forming a seed which 

 is the special character of the seed plants, which make up the 

 highest branch of the plant kingdom. 



Seventh. The supreme position which the sporophyte was 

 destined to occupy in the plant world is shown in the luxuriance 

 and immense quantity of vegetation during what is known as the 

 coal period, or Carboniferous Age. 



521. Deposits of coal formed by the fern-like plants. The 

 fern plants occupy a very minor position in the plant world at the 

 present time compared to their dominant position in past ages. 

 This has been revealed through fossil remains of plants dis- 

 covered in different strata of the earth's crust and through the 

 immense deposits of coal formed during what is known as the 

 coal period, or Carboniferous Age. The coal is formed by plant 

 remains covered by other strata, and subjected to such great pres- 

 sure and heat, in the absence of air, that carbon or carbonized 

 matter is formed since oxidation cannot ta';e place. The coal 

 is, therefore, laid down in strata, or seams, between other rock 

 layers. These layers of coal vary from one to three meters (three 

 to ten feet) in thickness in most of the regions where coal is 

 mined, and in some cases is much thicker, from twenty to 

 thirty meters or more in thickness. The pressure which is 

 necessary in changing plant material to coal reduces enormously 

 the thickness of the material, so that it would require beds of 



