386 GENERAL MORPHOLOGY OF PLANTS 



the coal seams. Alternate elevation and subsidence of the land, 

 it is believed, provided for the superposed strata of coal and other 

 kinds of rock. The fern plants of to-day are only a relic of the 

 grand fern vegetation of the Carboniferous Age. The fern plants 

 have gradually lessened in importance since that time, both in 

 size and in number. From being the dominant vegetation ele- 

 ments of that time they now occupy a subordinate place, while 

 the sporophyte of the seed plants has gradually risen to be the 

 dominant vegetation element first represented by the Gymno- 

 sperms, many of these being now extinct, and now represented by 

 the Angiosperms and some Gymnosperms. It is an interesting 

 picture to represent the rise and fall of these different classes of 

 plants. 



538. Formula for life history of the heterosporous 

 Pteridophytes.* The formula would be similar to that for the 

 ferns, but with two kinds of spores and gametophytes. It will 

 be more convenient perhaps to start the cycle with the sporo- 

 phyte. Then 



/ asexual microspore male gametophyte sperm v 

 Sporophyte C r / Fer- 



x asexual macrospore female gametophyte egg ' 



. mi sp MG s \ 

 tilized egg Sporophyte, etc. = S / ma ^ _ FG _ e / FE S., etc. 



* For reference. 



