THE PTEKIDOPHYTES 295 



This results in the production, from the small spore, of a small 

 gametophyte, which produces sperms; and from the large 

 spore, of a large gametophyte, which produces the egg, and 

 upon which thereafter the young embryo Selaginella plant 

 is developed. 1 All the groups of plants higher than pterido- 

 phytes have two kinds of asexual spores. 



272. Pteridophytes of past ages. The surface of the earth 

 has undergone many changes since plants began to live upon 

 it. In some periods of the earth's history conditions favored 

 certain kinds of plants and these flourished. When less favor- 

 able periods came these successful plants were greatly reduced 

 in number or completely exterminated. We have records of 

 what some of these former plants were. These records were 

 made by the plants themselves, for when they died they some- 

 times became fossilized, or made prints in soft mud or other 

 substances which afterward hardened. By means of fossils 

 much is being learned about the kinds of plants that used to 

 exist. In many cases great detail of structure has been pre- 

 served, and many important facts are thus established with 

 reference to the history of our existing plants. The study of 



1 In some cases it may be wise to go more into the details of the repro- 

 duction of Selaginella, and for such use the following facts are added : The 

 terminal cones or strobili of Selaginella resemble those shown in Lycopodium. 

 In Selaginella, however, two kinds of sporangia with two kinds of spores are 

 formed. One sporangium contains large numbers of small spores, while the 

 other sporangium contains four large spores. Both kinds are asexual, since 

 both are formed upon the sporophyte by cell division. The small spore is 

 called the microspore, meaning "little spore " ; the large one is called the mega- 

 spore, meaning "big spore.' 1 Similarly, the names of other structures relate 

 to the size of the spores ; as, the megasporangium which produces the mega- 

 spore is borne upon the megasporophyll, and the microsporangium which 

 bears the microspores is borne upon the microsporophyll. We have in Sela- 

 ginella different kinds of spores, or heterospory, as compared with similar 

 asexual spores, or homospory, as seen in the other ferns that we have studied. 

 Each spore produces a particular kind of gametophyte, one of which bears 

 the egg and the other the sperm. The egg is fertilized while within the 

 female gametophyte, and the new plant begins to grow from the same posi- 

 tion. If the structures around the embryo should become dry and hard, and 

 if the whole should undergo a resting period, we should have the structure 

 that we call the seed in the next great group of plants. 



