250 BOTANY: PRINCIPLES AND PROBLEMS 



sporangium was thus formed. The cells at the base of this 

 structure further developed into a support or stalk which length- 

 ened and carried the sporangium up into the air, whence the spores 

 might readily be dispersed. This general situation, where the 

 sexual plant produces a spore-case borne on a well-developed 

 stalk, is characteristic of the second main division of the plant 

 kingdom, the Bryophytes, which include the liverworts and the 

 mosses. 



C. In Pteridophytes. — The next stage in the development of 

 the "alternating" life cycle involved a radical change, and since 

 the plants in which this change took place have long since 

 perished and have left no descendants to our day, we can only 

 surmise as to what actually happened. The sporangium and its 

 related structures underwent a remarkable transformation, 

 developing chlorophyll-bearing leaves which manufactured its 

 food supply, and finally sending forth roots into the soil and thus 

 establishing a new and entirely independent individual. Instead 

 of producing but one sporangium, this now gave rise to many. 

 In short, a spore-bearing, non-sexual plant, the sporophyte, 

 had been evolved, entirely distinct from, and independent of, the 

 older gamete-bearing, sexual plant, which is known as the 

 gametophyte. * This is the situation as we meet it in the third great 

 division of the plant kingdom, the Pteridophytes, which include 

 the ferns, club mosses, horse-tails, and their allies (Fig. 138) . Here 

 the dominant, conspicuous plant, with which we are most famihar, 

 is the sporophyte. It produces thousands of sporangia and 

 perhaps milhons of spores, but sexual organs and gametes are 

 entirely absent from it. It has evolved from the primitive moss 

 sporangium and its associated organs. The gametophyte, 

 however — the structure which corresponds to the moss plant — is 

 small, inconspicuous and often hard to discover. It possesses no 

 true roots or leaves but bears the all-important sexual cells. 

 Here the fusion between gametes takes place, and from a fertil- 

 ized egg in one of these inconspicuous gametophytes is developed 

 a young spore-bearing plant. This soon develops leaves and 

 roots and becomes independent of the parent gametophyte, 

 which then withers away. Each of the many spores borne by 



* The terms sporophyte and gaynetophyte are also used among Bryophytes, 

 the former being applied to the spore-case and its related structures and the 

 latter to the plant itself. 



