304 EVOLUTION OF THE SPOROPHYTE 



The gametospore divides into two cells (Fig. 218, F), from the 

 lower of which a large foot is formed, and from the upper cell 

 the short stem and root arise. Later, the first leaf or cotyledon 

 is formed from the stem. This development of the sporophyte 

 goes on very slowly within the archegonium where it lives as a 

 parasite for a long time. Eventually the root ruptures the arche- 

 gonium or calyptra and comes in contact with the soil. The 

 cotyledon now grows upward and makes its appearance above 

 the ground as the first green leaf (Fig. 218, G). The sporophyte 

 thus becomes a self-supporting plant, although it probably re- 

 mains in part dependent upon the gametophyte for several years. 

 At this stage of development the young sporophyte is a very 

 simple type of fern, but gradually more efficient roots are de- 

 veloped and each year the stem sends up a larger and larger leaf 

 until the adult size is reached when the spore-bearing branch is 

 formed (Fig. 215), thus making possible again a new series of 

 gametophytes. 



(c) Compariso7i of the Adder Tongue Ferns with Preceding 

 Groups. — The most noteworthy difference between these simple 

 ferns and the Bryophyta is the larger development of the sporo- 

 phyte and its final independence of the gametophyte. This is 

 doubtless due to the development of true roots which made pos- 

 sible a continuous and abundant supply of the crude materials 

 from the soil. This change acted as a stimulus which promoted 

 variations in the sporophyte while the light and various climatic 

 factors also assisted to a very marked degree. Among the algae, 

 the gametophyte is the dominant generation, the sporophyte 

 being represented often by the gametospore. This is essentially 

 the relationship among the majority of the Hepaticae where the 

 gametophyte performs all the work of food construction and the 

 sporophyte is a minute parasitic plant upon it. In Anthoceros 

 and the mosses, the sporophyte is longer lived and more highly 

 organized, but the gametophyte is still the more important gen- 

 eration as it performs the major portion of the work. Among the 

 ferns, this relationship and the distribution of labor is completely 

 turned about. The sporophyte becomes the larger plant and the 

 principal center of photosynthesis, while the gametophyte re- 

 mains small and gradually becomes very short lived. 



