306 DECLINE OF THE GAMETOPHYTE 



in the Bryophyta and Pteridophyta of motile male gametes and 

 the consequent necessity of water for fertilization, points to the 

 inheritance of this peculiarity from aquatic ancestors. It is evi- 

 dent that there can be no considerable modification of the gameto- 

 phyte, as long as this feature is retained, without greatly decreas- 

 ing the possibilities of fertilization. Consequently the gameto- 

 phyte must remain of necessity a primitive structure. The sporo- 

 phyte, on the other hand, was accustomed from the first to ter- 

 restrial conditions, or to such conditions as were unfavorable 

 to the growth of the gametophyte. Among the algae we have 

 noticed that it may carry the plant over the more or less com- 

 plete drying up of the water or unfavorable temperatures. Per- 

 haps these exposures to a variety of stimuli which the gameto- 

 phyte never experienced or in a lesser degree enabled the sporo- 

 phyte to respond with more profound variations when it be- 

 came parasitic upon the gametophyte and exposed directly to 

 the light and air as was the case among the Bryophyta. Cer- 

 tainly we know that abundance of foods, light, temperatures, etc., 

 are among the important stimuli in causing variations. The 

 simple sporophyte of the Bryophyta was exposed to just such 

 forces as these and steadily gained in complexity until in the 

 Bryales it nearly equaled in importance the gametophyte. The 

 sporophyte of the Pteridophyta, owing to the development of the 

 root, experiences a new stimulus, i. e., that of the soil which, co- 

 operating with the stimuli of the light, etc., causes it to assume 

 much larger proportions than the gametophyte and become the 

 dominant generation in the life history of the plant. Perhaps 

 another factor played an important role in securing the supremacy 

 of the sporophyte. We have seen in paragraph 58, The Signifi- 

 cance of Fertilization, that the sporophyte is the result of the 

 fusion of two gametes which may vary in character. The con- 

 stant introduction of variant characters into the asexual genera- 

 tion in this way may have cooperated in its increasing complexity. 



Order 2. Filicales or Common Ferns 



113. General Features. — The great majority of plants popu- 

 larly known as ferns belong to this order. They are a highly 



