DEVELOPMENT OF PLANTS 321 



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 became 

 parasitic upon the gametophyte and exposed directly to the 

 light and air as was the case among the Bryophyta. Certainly 

 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 w^hich 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- 

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

 specialized group that have branched off from some primitive 

 fern stock in recent geological times and owing to their variations 

 being highly adapted to present conditions upon the earth, they 

 lave become very numerous and widely distributed — probably 

 exceeding 3000 species. In temperate climates the majority 

 live upon the ground in moist and shady regions and some are 

 aquatic or xerophytic. They attain their greatest abundance 

 in the mountainous district of tropical countries where they occur 

 in astonishing profusion and variety upon the moist rocks and 



