DEVELOPMENT OF PLANTS 291 



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 gametophyte 

 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 

 sporophyte 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. 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, ex- 

 periences a new stimulus, i. e., that of the soil which, cooperating 

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

 proportions than the gametophyte and become the dominant gene- 

 ration in the life history of the plant. 



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 

 specialized group that have branched off from the primitive ferns 

 in recent geological times and owing to their variations being 

 highly adaptive to present conditions upon the earth, they have 

 become very numerous and widely distributed. 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 coun- 

 tries where they occur in astonishing profusion and variety upon 

 the moist rocks and trunks of trees, as well as upon the earth. 



