DEVELOPMENT OF PLANTS 313 



was parasitic upon it. In the Pteridophyta, however, the 

 sporophyte is not at such a disadvantage since it actually becomes 

 independent of the gametophyte owing to the development of a 

 root which puts it in communication with the earth substances. 

 This radical departure in its mode of life acted as a profound 

 stimulus and cooperated in inducing marked variations that were 

 so beneficial in character as to cause the sporophyte to assume 

 large proportions and become differentiated into a highly or- 

 ganized plant. The stem elongates through the repeated divi- 

 sion of a single apical cell, as in the Bryophyta, but the stem and 

 also the leaves and roots in addition contain vascular bundles 

 and an arrangement of tissues already noticed in the higher 

 plants. 



The gametophyte in many of the Pteridophyta is very sugges- 

 tive of the simpler thalloid hepatics. Owing to the fact that it 

 is no longer permanently burdened with the nutrition of the 

 sporophyte, it becomes greatly reduced in size and length of life. 

 In fact, in several of the more specialized ferns, the gameto- 

 phyte may be reduced to a few cells and the entire development 

 may take place within a day. The reproductive organs and the 

 germination of the gametospore are suggestive of the correspond- 

 ing features noted in the Bryophyta. The more important orders 

 of the Pteridophyta are: I. Ophioglossales. 2. Filicales or Com- 

 mon Ferns. 3. Equisetales or Horsetails. 4. Lycopodiales or 

 Club Mosses. This sequence is followed because it brings out 

 more clearly the trends or tendencies that arose step by step in 

 the evolution of plant life. As a matter of fact the club mosses 

 appear to be the most primitive forms and they are the most 

 suggestive of the Bryophyta; while the common ferns are the 

 most recent in development. Enough has been said to show that 

 these forms are very remotely related to the ancestors of the Bry- 

 ophyta and that these four orders of Pteridophyta are very dis- 

 tantly connected. 



Order i. Ophioglossales 



112. General Characters of the Sporophyte. This order con- 

 tains three genera which are probably but a remnant of an 

 earlier and widely distributed group. Only two, Ophioglossum 



21 



