DEVELOPMENT OF PLANTS 299 



parture 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 pro- 

 portions and become differentiated into a highly organized plant. 

 The stem elongates through the repeated division 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 arrange- 

 ment 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. 



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 

 and Botrychium, are of common occurrence (Fig. 215). They 

 are of unusual interest because they present many features sug- 

 gestive of the liverworts and also of the more specialized ferns 

 and seed plants. This remark is not intended as necessarily 

 implying relationship between the three groups, but that analo- 

 gous structures have arisen in each, due either to inheritance 

 from allied ancestors or to the operation of stimuli upon plants 

 in no way related. The sporophyte consists of a short upright 

 stem with thick fleshy roots that are associated with mycorrhiza. 

 The leaves are simple or divided and usually appear singly, en- 

 sheathing the apex of the stem. One of the most remarkable 

 features about these ferns is the development in the stems, roots 



