PTERIBOPHYTES 



129 



(5) Many -celled sex organs. The antheridia and the 

 flask-shaped archegonia are very characteristic of Bryo- 

 phytes as contrasted with Thallophytes. 



76. General characters of Pteridophytes. The name means 

 " fern plants," and the Ferns are the most numerous and the 

 mctt representative forms of the group. Associated with 

 them, however, are the Horsetails (Scouring rushes) and 

 the Club-mosses. By many the Pteridophytes are thought 

 to have been derived from such Liverworts as the Antho- 

 ceros forms, while some think that they may possibly have 

 been derived directly from the Green Algre. Whatever 

 their origin, they are very distinct from Bryophytes. 



One of the very important facts is the appearance of 

 the vascular system, which means a "system of vessels," 

 organized for conducting material through the plant body. 

 The appearance of this system marks some such epoch in 

 the evolution of plants as is marked in animals by the 

 appearance of the "backbone." As animals are often 

 grouped as "vertebrates" and "invertebrates," plants are 

 often grouped as "vascular plants" and "non-vascular 

 plants," the former being the Pteridophytes and Spermato- 

 phytes, the latter being the Thallophytes and Bryophytes. 

 Pteridophytes are of great interest, therefore, as being the 

 first vascular plants. 



77. Alternation of generations. This alternation con- 

 tinues in the Pteridophytes, but is even more distinct than 

 in the Bryophytes, the gametophyte and sporophyte be- 

 coming independent of one another. An outline of the life 

 history of an ordinary fern will illustrate this fact, and will 

 serve also to point out the prominent structures. Upon the 

 lower surface of the leaves of an ordinary fern dark spots 

 or lines are often seen. These are found to yield spores, 

 with which the life history may be begun. 



When such a spore germinates it gives rise to a small, 

 green, heart-shaped thallus, resembling a delicate and sim- 

 ple liverwort (Fig. Ill, A). Upon this thallus antheridia 



