I'TEKllxirilVTEa 



321 



(5) Jfanij-cclh'd sex orfjaiis. — The ;mt]ieridi;i and the 

 flask-shaped sircliegonia are very characteristic of ISryo- 

 pliytos as contrasted with Tliallophytcs. 



210. General characters of Pteridophytes. — The name means 

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

 most representative forms of the group. Associated with 

 tliem, 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- 

 ccrOK forms, while some think that they may possibly have 

 been derived directly from the Green Algffi. A\hatever 

 their origin, they are very distinct from Bryopliytes. 



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 jilant 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 

 jilauts," the former being the Pterido}ihytes and Si>ermato- 

 phytes, the latter being the Tliallophytcs and Bryopliytes. 

 Pteridophytes are of great interest, therefore, as being the 

 first vascular plants. 



yil. Alternation of generations. — This alternation con- 

 tinues in the Pteridophytes, but is even more distinct than 

 in tlie Bryojihytes, 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 thallns, resembling a delicate and sim- 

 ple liverwort (Fig. 203, A). Upon this thallus antheridia 



