PTEKIDOPHYTES 
321 
(5) Many-celled sex organs. The antheridia and the 
flask-shaped archegonia are very characteristic of Bryo- 
phytes as contrasted with Thallophytes. 
210. General characters of Pteridophytes. The name means 
" fern plants," and the Ferns are the most numerous and the 
most 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 Algae. 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. 
211. 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. 293, A). Upon this thallus antheridia 
