350 PRACTICAL COURSE IN BOTANY 
different shapes, and in some species is branching and filamen- 
tous, like the protonema of a moss. Generally, however, it 
is flat and more or less two-lobed, as shown in Fig. 501. It 
is small and inconspicuous and very short-lived, being of 
importance only in connection with the work of reproduction. 
Look with your lens for a cluster of small, bottle-shaped 
bodies just below the deep cleft in the heart. If you can- 
not make out what they are, put a thin section through 
a part of the prothallium containing one under the micro- 
scope, and you will see that they are the archegonia. Lower 
down among the rhizoids, near the pointed base, will be 
found the antheridia. In some species the prothalli are 
dicecious, one kind bearing antheridia, the other archegonia, 
but this is rare among the true ferns. 
408. Fertilization. — This process is the same in all essen- 
tials as in the bryophytes. As in other cryptogams, it can 
take place only under 
water, — a circumstance 
which points to an aquatic 
origin for this subkingdom, 
and through them to the 
entire flora of the globe. 
The archegonia differ 
somewhat in shape from 
those of the liverworts and 
mosses, but a section under 
the microscope will show 
that they consist of essen- 
Fig. 503. — Young archegonium of a fern, tially the same parts. On 
magnified: K, neck canal cell; K’, ventral account of the similarity of 
canal cell; O, egg cell. 
these organs, the pterido- 
phytes and bryophytes are often classed together as Arche- 
goniates. 
409. Alternation of generations. — Among the section of 
ferns that we have been considering, the order of alternation 
corresponds in all essentials to that prevailing among the 
