Ferns: the Pterophyta

Ferns go back to the Carboniferous, occurring along side the lycopods and horsetails in the swamp forests. Still successful today, they occur worldwide, generally in more moist tropical or temperate forests, though species can be found in most every habitat. Over 12,000 species exist. Unlike the lycopods, they have true megaphylls generally arising off rhizomes. The sperm still need to swim to the egg, so water is still required for sexual reproduction.

The life cycle is as follows: how does the cycle compare with the previous families? In general it is quite similar to the lycopods. Most fern species are homosporous, the spores germinating into a bisexual gametophyte.

Fern sporangia are positioned in clumps called sori. In some species the sori are covered with a protective cap called the indusium. In the species pictured they are without an indusium.

An individual sporangium is pictured to the left. The annulus is hygroscopic, and when it dries out it contracts rather violently. The weak point, the lip cells break open releasing the spores with some velocity rather like a catapult would. A wind current can then carry them fairly far afield.

The spores germinate into a prothallus, which is photosynthetic though rather small ( 1/4" or more). It is generally heart-shaped and at the notch contains the antheridia and archegonia.The sperm swims through moist soil to fertilize the egg. The resulting zygote forms the sporophyte which grows out of the archegonium. as we saw in the earlier families.