Sphenophyta: Life History and Ecology

Sphenophyte Life History

The life cycle of sphenophytes is broadly similar to that of ferns and psilophytes. Presumably, this type of life cycle is plesiomorphic for the "trimerophytes" -- that is, it was present in the common ancestor of the clade containing sphenophytes, ferns, and most other vascular plants. Sphenophytes show the same pattern of alternation of generations found in all plants: asexually produced spores give rise to gametophytes, which produce eggs and sperm that fuse and grow into a sporophyte, which produces spores and continues the cycle. As is the case in almost all known vascular plants, the sporophyte is the dominant generation in sphenophytes.

Spores are produced in special sacs called sporangia, which are borne on the tightly packed sporangiophores that make up the "scales" of a sphenophyte cone, or strobilus, like the one shown at right. Living sphenophytes are homosporous, as were most of their fossil relatives; that is, they produced only one type of spore. Dispersed by wind, these small spores germinate into small, freeliving, disc-shaped gametophytes that produce either archegonia -- organs in which egg cells are formed -- or sperm-producing antheridia. Released from the antheridia under moist conditions, the sperm cells swim through water films to reach the egg cells on the female gametophytes.

Some of the large sphenophyte trees of the Paleozoic were heterosporous, producing large megaspores and small microspores, and probably retaining the megaspore in the strobilus. This type of life cycle is also seen in some of the giant fossil lycophyte trees, as well as in all seed plants; its presence in these three unrelated taxa is probably due to convergent evolution.

Sphenophyte Ecology

While a few species of the living genus Equisetum can grow in somewhat dry habitats, most species prefer moist soils; moisture is necessary for sperm to reach the egg cells. In riparian habitats -- that is, along the sides of rivers and streams -- Equisetum may be very common, forming extensive "thickets." Because of their ability to regenerate rapidly from pieces of rhizome, Equisetum species can survive well in environments that are often disturbed, such as riverbanks, or your lawn. Careful study of sphenophyte fossils and the paleoenvironments associated with them has shown that fossil sphenophytes had much the same environmental preferences as their living counterparts. Sphenophytes were present in the extensive coal swamps of the Pennsylvanian, but they were more common outside the swamps, in lowland areas without standing water.