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From Seed to Seed:
Plant Science for K-8 Educators

Mosses. Mosses belong to the phylum Bryophyta; this phylum also includes the liverworts and the hornworts. These primitive plants represent some of the earliest attempts to leave the water to colonize land. Members of this phylum share some common characteristics. They are relatively small,

and are most common in moist, warm regions.

Unlike higher plants, bryophytes lack specialized vascular tissue (xylem and phloem); without this supporting tissue, the plants remain small and low to the ground. Because they lack vascular tissue, mosses don't have "true" stems, roots, or leaves. Nevertheless, these terms are often used to describe the corresponding structures in mosses.

Most students will be familiar with mosses-they have probably seen them growing on rocks or trees. The familiar cushiony green mat

represents the gametophyte generation. This is the dominant and most conspicuous phase-responsible for producing the egg and sperm. If you and your students get down on your hands and knees for a close-up view, you may see some taller structures emerging from the green mat. The capsules at the top of these tall, delicate "stems" (technically termed seta) contain spores. These structures-the seta and capsules-are the sporophyte generation, which is initiated by fertilization.

Ferns. Ferns possess large, intricate leaves (fronds), vascular tissues, and well-formed root systems. They provide us with a good example of the evolutionary progression to the dominance of the sporophyte, or spore-producing, generation.

You probably have observed the neat rows of dots on the undersides of fern fronds and can easily point them out to students while on a field trip to the woods. These are spore capsules; when they dry and break open, huge numbers of tiny, dust-like spores are released. If a spore lands in a suitable spot and germinates, it will grow into a small, heart-shaped plant just ¼ inch across. These tiny plants (which generally remain unnoticed) represent the gametophyte generation, responsible for producing both male and female gametes-the sperm and eggs. Once united, these gametes form the zygote and eventually the familiar fern. The life cycle is complete when the mature fern produces spores. Unlike mosses, for which the gametophyte is the conspicuous phase, the dominant phase for ferns is the sporophyte generation.

Ferns are classified as seedless vascular plants. They contain vascular tissues (xylem and phloem), which distinguishes them from the mosses. However, as is the case with mosses, their developing embryos are not enclosed in seeds.

Fond of Fronds

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