Plants > Ferns
Vascular Non-seed plants: Ferns and their relatives
Some key differences between ferns and mosses:
- Ferns can live in drier places.
- Ferns have a highly developed vascular system with vessels that are reinforced with lignin (a woody material).
- In ferns, the sporophyte is much bigger and longer-lived than the gametophyte. When you're looking at a fern, you're usually looking at a sporophyte (the diploid phase).
- Ferns can grow tall: tree ferns can be several meters tall, while most mosses are limited to a few centimeters.
Ferns are vascular plants
Ferns and other vascular plants can grow much taller than nonvascular plants. Being tall is only possible for plants with a highly developed vascular system for transporting materials between the roots and the shoot, which is the part of the plant above the ground. Nonvascular plants such as mosses lack these structures, so their photosynthetic parts must be close to the ground.
In evolutionary history, the advent of vascular plants changed the way the world looked. Prior to the spread of vascular plants, the land had only plants that were a few centimeters tall; the origin of the vascular system made it possible for plants to be much taller. As it became possible for plants to grow taller, it also became necessary; otherwise, they would get shaded by their taller neighbors. With the advent of vascular plants, the competition for light became intense, and forests started to cover the earth. (A forest is simply a crowd of plants competing for light.) That's why most terrestrial habitats are dominated by vascular plants.
The earliest forests were composed of vascular non-seed plants such as ferns, though modern forests are dominated by seed plants.
Vascular tissue in ferns
In this cross-section of a stem of the whisk fern Psilotum, you can see that the tissue structure is considerably more complex than that of a moss or liverwort. Notice the bundle of vascular tissue in the middle; this bundle contains xylem and phloem, which will be discussed later.
Also note the presence of a clearly defined epidermis, a layer of cells protecting the outside of the plant. The epidermis protects the photosynthetic cells of the interior from drying out, but it also limits gas exchange; therefore, the plant must have pores to let gases in and out of the tissue. Air spaces between the cells allow for diffusion of oxygen and carbon dioxide.
This picture shows a vascular bundle from the rhizome (underground stem) of a fern (Pteridium). The vascular bundle includes two main kinds of tissue: xylem and phloem. The xylem cells are large and have very thick walls, stained red in this slide. This is important because xylem transports water and inorganic nutrients from the roots to the above-ground parts of the plant; this pressure relies on a powerful pressure gradient to make the water move upward. The xylem cells are reinforced to withstand this pressure. (The mechanisms of water transport will be discussed in lecture.) The phloem cells are smaller and have thinner walls, stained blue in this picture. Phloem is responsible for transporting sugars from photosynthetic parts of the plant to other areas; this transport doesn't require as much pressure.
Fern Life Cycles
Remember that all plants share certain features in their life cycles:
- Multicellular haploid gametophyte stage, which produces haploid gametes (egg and sperm).
- Egg and sperm fuse to form diploid zygote.
- Multicellular diploid sporophyte stage, which produces haploid spores by meiosis.
- Haploid spores grow into haploid gametophytes.
The biggest difference between fern life cycles and moss life cycles is that in ferns, the sporophyte is much bigger and longer-lived than the gametophyte. Fern gametophytes are typicaly only a few millimeters across. Many are hermaphroditic: they produce both sperm (which form in antheridia) and eggs (which form in archegonia). This is different from mosses, in which male and female gametophytes are separate individuals.
Fern sporophyte and spore production
The stage that people think of as a fern is the sporophyte (the image at the top of this page is a sporophyte). Sporophytes can grow for many years. On the underside of a fern sporophyte frond you can often find sori (singular: sorus), which are clusters of sporangia; the sporangia produce spores.
Fern sporophytes (like all sporophytes) are diploid. Haploid spores are produced inside the sporangia via meiosis. The spores are eventually released, and they can grow into new haploid gametophytes.
From the pictures on this page, you could make a rough estimate of how many spores are released by a single sporangium, how many sporangia are in a sorus, and how many spores could be released by a single fern frond in one season. That's a lot of spores.
This picture shows a single sporangium that has already released its spores.
Sporangium with spores
This picture shows a single sporangium, filled with spores. Each spore is a little less than 100 microns in diameter. Each one has the potential to grow into a new gametophyte, but these tiny spores will have a very small chance of survival.
This page updated September 17, 2011