Vascular Non-seed plants: Ferns

In this section of the lab, you should learn to contrast ferns, which are vascular non-seed plants, with mosses (nonvascular plants) and with vascular seed plants, which are covered in a later section.

Vascular plants are defined by the presence of vascular tissue, which forms vessels that allow for the transport of water, sugars, and other substances throughout the plant. The evolution of vascular tissue was an essential step in enabling plants to live on land, and it occurred before the appearance of seeds, pollen, and flowers. Before the evolution of seed plants, terrestrial habitats were dominated by vascular non-seed plants such as ferns.

In this lab, you'll look at ferns as a representative of the group known as vascular non-seed plants. This group also includes some other types of plants, including horsetails (which we might have in lab). See Seedless Vascular Plant Diversity in Campbell (fig. 29.13, 10th edition) for other examples.

Objectives

• Compare and contrast the structures of ferns and mosses in terms of how these structures determine a plant's ability to survive and grow in dry conditions and its ability to grow tall.
• Compare and contrast the life cycles of ferns and mosses, including the relative size and longevity of the gametophyte and sporophyte phases.
• Compare and contrast the life cycles of ferns and seed plants.

Specimens

• Live fern fronds, with sporangia.
• Pteridium rhizome (microscope slide). Pteridium is a genus of fern; the rhizome is more or less the same as a stem.
• Fern Prothallium separate male and female thallus w.m. (microscope slide). "Prothallium" is a specialized botanical term for the gametophyte of a fern; "thallus" simply means the whole body of the gametophyte. This slide could be better labeled "fern gametophyte." In some ferns, the gametophyte produces both eggs and sperm, in separate structures and typically at separate times. In other ferns, the gametophyte is either male or female. If the slide says "w.m.," it's a whole mount, meaning that the slide was prepared by mounting the whole specimen on the slide.
• Ferm prothallium antheridia sec (microscope slide). Antheridia are the sperm-producing structures of the gametophyte. "Sec" means section; this is a slice through the whole gametophyte. Ideally, it will give you a closer look at the antheridia, but you will have to search for them. This is the same kind of gametophyte that could be on the whole mount slide described previously, but this one is a cross section.

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.

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.

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. The outside of the epidermis is covered with a waxy cuticle, which reduces evaporation. The cuticle is on the outside of the cells, and it usually isn't visible in our microscope slides because the stains don't bind to it.

Because the cuticle seals the aboveground parts of a fern off from the air, the stems and fronds of ferns also need stomata, or openings to allow for some air movement for gas exchange.

The vascular tissue, epidermis with cuticle, and stomata of ferns are adaptations to terrestrial life which occur in vascular plants but not in nonvascular plants such as mosses.

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. They commonly grow on a forest floor in the wet season, and don't live long. 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.

Look at the slides of fern gametophytes (labeled ferm prothallium) and compare them to the fern life cycle diagram  in Campbell Biology (Chapter 29). 

Fern sporophyte and spore production

In contrast to the tiny gametophytes, fern sporophytes can be as large as trees and live for many years. 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 sporangia

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.

A single Sporangium

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 (0.1 mm) 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.

References & further reading:

Campbell Biology, Chapter 29: How Plants Colonized Land. In particular, study the diagram of the life cycle of a fern (fig. 29.11, 10th edition).

Sex and the single fern. Tai-ping Sun, 2014, Science Vol. 346 no. 6208 pp. 423-424. Fern gametophytes can be male, female, or both, but their sex isn't genetically determined. Older gametophytes secrete pheromones that can determine the sex of newly developing gametophytes that haven't determined their sex yet. This brief, accesible Perspective article is based on the research article Antheridiogen determines sex in ferns via a spatiotemporally split gibberellin synthesis pathway. For an even briefer news article, see Ferns communicate to decide their sexes in Nature.