Mosses and other nonvascular plants

This group includes mosses, liverworts, and hornworts. These are land plants, and show considerably more tissue complexity than the green algae. These plants were traditionally known as bryophytes; this term literally means "moss plant," and is used informally to refer to all the nonvascular plants. In this lab, you'll take a close look at a moss as a representative of nonvascular plants in general.

Mosses such as Mnium hornum are true land plants; they don't normally live underwater. Unlike the green algae, their bodies show a greater degree of tissue differentiation. However, they are only able to grow and reproduce in wet environments because they lack some of the more elaborate adaptations to dry environments that are found in the vascular plants.

Some key characteristics that land plants (including nonvascular plants) have, but algae don’t:

• Tissue and organ differentiation. Unlike algae, land plants are differentiated into two main parts: a root, usually growing underground and absorbing nutrients, and a shoot, usually growing above ground and absorbing sunlight to perform photosynthesis. Most plants have a variety of specialized tissues within these two regions of the body. For example, an organ such as a simple leaf involves several different kinds of cells, which you’ll study in a later lab. Algae are simpler; when you look at them under a microscope, you generally see the same kind of cell throughout the whole body (except for specialized reproductive cells). Mosses, being nonvascular, aren't considered to have true roots, but they do have rootlike rhizomes that help absorb water and nutrients and hold the moss in place.
• Growth at meristems. Plant growth normally occurs at meristems, which are localized regions of cells specialized for cell proliferation. There is a meristem at the apex of the shoot and one at the apex of the root; there may be other meristem regions as well. Since land plants are so highly differentiated, it makes sense that, for example, root cells should only be produced in the roots and not elsewhere. Algae are different; since the cells are less specialized, growth can occur anywhere.
• Alternation of generations. All eukaryotes have a haploid stage and a diploid stage. In land plants, both the haploid and the diploid stage are multicellular. This is called alternation of generations. Some algae also have this feature, but the algal ancestors of plants probably did not. Animals don't have alternation of generations, because only the diploid stage is multicellular. In animals, the only haploid stage is the unicellular gamete.
• Multicellular, dependent embryo. In plants, fertilization (the fusion of egg and sperm) creates a zygote, which develops into a multicellular embryo. This occurs inside the parent plant. In green algae, the zygote is on its own. It floats free of the parent and is independent.

Some characteristics that mosses lack, but vascular plants have:

• Lignified vascular tissue. Mosses have some water-conducting cells, but they do not have the empty, lignin-reinforced cells that allow vascular plants to transport water with strong pressure gradients. Thus, mosses have very limited water transport ability and can't grow very tall.
• Dominant sporophyte. In mosses, the larger, longer-lived part of the life cycle is the haploid gametophyte. In ferns and other vascular plants, the sporophyte is larger and lives longer.

Objectives

• Recognize moss specimens (whole or in microscope sections) and describe how they are structurally and functionally different from algae or from vascular plants.
• Recognize the structures in the moss life cycle slide and relate these structures to the life cycle diagram.
• Explain the life cycle of a moss, including alternation of generations.

Specimens

Although there are a lot of interesting nonvascular plants, I'll just use one kind of moss (Mnium) as our primary representative of nonvascular plants for this lab.

• Live mosses: green, leafy gametophytes with brown sporophytes attached. These could be Mnium or similar mosses, depending on what we get.
• Mnium life history (microscope slide). This slide contains several different sections, showing different parts of the life cycle.
• Mnium leaf cross section.
• Mnium stem cross section.

Moss body structure

Mosses are often leafy, but they lack the complex organization of vascular plant leaves, stems, and roots.

A cross section of the leaf shows that most of it is only one cell thick. There is no epidermis, no cuticle, and there are no stomata.

All the cells are lined with chloroplasts. The empty space in the center of each cell is the central vacuole, which is the largest feature of most plant cells. Each cell has a nucleus, but in this picture most of the nuclei are not visible. The nucleus is small compared to the cell, and in this slide most of the cells were not sliced though the nucleus.

The midrib (thickened area) in the middle of the leaf contains some water-conducting cells. These are not considered true vascular tissue; they cannot use a pressure gradient to transport water against the pull of gravity.

Since moss leaves lack a cuticle, they are subject to drying out. The lack of a cuticle also means that mosses can absorb water directly into their leaves in wet conditions. In fact, mosses can also absorb nutrients directly into their leaves (rather than through roots), which may be advantageous in an environment such as a wet forest floor.

Moss Life Cycle

Mosses and liverworts, like all land plants, have alternation of generations. Study the moss life cycle diagram in Campbell Biology (fig. 29.6, 10th edition) and compare it to the specimens you see under the microscope. For each specimen you look at, make sure you can identify it as gametophyte (haploid) or sporophyte (diploid), and recognize where it belongs in the life cycle. In a later lab, you'll contrast the life cycles of mosses with those of seed plants.

The slide labeled Mnium life history has multiple specimens on the same slide, showing different parts of the life history of this moss: young gametophyte, antheridial head and archegonial head (parts of male and female gametophytes), and capsule (part of the sporophyte). Make sure you see them all.

Liverworts

We may or may not have this as a bonus specimen in lab. I'm putting it in this page in case it shows up, but I won't test you on this specimen.

Liverworts are similar to mosses in some ways: both lack vascular tissue, and both have a large gametophyte with a smaller sporophyte that depends on the gametophyte for nutrition.

The picture at right shows Marchantia, a common liverwort. The whole structure shown here is about 2 cm tall, and everything you see here is part of the haploid gametophyte.

The thallus is simply the body of the plant. The term thallus is used for plants and algae with very simple, nonvascular structure. The thallus consists mainly of flat, leaf-like structures.

Rhizoids are thin, root-like structures. They aren't considered true roots, though, because they lack vascular tissue.

The archegonial head produces eggs. In this species, there are separate male and female gametophytes. Sperm are produced by a male gametophyte (which has an antheridial head), and the sperm must be carried in water to the archegonial head. Fertilization occurs inside, and the tiny diploid sporophyte undergoes meiosis to produce spores. The spores are dropped from the archegonial head. Thus, the entire lifespan of the sporophyte occurs inside the female gametophyte.

Liverwort thallus structure

Nonvascular plants such as liverworts are never very tall, but they do have some specialized tissues. In the cross-section diagam at right, notice that the liverwort thallus (body) is fairly flat -- no more than about 5 mm thick in this case. Unlike the green algae, though, it has a clearly defined top and bottom. The cells near the top have more chloroplasts; they perform most of the photosynthesis.

The liverwort is also covered by an epidermis -- a tough skin, one cell layer thick, that helps protect the plant. The epidermis provides some resistance to drying out, but it also makes it harder for the photosynthesizing cells to do the gas exchange they need. The solution to this problem is that there are pores in the epidermis and air spaces inside the thallus. This arrangement allows gas exchange while minimizing water loss due to evaporation.

The liverwort also has rhizoids -- thin, hairlike structures that function like roots and help the plant absorb nutrients and water from the soil. Each rhizoid is made by a single cell.

Now notice what the liverwort does not have: leaves, stems, roots, or specialized vascular tissue. Vascular tissue is specialized tissue for transporting water and nutrients. Vascular plants such as trees have vascular tissue reinforced with lignin, a complex polymer. Nonvascular plants such as liverworts and mosses don't have this kind of tissue. Without lignified vascular tissue, this liverwort cannot transport nutrients or water from its rhizoids to other cells that are more than a few millimeters away. That's why nonvascular plants are always short.

References & further reading

Campbell Biology, Chapter 29: The life cycle of a moss. As you look at the moss specimens, you should be comparing them to what you see in this chapter.

Introduction to Moss Morphology from Bio 321 at University of British Columbia. This page has a good description of moss form and function, with detailed photos of various mosses.