Plants > Nonvascular Plants
Nonvascular plants (bryophytes)
This group includes mosses, liverworts, and hornworts. These are land plants, and show considerably more tissue complexity than the green algae. The term "bryophyte" literally means "moss plant;" it is used informally to refer to all the nonvascular plants.
Mosses such as Mnium hornum (shown at right) are true land plants; they don't normally live underwater. Unlike the green algae, their bodies show a fairly high 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. (Moss picture borrowed from Andrew Spink.)
Some key characteristics that land plants (including bryophytes) 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).
- 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. Some algae also have this feature, but the algal ancestors of plants probably did not.
- 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.
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, and 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.
Moss Life Cycle
Mosses and liverworts, like all land plants, have alternation of generations. Study the moss life cycle diagrams in the lab manual and in Campbell.
As you examine the specimens of moss (Mnium) and liverwort (Marchantia), refer to those diagrams. For each thing you look at, make sure you can identify it as gametophyte (haploid) or sporophyte (diploid).
In a later lab, you'll contrast the life cycles of mosses with those of seed plants. Many of the key features of land plants are related to reproduction.
The gametophyte starts out as a haploid spore. Spores are released from capsules and grow into independent gametophytes. Each gametophyte is either male or female. This image shows a very young gametophyte, consisting of just a few dozen cells.
Gametophyte with sporophytes
Eventually the gametophyte grows large enough to reproduce. In mosses, the gametophyte is larger and than the sporophyte, and lives longer.
The gametophyte performs photosynthesis and provides most of the energy needed by the sporophytes.
At the top of each sporophyte is a capsule, which produces spores.
You will typically see numerous sporophytes growing from a single female gametophyte. Although the whole thing may appear to be a single plant, each sporophyte is a genetically distinct individual.
Antheridial heads form at the tips of male gametophytes, and they produce sperm.
This image shows numerous antheridia, each filled with sperm. The sperm appear as dark dots.
Archegonial heads form at the tips of female gametophytes; they produce eggs.
This image shows several archegonia, each containing a single egg. The eggs, much larger than sperm, are easily visible at this magnification.
When the egg is fertilized, a zygote is formed. The zygote grows to become a new sporophyte, which will grow as a stalk attached to the female gametophyte.
A capsule forms at the tip of each sporophyte. The capsule is part of the diploid sporophyte. Inside the capsule, some cells undergo meiosis to form haploid spores. Spores are not visible in this image.
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.
For more information on nonvascular plants, see the excellent website on bryophytes prepared by students in a botany class at the University of British Columbia.
This page updated September 30, 2013