Name___________________________________
As you saw earlier, the algae are primarily aquatic; the few which do live on land are mostly unicellular or are simple filaments and usually occur in a habitat that is almost continuously moist. Except for floating forms, they are limited in their distribution within lakes, oceans, etc. by the depth to which enough light can penetrate to allow sufficient photosynthesis to maintain life and growth. Light is not such a limiting factor on land, but essentially all algae exposed for any length of time to the atmosphere desiccate and die. Most fungi do live on land, but they are dependent on the occurrence there of plants and animals to supply the food materials necessary for their heterotrophic existence.
The simplest green plants which show a considerable degree of adaptation to a terrestrial environment are bryophytes: division Bryophyta. As you look at the diversity of plants which occur in this group and study the details of the life cycle of one representative, try to see in what ways the plants are adapted to life in air and in what ways they fall short of being well-adapted land plants. Also, see if you can recognize characteristics which different bryophytes have in common and characteristics which distinguish them from the algae and fungi. Make a record which gives some idea of the variety in size, form and structure that can be found among bryophytes.
Today we embark on a new tack in evolutionary botany; we leave primitive forms behind us, and begin to concentrate on the types of true plants on our planet. Our goal is not only to see the form of these organisms, but also to begin to understand their life cycles, and follow the evolution leading to higher plants on earth.
Three classes of plants are recognized within division Bryophyta: Hepaticopsida (liverworts), Anthocerotopsida (hornworts), and Bryopsida (mosses). Liverworts show us the transition from algae to bryophytes without conducting tissue. Hornworts show us the appearance of intercalary meristems in sporophyte evolution, hinting at the dominance of sporophytes to be observed in truly vascular plants. Since mosses have rudimentary conductive tissue, they may be thought of as marginally vascular. Mosses have apparently evolved little from the common ancestor(s) shared with the line of evolution to the vascular plants. Thus, we may observe in these plants the most primitive form of vascular anatomy on earth. In addition, the life cycle of mosses is likewise more primitive than truly vascular plants. Thus each class of bryophytes helps us understand some of the steps of evolution leading to higher plants.
The majority of liverworts grow in moist, shady locations; a few are essentially aquatic. Two forms occur: thallose and leafy liverworts. Thallose liverworts consist of a flat, green, ribbon-like, dichotomously-branched thallus. This is very reminiscent of some algae; particularly Fucus. The thallus is anchored to the substrate by rhizoids arising from the lower surface. Leafy liverworts are also horizontal, but the thallus consists of "stems" bearing small "leaves" and rhizoids. The leafy liverworts are the more numerous, but they are so similar to some of the mosses in appearance that one is not always aware of how common they are.
Observe the gross appearance of a representative of the thallose liverworts such as Marchantia, Lunularia, or Conocephalum. All of these have a somewhat more complex body structure than many of the other liverworts. Note the rhizoids and scales on the lower surface. A thin cross section of the thallus should show that the cells in different parts of the thallus are specialized for different functions. What cell type(s) can you recognize?
Do you see any signs of vascular tissue (xylem or phloem)? yes no
What is the structure of the upper part of the thallus?
How does it differ from the lower part?
Can you see any evidence of the air chambers when you look at the upper surface of a whole thallus? yes no
Observe the central pores formed by chimney cells and the photosynthetic cells inside the chamber.
Asexual reproduction occurs commonly by fragmentation. As the thallus elongates near the tips, branching repeatedly, the older portion dies and the many ends become independent plants. Some species, such as Marchantia and Lunularia, also produce special asexual reproductive bodies known as gemmae. Find a cup-shaped (Marchantia) or half-moon-shaped (Lunularia) structure on the surface of the thallus. This is a gemma-cup. Note the small green bodies borne in the base of the cup. Detach one of these and observe it under the microscope. The gemma is a miniature thallus, and in a favorable environment it will start to grow rapidly.
From your experience with these, are gemmae sensitive to the gravity vector? yes no
Locate the growing point; is there only one or more than one? one more than one
Examine the prepared slides of Marchantia showing a gemma cup and several gemmae in section for comparison.
Sexual reproduction is similar to that for mosses. Male thalli produce antheridia at the end of a vertical stalk (antheridiophore) which is umbrella-like. Female thalli produce archegonia underneath finger-like projections on the end of a vertical stalk (archegoniophore). The sperm must be splashed or swim up a film of water to reach the egg in the archegonium.
Examine the male and female Marchantia plants available. Place a drop of water on the broad surface of the umbrella-like end of the antheridiophore. If we are lucky, you should observe sperm shedding and might be able to get a wet mount of the liverwort sperm. If this happens, we might try to "flood" some of this sperm water on some of the archegoniophores to see if we can get some sporophytes later in the semester.
Look carefully at a specimen of a leafy liverwort if available. How are the "leaves" arranged?
Detach a "leaf" and observe it under the microscope. Use the fine focus knob to focus up and down to see several optical sections.
How many cell layers thick is it? one two many
Does it have a vein (vascular bundle)? yes no
Dissect a "stem" longitudinally. Do you find any evidence of vascular tissue? yes no
How does a rhizoid compare with a root?
Although we call these "leafy" liverworts, and speak of "stem" and "leaves,"
do these plants have a true stem, leaves, or roots? yes no
The hornworts are a small group in which the body form is similar to that in the simplest thallose liverworts. The gametophyte thallus consists of a narrow, green, ribbon-like structure with essentially no internal differentiation. They are separated from the liverworts because of certain unusual characteristics of the sporophyte often considered to be advanced. These have an intercalary meristem at the base of the seta, are photosynthetic, and have guard cells. If living specimens of these rare plants are available (Anthoceros is most likely), your instructor may direct you a deeper investigation of these interesting plants. If not, then examine any available prepared slides of these remarkable transitional organisms.
How many chloroplasts are in each cell of the thallus? one two many
Neither liverworts nor hornworts are as highly "vascularized" as advanced mosses, so we will expand our encounter with the mosses. The mosses include two groups--the true mosses (order Andreales and others) and the sphagnums (order Sphagnales).
Observe the plants of Sphagnum available. How does their color compare to that of the different examples of liverworts and mosses which are also available in the laboratory?
Remove an upright stem with leaves. Squeeze it between your thumb and forefinger. What happens?
Carefully remove a leaf from the plants you squeezed and study it under the microscope. What is its structure?
The overall color of the plant changed when you squeezed it because empty chambers reflect white light.
Sphagnum grows in very wet areas and often forms complete mats over the surface of open areas of water in bogs; it thus acts as a pioneer plant in the succession occurring in some aquatic habitats leading to the formation of soil and the establishment of vascular plants. It flourishes when the water is highly acid, a condition discouraging bacterial and fungal decay, so that dead plants accumulate, resulting in the formation of thick deposits of peat. Dried Sphagnum was used in the US Civil War as a dressing for wounds; the acids discouraged infectious bacteria. It was also used as stuffing for upholstery.
The life cycle which you will consider in detail is that of a true moss, so you will learn quite a bit about the structure of this group. Your study of the life cycle will include more than one species since you will use whichever species is most readily available and shows certain stages most clearly. The genera from which most of the material will be taken are Mnium and Polytrichum. As you work today, you will want to illustrate the various stages in the life cycle of a moss, making sure to keep clear the relationships of the various stages to one another.
1. Obtain a few fresh spores from a Polytrichum or Mnium capsule, mount in water, and observe them under the microscope.
What is their shape? ____________ Size? ______________ Color?________________
2. Examine the moss culture you started earlier in the term under a dissecting microscope. What is the appearance of the cells which have resulted from germination of the spores?
While watching under the microscope, remove several areas of protonema which show different stages in development, mount in water, and observe them under the compound microscope.
Can you find any small buds that are the start of the upright leafy shoots? yes no
Tease apart a small bit from the moss culture in your Petri dish with needles to isolate a single upright plant and place it in water on a slide.
What different parts can you distinguish under the microscope?
Remove a "leaf" from the plant and check its structure.
How thick is it? ______________ cells. Is it a true leaf? yes no
The "stems" are relatively thick. You may wish to try to section one from the wild to see if its cells are differentiated, but there are no tracheids or vessels and no sieve tube elements, although some cells may approach the latter in their characteristics.
Does the moss plant have any method of asexual reproduction? yes no
(If you can see no examples, suggest ways in which this might be accomplished).
3. Some species of moss are monoecious, that is, the same plant can produce both antheridia and archegonia, but both Mnium and Polytrichum are usually dioecious with antheridia and archegonia produced on different plants. Both types of gametangia are produced at the tips of leafy shoots which become expanded and bear many antheridia or archegonia intermingled with sterile multicellular hairs called paraphyses. Under a binocular microscope, dissect part of an antheridial head.
What is the shape of an antheridium?__________________________
Observe a prepared slide of a longisection of a similar antheridial head. These may show only pieces of antheridia, but having seen a whole antheridium, you should be able to interpret these.
How many cells compose an antheridium? One Few Many
Do all cells of the antheridium produce sperm or are some cells sterile? All Some
If some are sterile, which ones?___________________________________
Each small, heavily-stained cell becomes one sperm cell. Estimate how many sperm are produced by one antheridium. One Few Many
Each sperm is small, composed largely of an elongated nucleus with only a thin layer of cytoplasm, and is biflagellate. Sperm will be shed from the antheridium only when water is available.
How does the moss antheridium compare to an algal antheridium?
4. Similarly dissect part of an archegonial head and observe the characteristics of an archegonium. Prepared slides having longisections of an archegonial head show pieces of the archegonia and only rarely a complete one. Sections do help to understand the structure, however. Note that there are three regions in an archegonium: a moderately broad stalk, slightly broader venter, and a somewhat narrower, elongated neck. A single row of cells occurs centrally in the neck and venter; the largest cell, the egg, is at the basal end of the row, in the venter. Above this is a ventral canal cell then a long row of neck canal cells. At the time the egg is mature, and if water is available, the neck opens at the outer end, neck canal and ventral canal cells disintegrate, and one to several sperm swim down the neck to the egg which is fertilized by only one of the sperm.
Which gametangium has a higher ratio of sterile to fertile cells? antheridium archegonium
How does the moss archegonium compare to the oogonium of an alga?
5. The zygote remains within the archegonium and starts developing almost immediately. Initially the young spindle-shaped embryo is undifferentiated, but soon three parts become recognizable: a foot which grows downward into the apex of the leafy gametophyte shoot, a stalk (seta), and a terminal sporangium (capsule). The stalk elongates considerably, raising the capsule well above the top of the leafy shoot. There are some chloroplasts in the capsule wall and a few stomata occur in the epidermis of the basal portion, so it can make its own food. On the other hand, the young sporophyte's nutrition largely comes via an indirect connection: the foot embedded in the leafy gametophyte plant which bore the archegonium. During the early stages of growth from the zygote, the venter of the archegonium also grows to form a protective cover over the embryo. When the stalk elongates rapidly, this archegonial tissue is torn loose and is carried up as a covering (calyptra) for the capsule. What generation of the plant life cycle is
the calyptra? gametophyte sporophyteWhat is the relative chromosome number of the:the green leafy plant? gametophyte sporophyte
the plant composed of foot, stalk, and capsule? gametophyte sporophyte
| leafy plant? | 1N 2N 3N | archegonium? | 1N 2N 3N | |
| foot? | 1N 2N 3N | antheridium? | 1N 2N 3N | |
| stalk? | 1N 2N 3N | protonema? | 1N 2N 3N | |
| capsule? | 1N 2N 3N | sperm? | 1N 2N 3N | |
| calyptra? | 1N 2N 3N | egg? | 1N 2N 3N |
6. Obtain a moss plant bearing a sporophyte. Remove the calyptra. Note that the capsule has a distinct cap (operculum) at its outer end. Remove this, and then look directly down on the exposed end using a dissecting microscope or very low power of the compound microscope. Note the ring of jagged teeth (peristome) around the periphery of the opening. Wrap the capsule in a piece of wet paper toweling for a minute or two, then observe the position of the peristome teeth. Allow the capsule to dry under a lamp and observe again.
What happens to the peristome teeth with changes in humidity?
Squeeze the contents of the capsule onto a slide. What do you obtain?______________
What is the probable function of the peristome teeth?
Look at a prepared slide showing a longisection of a young sporophyte. Locate the three parts of the sporophyte. In the capsule, locate the operculum, sterile jacket cells, central column of sterile cells (columella) and the cylinder of sporogenous cells. With the formation of spores you are back to the cells with which you started this laboratory session. Be sure to complete your diagrams of the life cycle stages for the moss.
Mosses and liverworts are never large; one moss reaches about 6-8 inches in height, but most are 1 inch or less. Almost all grow in moist habitats, though a few mosses are found in dry areas.
Why do you think these plants do not get larger?
Why are they so restricted in habitat?
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