Lab 9
Fern Allies, Ferns, and Mosses

    Whereas the evolutionary relationships among the lower organisms studied in some previous laboratory exercises are in dispute, the evolutionary relationships of terrestrial plants are much clearer.  All land plants, by virtue of lacking or possessing vascular tissue (xylem and phloem), may be placed in either of two groups, the vascular plants or nonvascular plants.  Both are believed to be directly descended from green algae, with which they share several important characteristics.  Among these characteristics are possession of the pigments chlorophyll a and b, cell walls of cellulose, and starch as the medium of glucose storage.

    Survival on land required the development of physical attributes and physiological strategies quite different from those involved in living in the water.  Organisms which are well adapted to land have found ways to:
    1.  obtain enough water when fluid no longer bathes the entire surface of the body.
    2.  transport water and nutrients throughout the body of the organism.
    3.  prevent excessive loss of water by a protective covering.
    4.  support a large body against the pull of gravity.
    5.  carry out reproduction without dependence on external water.
    6.  maintain a constant internal environment.

    In all probability the nonvascular land plants evolved first and as a consequence are less well adapted to life on land than those with vascular tissue.  In this exercise we will examine selected taxa from each of these groups.  See figure 1.

Question 1:  What are some characteristics held in common by green algae and plants?

Question 2:  Why are nonvascular land plants not as well as adapted to life on land as are vascular plants?

OBJECTIVES:
    1.  To diagram the life cycle of each of the major taxa presented.
    2.  To identify the gametophyte and sporophyte generations of each major taxon presented.
    3.  To identify the anatomical parts indicated in the exercise.

I.  Division Bryophyta - mosses and liverworts

    The bryophytes were the first plants to live on land.  With very minor exceptions, bryophytes differ from higher plants in lacking a vascular system (xylem and phloem).  Because of this, none of the species becomes very large, although some may form extensive low mats.  They cannot grow or function very long without moisture; hence their association with damp habitiats.

Question 3:  Why can bryophytes not function long without moisture?

    The gametophyte (haploid) generation is dominant in these plants.  The gametophyte produces eggs within the archegonia, and motile sperm are produced in the antheridia.  Bryophytes are dependent on external moisture to ensure fertilization because the sperm must swim to the egg.  The zygote developing within the archegonium is protected from drying out, however.  The sporophyte (diploid) generation is also nonvascular and dependent upon the gametophyte generation.  Another adaptation to land exhibited by bryophytes is that spores which disperse the species are windblown.  See figure 2.

Question 4:  What is the function of the archegonia and antheridia?

ACTIVITIES:

    1.  Examine the liverworts available.  Note that the life cycle is similar to the life cycle of moss.  See figure 3.

    2.  Examine the clump of moss provided, and preserved specimens.  The clump consists of small green "leafy" gametophytic plants.  You may see sporopohytes (the moss capsule) emerging from the tips of some leafy shoots.  The gametophytes first appear as leafy shoots on the slender alga-like threads of protonema, which emerge from moss spores.  The archegonia give rise to the sporophytes, consisting of a stalk (seta) and capsule (sporangium).  Within the capsule, spore mother cells undergo meiosis to produce spores which are released through a toothed peristome at the top.

    3.  Utilizing the prepared slides of moss protonema, moss antheridial head, moss archegonial head, and mature capsule, locate all structures emphasized by your instructor and underlined above.  See figure 4.

    4.  Be prepared to label figure 4, indicating whether it is the gametophyte or sporophyte generation.

Question 5:  Does the moss have roots?  true leaves?

Question 6:  What is a zygote?

The Tracheophytes - Vascular Plants

II.  Division Psilotophyta - Whiskferns

    This division contains relatively simple plants.  The prototype organism, Psilotum, is a tropic and subtropic native but grows also in Texas, Louisiana, Arizona and Florida, and is commonly known as whiskfern.  They lack true leaves and roots and are anchored by a rhizome.  Psilotum is sometimes called a living fossil because of its relative simplicity and its remarkable resemblance to early vascular plants.  See figure 5.

    Psilotophytes are a division of seedless vascular plants.  The whiskferns produce separate gametophyte and sporophyte plants which require moisture for the flagellated sperm.  The reproduction process in these organism is very similar to that of ferns.

ACTIVITY:

    1.  Observe the Psilotum herberium specimens and other preserved specimens or illustrations.

Question 7:  What is a rhizome?

III.  Division Lycopodophyta - club mosses

    This division is an ancient one, which in past ages formed a much more important part of the vegetation of the earth than it does today.  During the Carboniferous period (around 250-300 mya), giant, tree-sized lycopods dominated the extensive forests.  It is from the remains of these forests that the extensive coal deposits of the Appalachian region (and other regions) were formed.

    Only a few lycopods have survived to the present day and all are small, inconspicuous plants confined to two genera.  Lycopodium includes the species with the common name club mosses, so named for their small, scale like leaves and the club-shaped strobili on which the spores are formed.  See figure 6.

    Selaginella (spike mosses) includes several hundred species, most of which are tropical.  Of special biological interest is the fact that some species of Selaginella are heterosporous, that is they produce two types of spores.  The small microspores give rise to a male gametophyte which produces sperm, while the large megaspores leads to the female gametophyte which produces the egg.  A variation of this pattern occurs in the higher vascular plants.  See figure 7.

Question 8:  What is meant by the term heterosporous?
                    homosporous?

ACTIVITIES:

    1.  Examine the herbarium and other preserved specimens of club mosses.

    2.  Examine the microslide of a Selaginella stobilus.  See figure 7 and locate the structures your instructor emphasizes.

Question 9:  What is a sporophyll?

IV.  Division Equisetophyta - horsetails

    Coexisting with the giant lycopods in the Carboniferous swamps were large plants called Calamites.  The size, number and importance of species closely related to the Calamites has diminished to the point that today only one living genus, Equisetum, remains in the subdivision Sephenopsida.

     Members of Equisetum go by the common name of horsetail or scouring rush.  The former name is derived from the general appearance of the plant, the latter name is owing to the relatively great amount of silica deposited in the stems, rendering them very tough and gritty.  See figure 8.

ACTIVITIES:

    1.  Examine the herbarium specimens and plastomounts, as well as any living specimens provided.  Note the branches, where they occur, emerge as whorls at regularly spaced nodes.  Also at the nodes, observe the whirl of minute, scale-like leaves.  Note the ridges on the stems.  Run your fingernail over the stems to feel the deposits of silica.  Note the strobilus located at the end of fertile shoots.

Question 10:  Where are photosynthetic chlorophylls located in the horsetail?

V.  Division Pteridophyta - ferns

    The ferns are generally small plant forms growing in woodland habitats under moist, temperate climatic conditions.  In dry areas the ferns attain their growth and reproduction during the brief rainy season and remain more or less dormant throughout the rest of the year.  Ferns exhibit their greatest abundance and diversity of form in the tropical rain forests, ranging from those having fronds (leaves) 1/4 to 3/4 inches in length, to 50-foot tree ferns with fronds measuring 20 by 5 feet.

Question 11:  Why are most ferns confined to moist environments?

ACTIVITIES:

    1.  Examine the herbarium specimens and plastimounts, as well as any living specimens provided.  Utilizing figure 9, locate the structures underlined as outlined below.
        a.  Note the rhizome with its brown scaley covering and adventitious roots.  Examine it carefully for any young leaves (fronds) which may be emerging.  These coiled structures are termed fiddleheads.
        b.  Now examine a mature frond.  It is composed of a petiole, rachis, and leaflets.  Most fronds are pinnate, with some being twice or three times pinnate.  On some of them should be found small, cinnamaon-colored fruit dots or sori.  Within these sori are the sporangia in which the spores are formed.  In some ferns, specialized fertile fronds bear all the spores and vegetative leaves do not have sori.

Question 12:  What are adventitious roots?

Question 13:  What does pinnate mean? 

    2.  View the sori on the fern leaflets which your instructor has set up under the dissecting microscope.

    3.  Select a slide of the fern sporangia.  Identify the sporangia and the spores.  The spores are dispersed when mature and moisture is present.  This phenomena may be demonstrated on the microscopes under 10X.

Question 14:  Is the fern heterosporous or homosporous?

    4.  The fern spore germinates and grows into a small, heart-shaped gametophyte called the prothallus.  Obtain "said" slide.  Utilizing figure 9, locate the fern archegonia, near the cleft of the prothallus.  Eggs are produced here.  Locate the several antheridia.  The motile sperm of the fern are produced in the antheridia.