Survey of the three domain systemPRIVATE

                The vast assemblage of diverse organisms that we call plants are grouped into up to 20 million species (depending on how you count). To facilitate study of these species, several classification systems have been proposed. Classifications, of course, are human inventions and may be designed to suit scientists' convenience or reflect their primary interest in particular groups of organisms.  Our contemporary understanding has been achieved by devising a classification that expresses the consensus among a variety of types of biologists- taxonomists (classifiers), molecular biologists (gene sequencers), morphologists (organismal structure), evolutionary biologists (theorists), and many others.  The system reflects, to the best of currently available knowledge, the course of evolutionary history.  Like many things in science, this system has its problems, but it is believed better and more accurate than prior systems and is always subject to evaluation and revision.  According to this classification scheme, life on earth is divided into three major groups: the domains bateria, archaea, eukarya.  Many of the organisms familiar to you are placed within the eukarya-  including the old-type “kingdoms” encompassing the algae, plants, fungi and animals.

This lab will provide some overview and the chance to make some careful observations of these groups in the context of the three-domain classification system.

I.               Domain: Bacteria:

A.      Heterotrophic bacteria

Most species of bacteria are unicellular and cannot be seen with the unaided eye. Even with the aid of a microscope, stains must be used to see details of bacterial structure.  Bacteria come in three shapes: bacillus (rod-like), spirillum (spiral) and coccus (spherical); classification is based primarily on the biochemical activities of the organisms.  Bacteria do not have a nuclear membrane and reproduction is primarily asexual.  Some bacterial have the ability to produce spores that are highly resistant to adverse environmental conditions; these spores are the bases for the food processing and disinfectant industries. A few bacteria are photosynthetic but this is the exception rather than the rule.

                Examine a slide marked "Bacterial Types" which has a sample of each of the three bacterial forms. Even though the organisms have been stained, they are at the limits of resolution of your microscopes. Focus carefully, and you should be able to see all three forms; the coccus forms are the smallest in size.  Recall your exercise of some weeks ago estimated the size of bacteria.

B.       Autotrophic bacteria (cyanobacteria)

                Anabena is a filamentous cyanobacteria that exists as coils.  By following a single coil along its length, you may find single, enlarged, thickwalled, apparently empty cells; these are heterocysts, involved in nitrogen fixation (what is that).  Cells of similar shape and size but filled with protoplasm are the akinite cells or resting spores that are a survival mechanism. Akinites have been known to germinate and produce new individuals after storage for 87 years.

(Differentiate the “way of making a living” between the heterotroph bacteria and autotroph cyanobacteria.)

II.             Domain: Eukarya

Make sure you a comfortable with the differences between the Bacteria and Eukarya. 

A.      The algae

The old-style kingdom “Protista” used to be used to collect and classify a very diverse of unicellular and multicellular though sometime colonial, generally photosynthetic, simple Eukarya.  This no longer exists by the simple observation that it attempted to collect too many varieties of organisms under one taxon and in the process could be thought of as a “miscellaneous box” for the classification of life.  That is, it was of very little practical use.

                These organisms have now been spread across the Eukarya to reflect their tremendous diversity.  We will focus on a few select Eukarya to give you some idea of this diversity.

                The algae are chiefly aquatic, occurring in freshwater lakes and streams, in swamps, and in the sea. Some algae occur in moist situations on land such as wet soil and moist surfaces of trees and rocks. They are variable in shape, unicellular vs. multicellular and most reproduce both sexually and asexually. The all contain chlorophyll (of some type) and are photosynthetic, and may contain other pigments that give them distinctive colors.

Green algae (Chlorophytes): Examine a specimen or slide of Spirogyra, a filamentous green alga. The most conspicuous internal structure in each cell is the chloroplast, which is in the form of a ribbon spiraling from one end of the cell to the other. Can you follow the chloroplast along one entire cell? Starch storing structures called pyrenoids are embedded in the chloroplast at regular intervals and should be conspicuous. The nucleus is suspended by cytoplasmic strands in the middle of the vacuole at the center of the cell.

Brown algae (Phaeophytes):  Many algae attain very large size. Fucus, or rockweed, is common along ocean coasts in the temperate zone and is exposed at low tide. Its branching, leaf-like thallus bears air bladders that lift the organism toward the surface. Within the tips of some of the branches are small spherical cavities called conceptacles, within which the eggs and sperm are produced. The position of each conceptacle is marked by a pore that opens to the exterior and is visible to the unaided eye. The eggs and sperm may be produced in the same conceptacle, but more commonly they are found in separate cavities on the same or on different plants. Examine a specimen of Fucus and identify the structures named above.

B.       The fungi

The generally filamentous, multicellular, heterotrophs (by external digestion), spore-producing, cell-wall-made-of-chitin organisms that reproduce both sexually and asexually have been placed in the “Fungi”.  Unlike the “Protista”, recent revisions in classification have changed this particular Kingdom little.  (The basic elements of the structure of fungi is shown in fig. 31.2 (pg. 609) of your text.)  The best known fungi are the more economically useful ones such as yeast, edible “mushrooms” of all sorts and damaging molds and mildews & rusts. 

Unicellular fungi: Yeasts are the fungi most widely employed industrially.  (In what industries?)  Examine a sample of yeast cake or culture in a wet mount of water or methylene blue. Do yeasts exactly fit the description of fungi given above? It may be possible to see structures inside the yeast cells; these are inclusions of oil droplets.  We will prepare a demonstration of the metabolic process of fermentation.

The zygote fungi (Zygomycota): Some fungi can reproduce asexually for indefinitely long periods of time. Perhaps the most important or these are Rhizophus stolonifer, the common bread mold, and Penicillium chrysogenum, the source of penicillin.  In Rhizophus, the asexual spores are produced inside sac-like structures called sporangia that are carried on stalks.   Each sporangium contains thousands of spores.  Examine a slide and identify the sporangia and stalks.  The name Penicillium is derived from penicillus, meaning "little brush."  Examine a slide and decide if this name is appropriate.

Although fungi are generally considered to be microorganisms, they often produce macroscopically visible structures; good examples are the fungal “sacs” and “mushrooms”. Throughout most of the year, the mushroom organisms exist as filaments within the soil; when environmental conditions are favorable, the fruiting structures or mushrooms are produced.

The sac fungi (Ascomycota): Examine some examples of fungal “sacs”.  Morels are highly prized by cooks for their delicate flavor.  These too are sac mushrooms.

The club fungi (Basiomycota):  Examine a more traditional “mushroom”. Identify the stipe (stalk), pileus (cap) and lamellae (gills). Make a wet mount of a piece of lamella and examine the spores.  The process of meiosis occurs in the lamellae to produce the haploid spores.  (What do all these terms mean?)

C.       The Plants

Before you start, review the following terms in your text: haploid, diploid, gamete, fertilization, sporophyte, gametophyte, & meiosis.

1. Bryophytes:      The Bryophytes are the simplest land plants. Their tissues are more highly differentiated than in the other photosynthetic eukarya, the algae.  Bryophytes lack of conductive or vascular tissue (xylem and phloem). Although structures resembling stems and leaves are present, they are not true stems and leaves due to the lack of vascular tissue, no roots are present, and the plants are anchored in the soil by elongate single cells or filaments of cells called rhizoids. Bryophytes produce true embryos, and they have been described as the “frogs” of the plant kingdom since they require water for their motile sperm to effect fertilization. The Bryophytes are commonly called mosses, liverworts and hornworts, although it must be pointed out that not all plants that are called "moss" in common parlance are true mosses; Spanish moss, for example, is not a true moss.

                The most characteristic aspect of the life of Bryophytes is their alternation of generations. Unlike higher plants and animals, in which the conspicuous organism of the life cycle is diploid (2N), the most conspicuous organism in the bryophyte life cycle is haploid (N) (the green-tinted portion of fig. 29.8 (pg. 581)), with the diploid states being much shorter-lived and dependent upon the haploid state for its support and nutrition in many cases.

                Using your textbook and the illustrative materials provided, examine the structures in the life cycle of a true moss. The green "leafy" plant that is the conspicuous part of the life cycle is haploid (as noted above) and is technically called the gametophyte because it bears the sex organs inside which the gametes are produced: eggs inside the archegonia and sperm inside the antheridia. Fertilization occurs when a sperm swims from the antheridium down the neck of the archegonium, and fuses with the egg cell. A diploid zygote is formed, and an embryo subsequently develops. A tapering, food-absorbing foot grows from the embryo and buries deeply in the tissue of the gametophyte, a slender stalk grows upward, with an enlarged capsule on its tip. The foot, stalk and capsule constitute the sporophyte and are all diploid. Meiosis occurs in parent cells within the capsule, giving rise to haploid spores. The spores, upon release from the capsule, germinate and develop into the haploid gametophyte to continue the cycle.

                Examine all structures named above.

2. Seedless vascular plants (ferms and their kin):  The ferns and their kin are better adapted to life on land, and tremendously successful.  However, they, like the mosses, are not true land plants.  They possess simple roots and leaf-like structures (called fronds).  Well-developed vascular tissues (xylem and phloem) transport substances from one part of the plant to another.

                In terms of life cycle, the sporophyte (diploid) phase is the dominant one, the gametophyte is small, inconspicuous and short-lived. In most species, the fusion of gametes still depends on moisture.

                There are about 9,000 living species of ferns. Ferns maintain the alternation of generations. Unlike mosses, the large plant that is conspicuous is the sporophyte, with haploid spores produced on the underside of fronds in specialized structures called sporangia.  The haploid spores are shed, and develop into small, heart-shaped gametophytes on the surface of the soil. The gametophyte bears archegonia (female reproductive organs) and antheridia (male reproductive organs) on the same plant. When moisture is adequate, mature sperm are released and swim to the archegonia, where fertilization occurs. As the diploid sporophyte develops, the gametophyte disintegrates.

                Specimens of various materials are available for examination.  Included are horsetails- a fascinating primitive member of fern “family”.

3. Seed plants: 

a. Gymnospermae: The gymnosperms have no true flowers, the seeds are borne naked on the surface of the cone scales. There are about 640 species of living gymnosperms, that are divided into four groups- cycads, ginkoes, Conifers (conifers, or common evergreen trees with needle-shaped leaves) and the very odd gnetales (tropical climbing shrubs).

                The pine (a conifer; fig. 30.6 (pg. 597)) will be used as an example of a gymnosperm life cycle. The pine tree is the sporophyte plant. It produces two types of haploid spores, which give rise respectively to male and female gametophytes. The "male" spores, called microspores, are produced in small cones, 1 to 2 cm in length, borne on the lower branches of the tree. The pollen grain is the male gametophyte, and is dispersed by wind or insects to the female gametophyte that is held in ovulate ("female") cones higher in the tree. The ovulate cones are the "pine cones" conspicuous on the trees. Unlike the male gametophyte, the female gametophyte never leaves the sporophyte plant and is entirely dependent on it. On fertilization of the egg by the sperm delivered by a pollen grain, a diploid sporophyte embryo is formed, which, provided with food and a protective coat, makes up the seed.

                On being planted, the embryo grows into the mature sporophyte. Various types of cones, both "male" and "female," will be available for observation.

b. Angiospermae:  The flowering plants are the most complex and successful land plants. They include about 75% of all living plant species. The reproductive systems of angiosperms achieve further refinement over the gymnosperms. The flowering plants that we see around us are all sporophytes; stems and leaves are modified to form flowers, which contain the gametophtye generation which has been reduced to the pollen and the egg and its associated cells. Fertilization of the egg within the ovary of a flower leads, as in conifers, to the growth of an embryo sporophyte, which, with its surrounding tissues and protective coat, constitutes the seed. The angiosperms, however, go one step further than the conifers, enclosing the seed in a fruit, which develops from the tissues of the flower. There are two sub-groups of Angiosperms:

                i) Monocots: The monocots have only one cotyledon (a food containing, leaf-like structure) within the seed. Vascular bundles are scattered in the stem and leaves have parallel veins. The flower parts (sepals, petals, stamens and carpels) are in threes or sixes.

                ii) Eudicots: The embryos of dicots possess two cotyledons within the seeds. Vascular bundles are in a ring in the stem and leaves have network venation. The flower parts are grouped in fives, fours or twos.

                Examples of monocots and dicots will be available for examination.