Plant Systematics Laboratory #3
Evolution AND DIVERSITY of GREEN AND Land Plants
OBJECTIVES FOR THIS LABORATORY
1. Be able to recognize and name apomorphies for monophyletic taxa.
2. Understand the function and adaptive significance of these apomorphies.
3. Be able to identify and name the major plant groups and their features.
Green Plants (Chlorobionta)
Chloroplasts & cell wall
Pull off a young leaf of the aquatic plant flowering Elodea (Hydrocharitaceae) and prepare a "wet mount" by placing in a drop of water on a microscope slide and covering with a cover glass. Observe under a compound microscope. Observe the cells, each having a 1o cell wall that is composed largely of cellulose. The cellulosic wall in Elodea is thin and appears as a thin whitish line viewed with the light microscope.) Note also the green chloroplasts, which are pushed to the periphery of the cell by a large central vacuole (the membrane of which is usually invisible). The green color is caused by selective absorbance/reflectance of the major photosynthetic pigments, chlorophyll a + b. The chloroplasts may be moving in a circular motion, a phenomenon called cytoplasmic streaming or cyclosis. Observe the electron micrographs of chloroplasts, noting the thylakoids (chlorophyll-containing membranes) that are stacked into units called grana. (A cellulosic cell wall and chloroplasts containing chlorophyll b, grana, and starch are major apomorphies for all green plants.) Draw and label a cell, showing the cell wall and chloroplasts.
Oogamy and Antheridia
Observe the available material of the genus Chara or Nitella of the Charales, among the closest relatives of the land plants. Note the plant habit from preserved material. These are small (up to 30 cm tall), erect, fresh water aquatics. Observe the Chara whole mount prepared slides of sexual organs. Observe the spherical, sperm-producing antheridia, which are surrounded by a sterile "jacket" layer of cells. Also note the egg cell, which in Chara is enclosed within a structure called an oogonium (surrounded by noticeable helically-oriented cells). This type of sexual reproduction, in which one gamete (the egg) has become larger and non-motile is called oogamy. The antheridium, oogamy, and retention of the egg cell (e. g., in an oogonium) on the plant body represent major apomorphies for some Chlorobionta, among them the Charales and the land plants. Draw an oogonium (showing the egg and outer helical cells) and an antheridium, showing the sterile jacket layer and internal sperm cells.
Observe the demonstration of the fossil oogonia of Charales, some dating from as far back as the Devonian (345-395 million years ago).
Some General Adaptations of the Land Plants
The non-vascular land plants, including mosses, liverworts, and hornworts, share a number of derived features (apomorphies) with the vascular land plants. All of these represent major adaptations to life on land.
Cuticle. Observe the available prepared microscope slides of leaf cross-sections (e. g., Cycas revoluta, a seed plant, which has a particularly thick cuticle). The cuticle appears as a translucent, homogeneous layer covering the outer epidermal cells. What is it composed of? What is its function? Draw, labeling the cell, cell wall, and cuticle.
Antheridium. Observe the microscope slide of the available moss (e.g. Mnium) or liverwort (e.g. Marchantia) antheridium. Draw and label an antheridium, with enclosed sperm cells.
Archegonia. Observe the microscope slide of the available moss or liverwort archegonium. Note the egg cell and nucleus, ventral canal cell, neck, and neck canal cells. Draw and label an archegonium, including neck, neck canal cells, ventral canal cell and egg.
Non-Vascular Land Plants ("Bryophytes")
Observe the live or preserved material of a thalloid liverwort, such as Marchantia. Virtually all of the material you see here is haploid (n) gametophyte, which in the non-vascular land plants is the dominant, long-lived life cycle phase. In Marchantia the gametophyte is a flat mass of tissue or thallus. On the upper surface look for "gemmae cups," which contain "gemmae." These structures function in vegetative (asexual) reproduction; when a droplet of water falls into the gemmae cup, the gemmae spores may be dispersed some distance away, growing into a genetic clone of the parent. Pores in the upper surface allow for gas exchange. These pores are not true stomata, as they have no regulating guard cells. Also look for antheridiophores and archegoniophores, if present. What do these structures produce? Draw a thalloid liverwort gametophyte, including (if available) gemmae cups and antheridiophores or archegoniophores.
Observe the material of a leafy liverwort. Note the very different appearance of the gametophyte, which consists of a terete axis bearing three rows of leaves (generally two above or dorsal and one below or ventral); these leaves evolved independently from, and are thus not homologous with, those of vascular plants. Draw a leafy liverwort gametophyte in upper (dorsal) and lower (ventral) views, showing the three ranks of leaves.
Observe the prepared slide of a liverwort sporophyte, e. g., Marchantia. Note that the sporophyte is attached to (and nutritionally dependent on) the gametophyte. Also note the numerous spores (each a haploid product of meiosis) and surrounding elaters, specialized structures that hygroscopically aid in spore release. Prepare an outline drawing of the sporophyte and a close-up drawing of some of the spores and elaters.
Stomates are derived features for all land plants, with the exception of the Liverworts. It is as yet unclear if stomates evolved once or independently two or more times. Make a wet mount in a drop of water of an epidermal peel of the lower epidermis of available material (e. g., Coleus, Commelina, Graptopetalum, or Zebrina). Observe the stomates (= stomata), containing two guard cells which control the size of the pore (stoma). Stomata are the only epidermal cells with chloroplasts. Draw and label a stomate, noting the two guard cells and stoma.
The true Mosses are a group of non-vascular land plants that have evolved a diversity of morphological forms. Observe various moss gametophytes, the haploid, green, leafy part of the life cycle. Draw and label a moss gametophyte.
Moss Sporophyte. All land plants have a specialized life cycle phase: the diploid (2n) sporophyte. A young sporophyte attached to the gametophyte is called an embryo, hence the alternate name Embryophyta for the land plants. In the non-vascular land plants ("Bryophytes"), the sporophyte is relatively small, short-lived, and generally non-photosynthetic, being nutritionally dependent on the gametophyte. Observe sporophytes of mosses. These appear as brownish, short to elongate structures that are attached to the leafy, green gametophyte. Sporophytes grow from the fertilized egg (zygote) within the archegonium. The sporophyte terminates in an expanded sporangium (also called a "capsule" in "Bryophytes"), a specialized organ where meiosis occurs, producing spores. Most moss sporangia have teeth-like structures, called peristome teeth, that are hygroscopic, opening and closing depending on atmospheric moisture, effecting spore dispersal in the process. Draw and label a moss sporophyte, showing the seta (stalk), capsule (sporangium), operculum (cap), peristome teeth, and spores. [Optional: observe the capsule longitudinal-section.]
Sphagnum PEAT MOSS. Observe this moss, which grows in wet bogs and chemically modifies its environment by making the surrounding water acidic. If available, make a wet mount of live Sphagnum leaves and observe with the compound microscope. The leaves of Sphagnum are unusual in having two cell types: chlorophyllous cells that form a network around large, clear hyaline cells, the latter having characteristic pores and helical thickenings. The pores of the hyaline cells give Sphagnum remarkable properties of water absorption and retention, making it quite valuable horticulturally in potting mixtures. Observe the sample of peat. Peat is fossilized and partially decomposed Sphagnum, and is mined for use in potting mixtures and as an important fuel source in parts of the world.
Draw and label a portion of a Sphagnum leaf, showing the chlorophyllous cells and hyaline cells, the latter with pores and helical thickenings:
Observe the prepared slide of germinating moss spores, which develop into a filamentous protonema. The protonema represents a ancestral vestige, resembling a filamentous green "alga." It soon grows into a parenchymatous gametophyte. The spores of mosses and basal vascular plants have a thick outer layer called a perine layer, which may constitute an apomorphy for mosses and vascular plants. Also, note the 3-lined structure on the spore wall, called a trilete mark, that is the scar from the junction of the 4 spores after meiosis. Draw and label a spore, showing the trilete mark, a filamentous protonema, and a young gametophyte, if present.
Observe the live, preserved, or whole mounts of a hornwort, e.g., Anthoceros. Note that the gametophytes of hornworts are thalloid, very similar to those of the thalloid liverworts. Draw a hornwort, showing the thalloid gametophyte and the cylindrical sporophyte with a stout foot embedded in the gametophyte and surrounding gametophytic collar.
If available, study the longitudinal-section of a hornwort sporophyte. Draw and label the spores, central columella, and pseudoelaters, the latter functioning in spore dispersal.