The following five laboratory periods are designed to introduce you to the study of plant biology. The majority of the organisms you will be studying belong to the Kingdom Plantae (Campbell 6th Ed. 575-614; 7th Ed. 573-606). In addition, you will be examining certain plant-like organisms belonging to the Kingdom Protista (Campbell 6th Ed. Chapter 28; 7th Ed. Chapter 28) and selected representatives of the Kingdom Fungi (Campbell 6th Ed. Chapter 31; 7th Ed. Chapter 31). You will be examining the various levels of organization of the plant body, from the simple unicell through to the more complex seed bearing plants. When studying these organisms, try to ascertain how an organism exhibiting a particular level of organization performs the functions of growth, reproduction, nutrient procurement and water relations.
FUNGI (Filamentous
Heterotrophs)
(Campbell 6th Ed. Chapter 31; 7th Ed. 31)
Modern biologists recognize the uniqueness of the Fungi and most authorities have relegated them to a separate kingdom composed of three divisions (phyla): Zygomycota, Ascomycota and Basidiomycota. The Fungi are a diverse group which include such organisms as the yeasts, molds, mildews, rusts and mushrooms.
Some fungi are unicellular (yeast used in lab 4) but most have a filamentous organization. The filament is a tube called a hypha (plural, hyphae) which may or may not be divided into compartments by septa.
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The septa are specialized cross-walls that possess pores which allows for the interconnection of cytoplasm between compartments. A mass of branching hyphae is termed a mycelium which forms the body of the fungus. The hyphal cell walls contain chitin (also a structural component in some animal groups) and other complex polysaccharides and proteins. Growth of hyphae and hyphal branches takes place only at their tips. Growth is indeterminate. Being heterotrophic the organism cannot manufacture its own organic compounds. Soluble nutrients are absorbed into the mycelia through the hyphal surfaces. Fungi obtain their food either as parasites or as saprophytes. Fungi are capable of storing food reserves with the most common reserves being glycogen and fats. Fungi occupy a wide variety of both aquatic and terrestrial habitats. |
Rhizopus, the common black bread mold, is a terrestrial saprophyte belonging to the division Zygomycota. Typical of many fungi, the young mycelium of Rhizopus exhibits a circular growth pattern with hypha radiating from the central point of inoculation.
Click to view the young cultures of Rhizopus.
Note that hyphae are tubular, branched and aseptate.
The mature mycelium of Rhizopus is characterized by the presence of rather large surface hyphae termed stolons. At various points along a stolon, branched hyphae termed rhizoids are initiated. Certain hyphae, termed sporangiophores, grow upwards from the stolons developing swollen spherical sporangia at the tips.
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The mature sporangium is cut off from the sporangiophore by a thimble- shaped wall enclosing a sterile projection of the sporangiophore termed the columella. Within the sporangia asexual spores, termed mitospores, develop. Upon dehiscence of the sporangia these spores are liberated and disseminated by air currents, land on a suitable moist substratum and germinate into a new mycelium. Note the new mycelia do not differ genetically from the parent mycelium? Click to view the mature cultures of Rhizopus under a dissecting microscope. |
Sexual reproduction:
Sexual reproduction in Rhizopus and related genera occur by means of conjugation (click to view). When hyphae from opposite mating strains come into contact, lateral growths called progametangia are formed. The tips of the progametangia fuse, septa are laid down and two gametangia containing haploid nuclei are produced. The common wall between gametangia dissolves allowing for the intermingling and ultimate fusion of haploid nuclei from the two strains. A thick walled zygote, often called a zygospore, results. Click here to view both a young zygote and mature zygote of Rhizopus.
Mushrooms, along with puffballs, bird's nest fungi, rusts and smuts belong to the division Basidiomycota. All exhibit a filamentous level of organization. The structure commonly called a mushroom is the above ground reproductive or fruiting body of the organism. This fruiting body is composed of a compact mass of interwoven hyphae that form the stalk, cap and gills. It is attached to, and produced by, an extensive vegetative mycelium which grows saprophytically or parasitically beneath the surface of the soil.
Click here to view a typical mushroom
Details of the relatively complex life cycle of Agaricus will not be presented in this course.
