Cycle Of Flowering Plants

Figure 1Q.14 A. A common Russula mushroom. B. Earth stars (Geastrum sp.) C. A shelf or bracket fungus (Phacolus). (B. Courtesy Perry J. Reynolds; C. Courtesy Richard Critchfield)

Chapter 19

Sexual Reproduction

Sexual reproduction in many club fungi mushrooms begins in the same way as it does for members of the two fungal phyla previously discussed. When a spore lands in a suitable place—often an area with good organic material and humus in the soil—it germinates and produces a mycelium just beneath the surface. The hyphae of the mycelium are divided into cells that each contain a single haploid nucleus. Such a mycelium is said to be monokaryotic. Monokaryotic mycelia of club fungi often occur in four mating types, usually designated simply as types 1, 2, 3, and 4. Only types 1 and 3, or types 2 and 4, can mate with each other.

If the growth of the hyphae of compatible mating types happens to bring them close together, cells of each mycelium may unite, initiating a new mycelium in which each cell has two nuclei. Such a mycelium is said to be dikaryotic. Dikaryotic mycelia usually have little walled-off bypass loops called clamp connections between cells on the surface of the hyphae (Fig. 19.15). The clamp connections develop as a result of a unique type of mitosis that ensures each cell will have one nucleus of each original mating type within it.

After developing for a while, the dikaryotic mycelium may become very dense and form a compact, solid-looking mass called a button. This pushes above the surface and expands into a basidioma (formerly known as a basidio-carp), commonly called a mushroom (Fig. 19.16). Most mushrooms have an expanded umbrellalike cap ( pileus) and a stalk (stipe). Some have a ring called an annulus on the

Clamp Connection Basidiomycetes

Figure 1Q.1S Development of a clamp connection. Left to right. A protrusion appears in the wall of a cell, and one nucleus migrates into the loop. Both nuclei undergo mitosis, and the loop carrying the daughter nucleus turns toward the cell wall. A clamp connection is established, and the daughter nuclei pair in two cells.

Figure 1Q.1S Development of a clamp connection. Left to right. A protrusion appears in the wall of a cell, and one nucleus migrates into the loop. Both nuclei undergo mitosis, and the loop carrying the daughter nucleus turns toward the cell wall. A clamp connection is established, and the daughter nuclei pair in two cells.

Life Cycle Typical Mushroom

Figure 1Q.1Ô Life cycle of a typical mushroom.

Kingdom Fungi 367

Death Angel Mushroom
Figure 1Q.17 A death angel mushroom (Amanita). Note the egglike volva at the base. (Courtesy Dr. T. Duffy)

Basiodiospore stalk. It is the remnant of a membrane that extended from the cap to the stalk and tore as the cap expanded. Some mushrooms, such as the notorious death angel and destroying angel mushrooms (Amanita spp.), also have a cup called a volva at the base (Fig. 19.17). Thin, fleshy-looking plates called gills radiate out from the stalk on the underside of the cap. Microscopic examination of a gill reveals it is composed of compacted hyphae, with large numbers of basidia oriented at right angles to the flat surfaces of the gill.

As each basidium matures, the two nuclei unite, and the diploid nucleus undergoes meiosis. The four nuclei that result from meiosis migrate through four (in a few species, two) tiny pegs at the tip of the basidium, walls forming around the nuclei in the process. The resultant cells are now basidiospores. The tiny pegs, called sterigmata (singular: sterigma), serve as stalks for the basidiospores (Fig. 19.18). One large mushroom may produce several billion basidiospores within a few days. These are forcibly discharged into the air between the gills. They then drift downward and blow away with the slightest breeze.

If you remove a mushroom stalk and place the cap gillside down on a piece of paper, covering it with a dish to eliminate air currents, the spores will fall and adhere to the paper in a pattern perfectly reflecting the arrangement of the gills. The dish and cap can be removed a day later, and the spore print (Fig. 19.19) can be made more or less permanent with the application of a little clear varnish or shellac. Such spore prints can be used as an aid to identification, employing white paper for dark-colored spores and black paper for white or light-colored spores.

In nature, some of the basidiospores eventually repeat the reproductive cycle. Often a dikaryotic mycelium radiates out from its starting point, periodically producing basidiomata in

Arrangement Basidiospores
Arrangement Basidiospores

Figure 1Q.18 A small part of a mushroom gill showing basidiospores produced on the club-shaped basidia, x500. Figure 1Q.1Q A spore print.

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so-called fairy rings (Fig. 19.20). If conditions are favorable, the mycelium continues to grow at the edges for many years while dying in the center as food resources are depleted. Some mycelia have been known to grow in this fashion for over 500 years.

Some mushrooms such as boletes produce their spores on the surfaces of thousands of tiny pores instead of on gills (Fig. 19.21).

Shelf, or bracket, fungi (Fig. 19.22) grow out horizontally from the bark or dead wood from which they have grown, some adding a new layer of growth each year. Perennial species can become large enough and so securely attached that they can support the weight of a human adult.

Figure 1Q.20 A fairy ring of mushrooms (Pholiota sp.). The mushrooms appear periodically toward the edge of a dikary-otic mycelium that grows out from a point where the first dikary-otic cell was initiated.

Figure 1Q.21 A bolete (Boletus sp.) mushroom. The basidiospores are produced at the margins of pores instead of along gills.

Other members of this phylum produce spores within parchmentlike membranes, forming somewhat ball-like basidiomata called puffballs. Puffballs, which prior to developing their spores are generally edible, range in diameter from a few millimeters to 1.2 meters (0.125 inch to 4 feet) (Fig. 19.23). They have minimal or no stalks and rest in contact with the ground. Literally trillions of spores may be produced by a large puffball. These are released through a pore at the top or from random locations when the outer membrane breaks down. Earth stars (see Fig. 19.14B) are similar to puffballs but differ from them in having at the base an additional membrane that splits and resembles a set of woody flower petals.

Bird's-nestfungi (Fig. 19.24) grow on wood or manure and form nestlike cavities in which small egglike bodies containing basidiospores are produced. In some species, each "egg" has a sticky thread attached to it. When raindrops fall in the nests, the eggs may be splashed out, and as they fly through the air, the sticky threads catch on nearby vegetation, whipping the eggs around it (Fig. 19.25). When animals graze on the vegetation, the spores pass unharmed through the intestinal tract.

Smuts are parasitic club fungi that don't form basi-diomata. They do considerable damage to corn, wheat, and other grain crops. In corn smut (Fig. 19.26), the mycelium grows between the cells of the host. The hyphae absorb nourishment from these cells and also secrete substances that stimulate them to divide and enlarge, forming tumors on

Tree Trunk With Shelf Fungus
Figure 1Q.22 A shelf, or bracket, fungus growing out from the trunk of a tree.

Kingdom Fungi 369

Corn Fungus Cells

Figure 1Q.23 Agiant puffball. (Courtesy John Hardy)

Basidiospores Formed Cedar

Figure 1Q.24 Bird's nest fungi. Each "egg" in a nest contains basidiospores that are dispersed when raindrops splash them out.

the surfaces of the corn kernels. These eventually break open, revealing millions of sooty black spores, which are blown away by the wind. Some smuts affect only the flowering heads or grains, while others infect the whole plant.

Rusts are also parasites that don't form basidiomata. They attack a wide variety of plants. Some rusts grow and reproduce on only one species of flowering or cone-bearing plant. Others, however, require two or more different hosts to complete their life cycles. Black stem rust of wheat, which has reduced wheat yields by millions of bushels in a single year in the United States alone, has plagued farmers ever since wheat was first cultivated thousands of years ago. More than 300 races of black stem rust are now known. This rust requires both common barberry plants and wheat to complete its life cycle (Fig. 19.27).

Life Cycle Birds Nest Fungi

Figure 1Q.2S How the "eggs" in a bird's-nest fungus are dispersed. After Harold J. Brodie. 1951. "The splash-cup dispersal mechanism in plants." Canadian Journal of Botany 29:224-34. Reproduced by permission of the National Research Council of Canada from the Canadian Journal of Botany, 29:224-34.

Corn Rust Life Cycle
Figure 1Q.2Ö Corn smut fungus on an ear of corn.

Since two hosts are necessary for black stem rust of wheat to complete its life cycle, it was believed that control of the disease could be accomplished through eradication of common barberry bushes. In an attempt to eradicate the disease, an estimated 600 million such plants were destroyed in the United

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fertilization n + n mycelium gives rise to teliospores uredinium with uredospores basidiospores (+ and fertilization uredinium with uredospores

Uredospore Ploidy

basidiospores (+ and upper area of barberry leaf lower area of barberry leaf aecium with aeciospores (n + n)

union of cells (without union of nuclei)

spermagonia with spermatia and receptive hyphae

Figure 1Q.27 Life cycle of black stem rust of wheat.

upper area of barberry leaf lower area of barberry leaf aecium with aeciospores (n + n)

union of cells (without union of nuclei)

spermagonia with spermatia and receptive hyphae

Figure 1Q.27 Life cycle of black stem rust of wheat.

States between 1918 and 1990, but it has proved impossible to eliminate the species altogether, partly because the uredin-iospores (binucleate spores produced in the summer) can overwinter in southern fields and reinfect wheat seedlings in the spring. Producing rust-resistant strains of wheat has helped, but even as new strains are developed, the rusts themselves hybridize or mutate, producing new races capable of attacking previously resistant varieties of cereals—a striking example of adaptation and natural selection in action.

Another serious rust with two hosts is the white pine blister rust, which has caused huge losses of valuable timber trees in both the eastern and western United States. Basidiospores infect the pine trees, and when the basidiospores germinate, other types of spores are produced. These different spores, in turn, infect currant and gooseberry bushes, and the spores formed on the currants and gooseberries eventually give rise to new basidiospores, completing the cycle. The U.S. Forest Service had a program of gooseberry bush eradication in operation for many years in an attempt to alleviate the problem, but the program was only partially successful. Spraying programs have more recently been implemented with some success, and rust-resistant trees also are being selected and bred as alternatives.

Other rusts with two hosts include apple rust (alternate host: cedar trees), poplar leaf spot (alternate host: larch or tamarack trees), and corn rust (alternate host: sorrel).

A relative of black stem rust was recently discovered in the Rocky Mountains of Colorado. This rust causes its rock cress host plants to produce fake flowers that look and smell so real many insects are fooled by them. When bees and butterflies visit the fake flowers, they find a sugary, nectarlike secretion. While gathering the sticky fluid, they inadvertently also pick up fungal sex cells, which are spread to other rock cress plants.

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  • mulu
    Does blister cress reproduce?
    6 years ago

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