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Biology 103
2001 First Web Report
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What IS this stuff?: The World of Slime Molds

Jessica Blucher

Introduction

WordNet (r) 1.6 (1) defines a slime mold as follows:

slime mold n : a naked mass of protoplasm having characteristics of both plants and animals; sometimes classified as protoctists.

What on earth does that mean?! In this paper, we will explore what slime molds are. Scientists have written a great deal of literature about this strange and interesting life form. There are two kinds of slime molds: plasmodial slime molds and cellular slime molds. There is also something called Labyrinthulomycota, also known as slime nets, but they are not considered slime molds. (2) Over 700 species are reported as existing. (3)

What Are the Differences in Slime Molds

Plasmodial slime molds are basically enormous single cells with thousands of nuclei. Cellular slime molds, spend most of their lives as separate single-celled amoeboid protists, but upon the release of a chemical signal, the individual cells aggregate into a great swarm. (2)

What Slime Mold Looks Like

Slime mold takes many forms. The most common forms on turf resemble small purple or black ball attacked to a blade of grass or a readily noticeable creamy-white, yellow-orange, purple, or gray jelly-like mass situated on the lawn. The colonies of slime mold living on logs and bark mulch can be strikingly colorful in yellow, orange or red. (3), (4)

What Slime Mold Eats

It feeds on fungi, bacteria, protozoa, other micro-organisms, and decaying organic matter. (4), (5)

Where Slime Mold Lives

They live in moist terrestrial habitats, such as on decaying wood or fresh cow dung, leaves or other organic matter retaining abundant moisture. Bark mulch in a flower garden or shrub bed certainly fits that description. The same type of organism is often seen in the woods on decaying logs. (2), (3)

History:

Slime molds were once considered to be animals due to their creeping phase. One of the founders of mycology called them Mycetozoa, from the Greek words myketes (fungi), and zoon (animals). This name continued to be used until the 1970's. Some previous researchers classified the slime molds in the phylum Protozoa of the animal kingdom. (4)

Mycologists (those who study fungi) now consider these strange organisms to belong to a class called Myxomycetes; myxa (slime) and myketes (fungi). Even today, the true relationship of the Myxomycetes to fungi remains obscure. (4)

Slime molds were once regarded as a fungus but later classified with the Protista. In a recent system of classification based on analysis of nucleic acid (genetic material) sequences, slime molds have been classified in a major group called the eukarya (or eukaryotes), which includes plants and animals. (7)

The Life Cycle of a Slime Mold

The following is a summary of the life cycle of slime molds. Before entering the reproductive stage, a plasmodium moves to a drier, better-lit place, such as the top of a log. In the amebalike, or cellular, slime molds, up to 125,000 individual cells aggregate and flow together, forming a multicellular mass called a pseudoplasmodium that resembles a slug and crawls about before settling in a location with acceptable warmth and brightness. (6) When conditions become unfavorable, these slime molds form sporangia - clusters of spores, often on the tips of stalks. Spores from the sporangia are dispersed to new habitats, "germinate" into small amoebae, and the life cycle begins again. (2) That's what my second source said. In a language somewhat closer to common English, we could say that the molds form spores, the spores spread, the spores turn into amoebas and then they become slime.

The Anatomy of a Slime Mold

Fruiting Bodies

There are four types of myxomycete fruiting bodies. The most common type is the sporangium. The sporangium is actually a small spore container which may be sessile or stalked, with wide variations in color and shape. Sporangia usually occur in groups, since they form from separate portions of the same plasmodium. The second type is the aethalium, a cushion-shaped, sessile structure. Aethalia are presumed to be masses of completely fused sporangia and are relatively large, sometimes exceeding several centimeters in size. The third type is the pseudoaethalium (false aethalium). This fruiting body is composed of sporangia closely crowded together. Pseudoaethalia are usually sessile, although a few may be stalked. The fourth type of fruiting body is called a plasmodiocarp. Usually sessile, plasmodiocarps take the form of the plasmodial veins from which they were derived. (8)

Fruiting Body Structure

Fruiting bodies are usually composed of 6 parts: hypothallus, stalk, columella, peridium, capillitium and spores. In some fruiting bodies a pseudocolumella or a pseudocapillitium may be present. Not all of these components are present in all fruiting body types. (8)

In case after reading all of this you don't know what a fruiting body is, it's used to make, hold, and release spores.

Hypothallus

The hypothallus is a plasmodial remnant forming the base for one or more fruiting bodies. The hypothallus connects the stalk or stipe to the substrate. It may be dull or brightly colored, thin and delicate or coarse. It is not always in evidence. In some instances, the hypothallus may be composed of calcium carbonate. In the case of the transparent type, the hypothallus may be proteinaceous in composition.  (8)

Stalk

The stalk or stipe is an important identification characteristic. The stalk may vary in length and color and texture. In some species the stalk is opaque, while in others it is translucent. The stalk may also be coated with lime or filled with granular or sporelike structures. (8)

Columella and Pseudocolumella

The columella appears as an extension of the stalk into the spore mass, although it may not resemble the stalk. In a sessile fruiting body, the columella may be an area on the inside of the peridium where it contacts the substrate or appears as a dome-shaped structure. A pseudocolumella (pseudo=false) is a columella that does not attach to the stalk. The pseudocolumella is found only in the order Physarales, existing as a lime mass within the spore mass. Capillitial elements may be attached to the columella or pseudocolumella.  (8)

Peridium

The peridium is a covering enclosing the spore mass. It may or may not be evident in a mature fruiting body. In some species the peridium persists as a calyculus, a cup-like structure holding the bottom of the spore mass. The presence or absence of the calyculus may be used as a diagnositic feature along with the manner in which the fruiting body opens. The peridium may split open along lines of dehiscence, as a pre-formed lid, or in an irregular pattern. In an aethalium, the relatively thick covering over the spore mass is referred to as a cortex rather than a peridium.  (8)

Capillitium and Pseudocapillitium

The capillitium consists of threadlike elements within the spore mass of a fruiting body. Many species of myxomycetes have a capillitium, either as a single connected network, or as many free elements called elaters. Capillitial elements may be smooth, sculptured or spiny or they may appear to consist of several interwoven strands. Some members of the Physarales have limy capillitial elements (a badhamioid capillitium), while others have limeless tubules connecting to lime nodes (a physaroid capillitium). The capillitia elements are separate from the spores within the spore mass and are not connected to them. Some elements may be elastic, allowing for expansion when the peridium opens, while other types are hygroscopic and capable of dispersing spores by a twisting motion. A pseudocapillitium is present in some aethalia and pseudoaethalia producing species. Pseudocapillitial elements are highly variable in size and shape,
and may appear as bristles, threads or perforated plates.  (8)

Spores

Spores range in size from almost 5 to 15 micrometers. Nearly all of them appear to be round and most are ornamented to some degree. In fact, entirely smooth spores may not exist. Spore ornamentation can be reticulate (covered by a network of ridges), echinate (spiny), verrucose (warted), or asperulate (finely warted). Spore shape and size are very important in identification. Spores can be classified as either dark (found in the Stemonitales and Physarales) or light to brightly colored (all of the other orders). Dark spores include the colors black, violet, brown, and purplish brown. Brightly colored spores may be red, yellow, orange, white, pale gray, pink, light or rusty brown. In some species of Badhamia and Dianema corticatum, the spores appear clustered into "spore balls".  (8)
 

WWW Sources

(1)The DICT Development Group

(2) Introduction to the "Slime Molds" on the Berkley website.

(3) Slime Mold - Myxomycetes - Colorado State University Cooperative Extension - Tri River Area on the Colorado State University website.

(4) Slime Mold: The Blob on the Lawn, on the College of Agriculture University of Saskatchewan website.

(5) Slime Mold - Turfgrass - Colorado State University Cooperative Extension - Tri River Area on the Colorado State University website.

(6) Encyclopedia.com - Results for slime mold

(7) slime mold on the Information Please website.

(8) What are Myxomycetes?
 
 


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