Encyclopedia of Environmental Science and Engineering, Volume I and II

690 MICROBIOLOGY

ELEVATION

EDGE

Flat Raised Low Convex

High Convex

Entire

Umbonate Convex withpapillate surface

Erose Crenated

Undulate Lobate Rhizoid

FIGURE 5 Diagrammatic representa-

tion of types of bacterial colonies. These

shapes are specific for individual types

and are therefore quite useful as a step in

the process of identification of unknown

organisms.

SPORE FORMS

FIGURE 4 Diagrammatic represen-

tation of spores (clear areas) inside

rod-shaped bacteria. Note (bottom

row) that free spores may be ball or

egg-shaped.

hyphae, microscopic cytoplasm-fi lled tube-like branches

(lacking crosswalls), which together make a felty mat called

a mycelium. Individual hyphae are microscopic but the

mycelium, equivalent to a bacterial colony, is macroscopic.

Growth takes place by extension of the hyphae. Specialized

spore-containing bodies called sporangia can form at the

ends of some hyphae. Sexual reproduction requires fusion

of hyphae from two different mycelia to form a specialized

zygospore.

It is more common now to discard the term Phycomyetes

and instead subdivide the group into 4 classes in which affi n-

ities are much clearer. However, at present, the literature is

divided in its use of the older and newer terminology. As

with bacteria, chemical analyses of structures and metabolic

pathways followed are important in defi ning the classes.

These four classes are:

1) Chytridiomycetes lack true mycelia. They are aquatic,

have posteriorly uniflagellated zoospores and cell

walls composed of chitin.

2) Hyphochytridiomycetes have true mycelia. They are

aquatic, have anteriorly uniflagellated zoospores and

cell walls composed of chitin.

3) Oomycetes have true well developed mycelia and

cell walls composed of cellulose.

a) Saprolegniales are generally aquatic and have

asexual spores on specialized mycelear structures.

Only male gametes are motile.

b) Peronosporales are generally terrestrial. Sporan-

gia either produce asexual zoospores or may

germinate directly to form hyphae. Both gametes

are nonmotile.

4) Zygomycetes are terrestrial and have large and

well developed mycelia and nonmotile spores.

Asexual spores are produced in sporangia. Cell

walls are made of chitosan or chitin.

There are two classes included in the higher fungi.

1) The Ascomycetes are the best known and largest

class of fungi. Ascomycetes have hyphae divided by

porous crosswalls. Each of these hyphal compart-

ments usually contains a separate nucleus. Asexual

spores called conidia, form singly or in chains at

the tip of a specialized hypha. The sexual structure

called ascus, is formed at the enlarged end of a spe-

cialized fruiting structure and usually contains eight

ascospores. Some important microscopic members

of this group include yeasts, mildews, the common

red bread mold and many species which produce

antibiotics. On the other hand macroscopic forms

include Morchella esculenta or morels which are

highly regarded as a delicacy by gourmets.

2) The Basidiomycetes are entirely macroscopic and are

commonly known as mushrooms and toadstools.

Slime Molds

The slime molds, Myxomycetes, are at times classifi ed with

either true fungi or protozoa or, as here, treated separately.

They produce vegetative structures which look like ameboid

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