Articles written by Dennis Holley
Showing 1-100 of 137 Articles
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Medical and Cultural Importance of Amphibians
Humans have interacted with amphibians since antiquity. Some of these exchanges are direct and easily understood while others are more difficult to discern.
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Economic and Ecologic Importance of Amphibians
Humans have interacted with amphibians since antiquity. Some of these exchanges are direct and easily understood while other connections are more difficult to discern.
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The Ecology and Taxonomy of Reptiles
Although most abundant in the tropics and subtropics, the nearly 8,000 species of living reptiles inhabit every continent except Antarctica.
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The Ecology and Taxonomy of Amphibians
At around 5,400 species of frogs and toads, salamanders and caecillians, the extant (living) amphibians are the smallest group of tetrapod vertebrates.
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The History of Reptiles
The species of modern reptiles are but mere shadows representing the shattered remains of what was once the grandest collection of animals on the planet.
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The History of Amphibians
Both in their ontogeny (biological development of an individual) and phylogeny (evolutionary history of a group) amphibians reveal their watery origins.
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Venomous Snakes
Only about 250 species of snakes have venom powerful enough to kill a human. Those snakes account for the deaths of an estimated 30,000-40,000 people per year worldwide.
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The Characteristics of Snakes
"Always carry a small flagon of whiskey in case of snakebite and furthermore always carry a small snake."
(W. C. Fields)
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The Characteristics of Lizards
Lizards and snakes are so morphologically similar that they are classified together into the same order and suborder.
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The Characteristics of Frogs and Toads
"Theories pass. The frog remains." (Jean Rostand)
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The Characteristics of Salamanders
"Salamander - borne in the sun-kissed flame. Who was it lit your candle and branded you with your name?"
(Lyrics from the Jethro Toll song "Salamander")
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The Characteristics of Caecilians
Lurking on or in the floor of tropical forests are the most unusual of all the amphibians-the legless caecilians.
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The Characteristics of the Classes of Sponges
Sponges have traditionally been grouped taxonomically into three classes.
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The Characteristics of Phylum Porifera
As the sole members of the subkingdom Parazoa, sponges represent the lowest level of organized animal life.
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The Ecology and Classification of Sponges
Sponges come in an incredible array of colors and an amazing array of body shapes. Their simple structure belies their wide spread distribution.
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The Mesosphere and Thermosphere
At the outer reaches of the Earth's atmosphere lie the little understood mesosphere and the highly charged thermosphere.
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The Stratosphere and Ozone Layer
Sandwiched between the troposphere layer below and several layers above, the stratosphere layer is a place of calm that contains the ozone layer within it.
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The Troposphere
Like the planet itself, the atmosphere is layered. Working upward from the surface, the first layer is the one humans live in, the troposphere.
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The Characteristics of Alligators and Crocodiles
Crocodilians are "living fossils" in the sense that they are the only remaining reptiles of the same lineage from which the dinosaurs and birds descended.
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The Characteristics of Turtles
Turtles, the most ancient lineage of the living reptiles, have scarcely changed over the course of their 260 million years history.
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The Characteristics of Amphibians
Amphibians are fishes out of water leading what amounts to a double existence. This duality is acknowledged even in their scientific name - Gr. amphi, dual + bios, life.
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The Characteristics of Reptiles
Through the evolution of a unique set of characteristics, reptiles were the first true terrestrial animals to appear.
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The Origin of Sponges
Poriferans are ancient animals. Fossils and molecular data put their origin at somewhere around 650 million years ago, nearly back to the dawn of animal life.
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Plant Taxonomy
"What's in a name? That which we call a rose by any other name would smell as sweet."
(William Shakespeare)
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Psychoactive Plants
Historically plants have been valued not only for their medical value but also for their psychoactive effects.
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Plants as Pharmaceuticals
At least a quarter of the prescriptions written in the United States contain at least one product that has been derived from a plant.
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Poisonous Plants Found Around the Home
There are nearly 70,000 reported plant poisonings a year in this country on average.
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Poisonous Plants of Field and Forest
"Everything is poisonous, nothing is poisonous, it is all a matter of dose."
(Claude Bernard)
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Wood as Fuel and Fiber
"My aunt in Knoxville would bring newspapers up for us to use as toilet paper. Before we used it, we'd look at the pictures." (Dolly Parton)
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Wood – The Perfect Building Material
"People love chopping wood. In this activity one immediately sees results." (Albert Einstein)
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Plant Fibers – Stuff of Paper, Ropes, and Cloth
"Half the world does not know the joy of wearing cotton underwear." (Phil Gramm)
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Chocolate – Food of the Gods
"All I really need is love, but a little chocolate now and then doesn't hurt!" (Lucy van Pelt from the Charlie Brown comic strip)
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Tea - The Drink of Ceremony and Custom
"There is no trouble so great or grave that it cannot be much diminished by a nice cup of tea."
(Bernard-Paul Heroux)
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Coffee - From Bean to Brew
"Coffee...the finest organic suspension ever devised." (From an episode of Star Trek: Voyager)
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Angiosperm Fruits as Human Food
"Botanically speaking, tomatoes are the fruit of a vine, just as are cucumbers, squashes, beans, and peas." (Horace Gray)
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Roots, Stems, and Leaves as Human Food
"This cabbage, these carrots, these potatoes, these onions all will become me. Such a tasty fact!" (Mike Garofalo)
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Legumes as Human Food
"Gardening is an art that uses flowers and plants as paint, and the soul and sky as canvas."
(Elizabeth Murray)
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Cereal Grains as Human Food
"Agriculture not only gives riches to a nation, but the only riches she can call her own." - Samuel Johnson
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Aggregate and Multiple Fruits
Aggregate and multiple fruits are fruits that develop in clusters.
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The Simple Fruits of Angiosperms
A fruit is defined botanically as a mature ovary containing one or more seeds.
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The Dispersal of Seeds
Once seeds and fruits are formed, they must be dispersed away from the parent plant.
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Seed Dormancy
Seeds are said to be dormant when they will not germinate even when placed in optimal environmental conditions.
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The Structure and Function of Seeds
"I have great faith in a seed. Convince me that you have a seed there, and I am prepared to expect wonders." (Henry David Thoreau)
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Plant Tissue Culture
Plant tissue culture is a method of vegetative propagation in which clones of select plants are produced in sterile conditions.
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Plant Layering in Agriculture and Horticulture
Layering can occur when plant stems that are still attached to their roots come in contact with the soil or other rooting medium.
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Plant Grafting in Agriculture and Horticulture
Grafting, a horticultural process known and practiced for centuries, is the process of attaching a cutting from one plant to the rooted stem of another.
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Plant Cuttings in Agriculture and Horticulture
Vegetative propagation of plant structures is an important tool in horticulture and gardening. The use of cutting is one method of propagating plants vegetatively.
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Vegetative Reproduction in Flowering Plants
While all flowering plants reproduce sexually, there are some species that are able to fall back on vegetative reproduction if needed.
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Pollination and Fertilization in Flowers
In flowering plants the production of seeds depends on the transfer of pollen and the fusion of male and female gametes.
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Wind as an Agent of Pollination
All gymnosperms and many angiosperms are pollinated by the wind.
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Insects as Agents of Pollination
Insects were the first and are still the most prolific pollinators and plants have evolved a number of adaptations to attract them.
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Birds and Mammals as Agents of Pollination
Most flower types enlist the services of animals to carry their pollen to receptive female structures.
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The Process of Pollination in Flowering Plants
Once female and male gametes (egg and pollen) have formed, pollination is the process that brings them together so fertilization may occur.
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The Function and Structure of Flowers
"The Earth laughs in flowers." (Ralph Waldo Emerson)
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The Structure and Diversity of Angiosperms
"Creatures from a planet without flowers would think we must be mad with joy the whole time to have such things about us."
(Iris Murdock)
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The Structure and Reproduction of Gymnosperms
In North America nearly all gymnosperms display an evergreen tree body form and produce seeds within hard comes.
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Types and Diversity of Gymnosperms
The gymnosperms are cone-bearing seed plants that once dominated ancient landscapes
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Structure and Reproduction of Ferns and Others
Ferns and their taxonomic relatives are complex of body but primitive in their mode of reproduction.
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The Diversity of Ferns and Their Relatives
Vascular spore plants such as the ferns and related species possess complex bodies but are primitive in their reproductive patterns.
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The Structure and Reproduction of Bryophytes
With primitive bodies that cannot conduct water, bryophytes must be inhabitants of shade and shadow where moisture levels are high.
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Bryophyte Diversity
Collectively known as bryophytes, the liverworts, hornworts, and mosses are primitive nonvascular spore plants.
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Photosynthesis
Through the process of photosynthesis plants feed not only themselves but indirectly the rest of the living world.
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The Functions of Plant Leaves
"A leaf is the only wealth that can actually feed people." (Alan Savory)
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The Structure and Appearance of Plant Leaves
The external appearance of leaves is as varied as the species that produces them. However, all leaves possess about the same internal configuration.
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The Structure and Types of Plant Stems
Stems come in two varieties: soft, green herbaceous stems or hard woody stems.
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Fluid Transport in Plant Stems
All stems perform three basic functions: support of the leaves and flowers, fluid transport between leaves and roots, and food storage.
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Support and Food Storage in Stems
All stems perform three basic functions: support of the leaves and flowers, fluid transport between leaves and roots, and food storage.
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Patterns of Growth in Plant Roots
Roots grow both in length (primary) and in thickness (secondary) and can spread great distances both horizontally and vertically.
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Types of Root Systems
As a seed germinates and the plant grows, the type of root system that develops reflects the evolutionary adaptations of that species.
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The Structure of Plant Roots
The internal structure of a root is relatively simple with the arrangement of the primary tissues showing very little difference from one level to another.
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The Function of Plant Roots
In most vascular plants, the roots compose the underground portion of the plant and perform several vital functions: absorption, anchorage, and food storage.
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What is Comparative Psychology?
Comparative psychology is the study of the mental existence (cognition) of animals other than humans.
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Behavioral Genetics
Behaviorists have come to realize that even though behavior is strongly influenced by environmental influences and learning, it also has a genetic component.
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Altruism in Animals
Behavioral ecologists define altruism as an animal interaction in which one animal at cost to itself acts in such a way to benefit another animal(s).
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Animal Communication
Animal communication is any behavior on the part of one animal that has an effect on the current behavior of another animal.
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Foraging Behavior in Animals
Behavioral ecologists use foraging theory to study the foraging behavior of animals in relation to the habitat of the animal.
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Mating Behavior in Animals
The patterns of mating and the types of mating systems found throughout the animal realm are quite diverse.
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Territorial Behavior in Animals
Behavioral ecologists define territory as any space that an animal defends against intruders of the same species.
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Agonistic Behavior
Behavioral ecologists view agonistic behavior as any behavior associated with competition and fighting.
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Learning by Instruction
Learning by instruction or teaching occurs when one individual (the teacher) sets an example that causes another individual (the pupil) to change its behavior (learn).
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Learning by Imprinting
As an animal matures, it recognizes and socially bonds with others of its species through a process known as imprinting.
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Learning by Conditioning
Behaviorists explain learning by conditioning as the association and response an animal makes when two stimuli are paired.
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Learning by Imitation
Long seen as the lowest form of learning, imitation is now regarded as an important manifestation of intelligence in nonhuman species.
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Associative Behavior
Learned or associative behavior develops from an interaction between the genotype of an animal and repeated inputs from a changing environment.
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Innate Behavior
Innate or instinctive behavior patterns are "hard-wired" into the nervous system of animals. These behaviors are triggered by external and/or internal stimuli.
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Animal Behavior
Behavior is the organized and integrated patterns of activity by which an organism responds to its environment.
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Life in the Clouds
Microorganisms floating in the air and clouds have been known for decades but are they passive passengers or active residents?
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The Doctrines of Modern Zoology
The biological principles of organic evolution, genetics, and ecology serve as the foundation of modern zoology.
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The History of Modern Zoology
Humans have always attempted to understand animals, to enslave animals, and to capture the strength and power of animals.
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Is Aging Inevitable?
Do animals age because they just wear out metabolically or is aging genetically programmed into each animal?
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Animal Nomenclature
There may be tens of millions of species of animals on the planet. What systems have animal taxonomists developed to scientifically name this multitude of animal types?
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Animal Classification
The field of animal systematics (or taxonomy) devises schemes for grouping (classification) and naming (nomenclature) animals.
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How Many Animals Inhabit the Earth?
"The most wonderful mystery of life may well be the means by which it created so much
diversity from so little physical matter."
(E. O. Wilson)
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Game Theory Helps Explain Animal Behavior
Game theory is best known for its economic applications. Behavioralists are now realizing its potential in biological situations as well.
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The Characteristics of Animals
Animals come in bewildering array of different sizes and forms but do they all have anything in common?
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The Hierarchy of Life
There is an ordered regularity to the pandemonium of life on Earth.
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Sunspots and Climate Change
Several times over the last 500 years low sunspots numbers have coincided with much cooler global temperatures. Is there a connection or is it coincidence?
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Sunspots Seem to Have Vanished
In 2008, no sunspots were observed on 266 of the year's 366 days (73%), a low surpassed only in 1913, which had 311 spotless days (85%).
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Regulating Gene Action
Geneticists have long used genetic switches to turn genes on and off, but until now they have struggled to control the intermediate levels.
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The Emerging Field of Synthetic Biology
The quest to create life is being taken to the next level with the fusion of organic chemical engineering principles and the principles of genetic engineering.
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