Ground tissue system The plant tissue system that forms most of the photosynthetic tissue in leaves, composed mostly of parenchyma cells and some collenchy-ma (elongate and thick walled in strands) and sclerenchyma (thick, rigid, secondary walls with lignin to provide support) cells. It is found between the epidermis and surrounding the vascular tissue system. Acts also as supporting tissue and in water and food storage.
growth factor A complex family of organic chemicals, especially polypeptides, that bind to cell surface receptors and act to control new cell division, growth, and maintenance by the bone marrow.
Synthetic growth factors are being used to stimulate normal white blood cell production following cancer treatments and bone marrow transplants.
Examples of growth factors are insulin (including insulinlike growth factor (IGF), GF1, II, all of which are polypeptides similar to insulin); somatomedins, polypeptides made by the liver and fibroblasts that, when released into the blood (stimulated by soma-totropin), help cell division and growth by incorporating sulfates into collagen, RNA, and DNA synthesis; HGH (human growth hormone), also called soma-totropin, a proteinlike hormone from the pituitary gland that stimulates the liver to produce somato-medins that stimulate growth of bone and muscle; platelet-derived growth factor (PDGF), a glycoprotein that stimulates cell proliferation and chemotaxis in cartilage, bone, and other cell types; fibroblast growth factor, which promotes the proliferation of cells of mesodermal, neuroectodermal, epithelial, or endothe-lial origin; epidermal growth factor (EGF), important for cell development as it binds to receptors on cell surface to create a growth signal; and granulocyte colony-stimulating factor (G-CSF), a growth factor that promotes production of granulocytes, a type of white blood cell.
Guanylate cyclase An enzyme catalyzing the conversion of guanosine 5′-triphosphate to cyclic guano-sine 3′,5′-monophosphate, which is involved in cellular regulation processes. One member of this class is a heme-containing enzyme involved in processes regulated by nitrogen monoxide.
Guard cell Specialized epidermal cells; two crescent-shaped cells on either side of the pore of a stoma in the stem or leaf epidermis. By changing shape, i.e., by opening and closing via changes in turgor, they regulate gas exchange and water loss by covering or uncovering the pore, which lets oxygen out and carbon dioxide in.
Gullstrand, Allvar (1862-1930) Swedish Ophthalmologist Allvar Gullstrand was born on June 5, 1862, in Landskrona and was the eldest son of Dr. Pehr Alfred Gullstrand, principal municipal medical officer, and his wife Sofia Mathilda Korsell. He was educated at schools in Landskrona and Jonkoping, and he attended Uppsala University, but left in 1885 to spend a year at Vienna. He then continued his medical studies at Stockholm and presented his doctorate thesis in 1890. He was appointed lecturer in ophthalmology in 1891 and was appointed the first professor of ophthalmology at Uppsala University in 1894. He stayed in that position until 1914, when he was given a personal professorship in physical and physiological optics at Uppsala University, becoming an emeritus professor in 1927.
Gullstrand contributed a great deal to the knowledge of clinical and surgical ophthalmology and of the structure and function of the cornea of the eye, as well as research on astigmatism, and was self-taught in this area. He laid out his ideas in his doctoral thesis in 1890, Bidrag till astigmatismens teori (Contribution to the theory of astigmatism) and further refined them in Allgemeine Theorie der monochromatischen Aber-rationen und ihre nachsten Ergebnisse fur die Oph-thalmologie (General theory of monochromatic aberrations and their immediate significance for ophthalmology), 1900; Die reelle optische Abbildung (The true optical image), 1906; and Die optische Abbildung in heterogenen Medien und die Dioptrik der Kristallinse des Menschen (The optical image in heterogeneous media and the dioptrics of the human crystalline lens), 1908. Further important works included Tatsachen und Fiktionen in der Lehre von der optischen Abbildung (Facts and fictions in the theory of the optical image), 1907; and Einfuhrung in die Methoden der Dioptrik der Augen des Menschen (Introduction to the methods of the dioptrics of the human eyes), 1911.
In 1911 he invented the slit lamp and the reflex-free ophthalmoscope to help study the eye. He introduced a surgical technique for the treatment of symblepharon (a fibrous tract that connects the bulba conjunctiva to conjunctiva on the eyelid) and redefined the theory of accommodation. In 1911 he received the Nobel Prize for his research on dioptrics of the eye, although at first he declined it. Gullstrand died in Stockholm on July 28, 1930. Gullstrand is seen as one of the founders of modern ophthalmology.
Guttation A way for plants to expel water in excess of transpiration—principally through hydathodes, a special epidermal structure at the terminations of veins around the margins of the leaves—and especially under conditions of relatively high humidity. The expelled water appears on the ends of leaves mostly on moist cool nights and resembles dew.
Gymnosperm A group of flowerless plants that includes pines, conifers, redwoods, firs, yews, and cycads. Gymnosperm means "naked seed." Their ovules and the seeds, which develop in them, are born unprotected on the surface of megasporophylls, and are often arranged on cones instead of being enclosed in ovaries as in the flowering plants. They release pollen directly into the air, which finds its way to female ovules and fertilizes them. The earliest examples of gymnosperms appeared during the Devonian period some 408 million years ago and were seed ferns. Gymnosperms have reached as high as 350 feet to as small as 4 inches in height. Worldwide there are 20 plant families with at least 50 percent of their species threatened. Of these, eight are gymnosperm families (including cycads and conifers). This may be due to the fact that many of their species are widely exploited for both timber and horticultural purposes, and, because they are an ancient group, they may not adapt easily to the rapidly changing environment around them. There are only 720 species compared with 250,000 species of angiosperms.
Gynandromorphism A female that develops partly or completely male characteristics. A lateral gynandro-morph on one side has the external characters of the male, and the other side has those of the female. Common in bees and silkworms.
Haber-Weiss reaction The Haber-Weiss cycle consists of the following two reactions:
The second reaction achieved notoriety as a possible source of hydroxyl radicals. However, it has a negligible rate constant. It is believed that iron(III) complexes can catalyze this reaction: first Fe(III) is reduced by superoxide, followed by oxidation by dihy-drogen peroxide.
Habituation Reaction to the repeated presentation of the same stimulus that has no special significance and that causes reduced attention to the stimulus. Short-term habituation lasts for only minutes, while longer-term habituation could last for weeks.
Haeckelian recapitulation (embryonic recapitulation) A clade goes through, in its development, an ontoge-netic stage that is present in the adults of its sister group. Named for Ernst Haeckel (1834-1919), who promoted the idea that the development of the human embryo in the womb is a rerun (recapitulation) of the steps in humanity’s rise from a primitive creature. Haeckel’s views about evolution have been refuted by most scientists.
Half-life For a given reaction the half-life, t1/2, of a reactant is the time required for its concentration to reach a value that is the arithmetic mean of its initial and final (equilibrium) value. For a reactant that is entirely consumed, it is the time taken for the reactant concentration to fall to one-half of its initial value. For a first-order reaction, the half-life of the reactant can be called the half-life of the reaction. In nuclear chemistry, (radioactive) half-life is defined, for a simple radioactive decay process, as the time required for the activity to decrease to half its value by that process.
Haloperoxidase A peroxidase that catalyzes the oxidative transformation of halides to XO- (X being Cl, Br, or I) or to organic halogen compounds. Most are heme proteins, but some bromoperoxidases from algae are vanadium-containing enzymes.
Halophyte A plant that has adapted to grow in salt-rich soils or salt-rich air, e.g., glassworts (Salicornia vir-ginica). Obligatory halophytes need salt, whereas facultative halophytes can live in freshwater conditions as well.
Hansch analysis The investigation of the quantitative relationship between the biological activity of a series of compounds and their physicochemical substituent or global parameters representing hydrophobic, electronic, steric, and other effects using multiple regression correlation methodology.
Haploid cell A cell containing one set of chromosomes. Sperm and egg cells are haploid.
Haplozoans Extinct echinoderms of the Cambrian era that include two classes, Cycloidea and Cyamoidea. Cyamoids have bilateral symmetry, and cycloids are dome-shaped with radial symmetry. They are quite uncharacteristic of echinoderms, although they have a skeletal structure that relates them. Otherwise, they have uncertain affinities to all other echinoderms and may provide a link between echinoderms and fish.
Hapten A molecule (usually a small organic molecule) that can be bound to an antigenic determinant/epi-tope. Usually they are too small to give a response of their own. They become antigenic if they are coupled to a suitable macromolecule, such as a protein.
Hapto The hapto symbol, n (Greek eta), with numerical superscript, provides a topological description for the bonding of hydrocarbons or other n-electron systems to metals by indicating the connectivity between the ligand and the central atom. The symbol is prefixed to the ligand name or to that portion of the ligand name most appropriate. The superscript numerical index on the right indicates the number of coordinating atoms in the ligand that bind to the metal. Examples:
Hard acid A lewis acid with an acceptor center of low polarizability. It preferentially associates with hard bases rather than with soft bases, in a qualitative sense (sometimes called HSAB rule). Conversely, a soft acid possesses an acceptor center of high polarizability and exhibits the reverse preference for a partner for coordination.
Hard base A lewis base with a donor center of low polarizability; the converse applies to soft bases.
Hard drug A nonmetabolizable compound, characterized either by high lipid solubility and accumulation in adipose tissues and organelles, or by high water solubility. In the lay press, the term hard drug refers to a powerful drug of abuse such as cocaine or heroin.
Hardy-Weinberg theorem A mathematical theorem that describes a population that is not evolving under certain conditions. First proposed in 1908 by G. H. Hardy, an English mathematician, and B. W. Weinberg, a German physician, although both were working independently of each other. The theory describes the frequencies of various genotypes in a population and the ability to predict how the frequencies will change in future generations based on certain assumptions not being violated, such as the absence of mutation, the absence of migration or selection, random mating, and an infinite population size. In other words, in the absence of any of these factors, the allele frequencies and the frequencies of the different genotypes will remain the same from one generation to the next, and genetic recombination due to sexual reproduction will not result in any changes in allele frequencies or genotype frequencies.
Harelip (cleft lip) A congenital (birth) deformity caused by a failure of developing tissues to fuse; a cleft in the upper lip, found on one or both sides of the mid-line. Often associated with a cleft palate.
Haustorium A specialized structure of a parasitic plant or entophyte, an organism that lives at least part of its life cycle within a host plant, that penetrates the living host and absorbs nutrients. In parasitic fungi, it is a hyphal tip that penetrates the host but not the cell membrane.
Haversian system (osteon) A structural composition of compact bone that contains a Haversian canal, a longitudinally arranged vascular channel in the center of the osteon containing blood vessels and nerves; Haversian lamella, the circumferentially arranged layers of osteoid and osteocytes surrounding the canal; and the canaliculi, fine channels through the bone that provide a link between osteocytes.
Heart A muscular organ composed of cardiac muscle that pumps blood throughout the body.
Heat Kinetic energy in the process of being transferred from one object to another due to a temperature difference. It moves in one of three ways: radiation, conduction, or convection.
Heath hen Along with the passenger pigeon, the heath hen (Tympanuchus cupido cupido), a relative of the prairie chicken, is an example of extinction by human disturbance. Last observed on March 11, 1932, at James Green’s farm in West Tisbury, Martha’s Vineyard, Massachusetts, it became the victim of habitat conversion to crops and pastureland, as well as being hunted as game and for sport. It was a ground-dwelling/nesting meadows seedeater ranging from Maine to Chesapeake Bay.
Heat-shock proteins (HSPs) A family of closely related proteins, widely distributed in virtually all organisms (plants, animals, microorganisms, and humans). Even though they are found in widely different sources, they show structural similarity. HSP expression increases in response to physiological stresses such as rise in temperature, pH changes, and oxygen deprivation. Many of these stresses can disrupt the three-dimensional structure, or folding, of a cell’s proteins, and so HSPs bind to those damaged proteins, helping them refold back into their proper shapes.
Illustration of a section through a whole human heart showing the internal organization of chambers, valves, connecting arteries, and veins. The heart can be regarded as being two pumps: the right side (left on image) serving the lungs and the left side pushing blood around the general circulation. Oxygenated blood entering the heart from the lungs passes through the left atrium into the left ventricle (bottom right) before being distributed via the aorta (top, center). Deoxygenated blood enters the right atrium through the venae cavae (top left) prior to being pumped to the lungs from the right ventricle.
They also help newly synthesized polypeptides fold and prevent premature interactions with other proteins. HSPs, also called chaperones, aid in the transport of proteins throughout the cell’s various compartments and aid in the destruction of peptides specific to tumors or pathogens.
Hectare A unit of land measurement. one hectare (ha) is equal to 2.5 acres.
Insects and Man—An Exotic Dilemma
Insects have long been recognized for their benefits as pollinators, honey producers, predators, and parasitoids, as well as for pure aesthetics, such as admiring the grace and beauty of a butterfly. They have also been recognized as vectors of disease, agricultural pests, defoliators of forests, bloodsuckers, and as having other characteristics unwanted or not admired by humans.
Insects have only recently come to be appreciated by the scientific community and public for what they can tell us about the health of the environment. Their sheer diversity, estimated at 30 million species, and numbers (the world’s ant biomass equals the total human biomass) are evidence of their significance. In recent times, insects have been used for biocontrol, i.e., using one species to control the population of another. Klamath weed (Hypericum perforatum, also known as St. John’s-wort) control by a leaf beetle is a famous example. The Klamath weed leaf beetle, Chrysolina quadrigemina, intentionally introduced in 1946, feeds on the foliage of this weed in both the adult and larval stages. The Klamath weed leaf beetle has dramatically reduced the abundance and density of this weed in areas where fall temperatures are mild. This biological control agent is responsible for contributing to the impressive decline in St. John’s-wort on the Pacific coast.
Introductions have led to mistakes that are equally dramatic. Another leaf beetle, the Colorado potato beetle (Leptinotarsa decemlineata), was introduced from the United States to Europe in the late 1920s, where it has become a serious potato pest. A parasitic fly (Compsilura) was intentionally introduced to the United States in the 1920s to control a newly established and very serious forest threat, the gypsy moth (Lymantria dispar’). It took a long time for the fly to become established, and now it is an integral part of our fauna. However, this species of fly has five broods per year, while the gypsy moth only has one. The fly survives for the remainder of the year by attacking hundreds of species of native moths and butterflies, unrelated to the gypsy moth, and has suppressed their numbers to unprecedented low levels. The population of this fly has been estimated at 16,000 individuals per acre in Connecticut. Clearly this "cure" for the gypsy moth was worse than the "disease," having totally unexpected side effects.
The honeybee is widely acclaimed for its ability to pollinate and for its honey production. However, this introduced bee, originally an Old World species, has displaced and probably caused the extinction of several bees native to the New World. The tracheal bee mite, a recent accidental introduction, possibly from the Philippines, has had a catastrophic effect on the honeybee and honey production. The secondary impact of tracheal mites on native bees is an ongoing concern. The "killer" bee, an introduced strain of the honeybee, is another matter entirely.
Not all introductions need be derived from another continent. Geographic barriers, such as the Rocky Mountains and the Great Plains, prevent the natural spread east and west on the North American continent. The western conifer seed bug (Leptoglossus occidentalis) was accidentally introduced from the West to a tree nursery in Iowa in 1956. From Iowa, the bug spread throughout the northeastern United States, reaching New York in 1992, where it has become a nuisance due to its proclivity to overwinter in houses.
Insects are continually on the move. Even native populations exhibit periodic instability and can rapidly expand (or contract) a range that had been stable for decades, even centuries. The plain ringlet butterfly (Coenonympha tullia) has been moving south from Canada at a pace of nearly 40 miles per year with no end in sight (except eventually an ocean).
Even exotic plants have a profound impact on native plants and consequently on the associated insects. Glaciers "erased" the flora and fauna of the northern states thousands of years ago. Species with poor dispersal mechanisms have still not reinvaded since the glacial retreat 15,000 years ago. Glaciers wiped out earthworms, but introduced earthworms have filled the void. One exotic earthworm species is so adept at removing organics that it actually mineralizes the soil in the process of feeding. Garlic mustard, also an introduction, loves mineralized soils and follows the earthworms. The Virginia white butterfly (Pieris virginiensis), a species threatened by competition with the introduced cabbage white butterfly (Pieris rapae), has a caterpillar that feeds on a native mustard, but the adult butterfly demonstrates an ovipositional preference for the introduced garlic mustard. Its caterpillar cannot survive on the garlic mustard, and as a consequence of this oviposi-tional preference, this species is disappearing in the wake of the encroaching garlic mustard. As an added insult, the diminished quality of the forest litter beneath the garlic mustard has led to a dramatic (more than 90 percent) decline in ground beetles, native denizens of the forest floor. This impact makes its way up the food chain as we witness declines in red-backed salamanders that feed on insects in the forest litter.
Ladybugs have been intentionally introduced with wild abandon because of their reputation as voracious aphid eaters. Nearly half of North America’s ladybugs are not natives, but introductions. Our native nine-spotted ladybug (Coccinella novemnotata), the "official" New York State insect, was eradicated from New York by the seven-spotted ladybug (Coccinella septempunctata), an intentionally introduced European species. No nine-spotted ladybug has been collected in New York State since the 1940s, yet it had been a dominant species before the introduction of this competitor. Another exotic introduction, the Asian ladybug (Harmonia axyridis), made its way to the Northeast in the 1990s and has partially displaced the previously introduced seven-spotted ladybug.
It is now somewhat of a rarity to see a native ladybug in New York State. The level at which the various ladybugs compete is not that well understood. The nine-spotted and the seven-spotted are very close relatives, so the fact that they compete directly should not have surprised scientists. What is more surprising is that the distantly related Asian ladybug displaced the nine-spotted and many others, despite the lack of any close relationship. Parasites and diseases of introduced insects are some of the baggage that comes with these introductions. The introductions have evolved a resistance or at least a tolerance to these coevolved agents to which our native species have never been exposed. It is smallpox at an insect level.
All the aforementioned ladybugs eat aphids, but they can be particular about which species and in what habitats. The diversity of native ladybug species was achieved through natural selection. Introductions upset the balance. We can never take back the dozens of introduced ladybugs.
If all life (not just ladybugs) from all of the earth’s continents were mixed together tomorrow, it would result in a die-off of diversity that would parallel that of the Devonian extinction that occurred millions of years ago. This is what we are doing, albeit slowly, with accidental and purposeful introductions. Once an undesirable exotic has gained a foothold, we are caught up in a cycle of introducing predators, parasitoids, and diseases in an attempt to suppress or eradicate the original introduction. As shown by these examples, it often backfires.
Helium An inert gas; an element with atomic number 2. Helium is produced in stars and is the second most abundant element in the universe. Its atom contains two protons, two neutrons, and two electrons.
