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Glossary: H

Habgood nomenclature
This nomenclature uses the numbers of the binomial expansion (i.e., 20, 21, 22, 23, etc., with arithmetic values of 1, 2, 4, 8, etc.). Each binomial number has an arithmetic value that is double that of its predecessor. The sum of any combination of binomial numbers is unique. For example, the sum 21 can be obtained only by adding 16 + 4 + 1, and no other combination of binomial numbers can add up to this sum.
The nomenclature can be applied to matching pairs of vertical genes. Each pair of matching genes is then labelled with the binomial numbers 1, 2, 4, 8, etc., in order of discovery. The name of each pair of genes is the primary Habgood name, and it is a single binomial number. Any combination of genes is named with the sum of their binomial numbers, and this is a secondary Habgood name.
Any combination of genes, in either the host or the parasite, is named with a single number, and exactly matching vertical resistances and vertical parasitic abilities have the same name.
The composition of a secondary Habgood name is easily determined. Suppose the secondary name was 29. The largest possible binomial number is subtracted from it. In this case, this would be binomial 16. This means that gene 16 is present. The remainder is 13, from which 8 can be subtracted, indicating that gene 8 is present. The remainder is now 5, showing that genes 4 and 1 are also present. These gene names 16 + 8 + 4 + 1 add up to 29, and no other combination of binomial numbers can add up to this sum.
Habitat
The natural home of an organism, usually with living conditions that are closely similar to those of its original environment.
Hand-pollination
The artificial pollination of a flower, usually involving cross-pollination in order to obtain a sexual recombination of two chosen parents. See also: Emasculation.
Haploid
A cell or plant that has only one set of chromosomes. A sex cell (i.e., pollen and ovules in plants, sperm and ova in animals) is normally haploid, and the fusion of two sex cells produces a normal diploid with two sets of chromosomes. Haploid plants can be produced artificially, and their single set of chromosomes can be doubled to produce a doubled monoploid. The terms haploid and monoploid are synonymous. See also: tetraploid, triploid.
Hardy-Weinberg law
The law that states that gene frequencies will remain constant from generation to generation, provided that no other factors, such as selection or mutation, are operating.
Hardwoods
Timber trees that are Dicotyledons. The timber of these trees is suitable for fine furniture and cabinet making. See also: Softwoods.
Haricot bean
See: Phaseolus vulgaris.
Harvesting
The process of gathering in a crop. Commercial harvesting of grain crops is usually undertaken with a combine harvester. Many horticultural crops, and all subsistence crops are harvested by hand. The harvesting of a plant breeder’s screening population usually involves carefully selected individual plants.
Hashish
See: Cannabis sativa.
Hay
Pasture grasses and/or pasture legumes that have been cut and dried in the field for use as animal feed. “Making hay while the sun shines” is a traditional method of providing winterfeed for farm livestock.
Hazel nut
See: Corylus avellana.
Head to row selection
See: Family selection.
Hectare
A measure of land area. One hectare is 10,000 square metres, or 2.471 acres.
Helianthus annuus
The sunflower, which is now a valuable oil crop. The Church in Russia forbade the use of a long list of cooking oils on many fast days each year. Being an unknown New World plant, sunflower was not on that list of proscriptions. It consequently became very popular in Russia where the first cultivars were developed. Dwarf varieties are now grown for combineharvesting in many countries. The species is open-pollinated and amenable to selection for horizontal resistance by amateur breeders Sunflowers, and the closely related Jerusalem artichoke, are the only crop species of any significance to originate in North America.
Helianthus tuberosus
The Jerusalem artichoke. A close relative of the sunflower, it is open-pollinated and amenable to recurrent mass selection for horizontal resistance by amateur breeders who might have a special interest in this rather unimportant crop.
Heliotropic
(= phototropic) A directional growth or movement towards light.
Helminthosporium
A genus of fungi which cause disease on a number of crops. While mostly imperfect fungi, a few species belong to the Ascomycete genera Pyrenophora, Ophiobolus, Gaeumannomyces, and Cochliobolus.
Hemileia vastatrix
Coffee leaf rust. This disease is of interest because, when arabica coffee was taken as one pure lines to the New World, all of its pests and diseases were left behind in the Old World. This gave Latin America a commercial advantage and it now produces about 80% of the world’s coffee. When the rust was accidentally introduced into Brazil in 1970, there were fears of a major disruption of the world supply. Fortunately these fears proved groundless, as the disease was easily controlled.
Leaf rust is also interesting in that its spores can be either wind-borne or water-borne. The former method of dissemination is clearly for the process of allo-infections from tree to tree. The latter is for Auto-infection from one leaf to another within one tree.
This disease is an apparent exception to the rule that vertical resistance will evolve only in the seasonal tissue of a discontinuous pathosystem, because coffee is an evergreen perennial. However, infection can only occur if there is free water on the leaf surface. During the tropical dry season, all infected leaves are shed, and the fungus dies with them. Consequently, arabica coffee is functionally deciduous with respect to rusted leaves only.
Hemiptera
An order of insects usually called ‘bugs’. Many leaf bugs are important crop pests. The order also includes the bed bug, which is of interest in demonstrating the stability of natural pyrethrins.
Hemp
See: Cannabis sativa.
Hemp, Deccan
See: Hibiscus cannabis.
Hemp, Manila
See: Musa textilis.
Hemp, sunn
See: Crotalaria juncea.
Herb
Any flowering plant that lacks woody tissues.
Herbicide
Any chemical that kills weeds. Modern herbicides are often selective in the sense that they will kill some types of plant but not others. Typically, 2,4-D kills Dicotyledons and may be used safely on Monocotyledons crops such as cereals. Further selectivity has been obtained by the use genetically modified crops that are resistant to a specific herbicide.
Herbicide injury
Traces of herbicide in sprayers or other equipment can cause injuries to crop plants. The symptoms can be very confusing and, if suspected, a specialist should be consulted.
Herbivores
An animal that lives on plants, mostly grasses that can withstand grazing because their leaves grow from the base, and not the tip. The appearance of grasses, some 25 million years ago, led to an explosive evolution of herbivores. The evolution of humans, as hunter-gatherers, depended on the fact that the African savannah carries up to 20,000 kilograms of herbivores per square kilometre. At the other extreme, tropical rain forest carries only 5-10 kg/sq.km. It is no accident that rain forests have the fewest archaeological remains of hunter-gatherers, or that our hominid ancestors favoured open grasslands.
Herders
Nomadic people who had domesticated a social (i.e., herding) species of animal. Surviving herder societies include the Laplanders who herd reindeer, and the Masai, who herd cattle. The earliest known herders date from 20,000 years ago, and they herded Barbary sheep in North Africa.
Heritability
The percentage of a plant’s quantitative variable that is due to genetics, the remaining percentage being due to environment. For example, a plant may have a zero level of parasitism because the parasite is absent from the area in question. It appears to have 100% resistance. However, if the parasite were present with maximum epidemiological competence, the plant might have a 50% level of parasitism. The heritability of that apparent 100% resistance would then be only 50% (i.e., half of the original apparent resistance is inherited and can be inherited by the progeny, while the other half is an environmental effect that cannot be transmitted to the progeny).
Hermaphrodite
Having both sexes in one individual. In plants, this means having both sexes in one flower. If both sexes occur in separate male and female flowers on one plant, this arrangement is termed dioecious. See also: monoecious.
Hessian fly
See: Mayetiola destructor.
Heteroecious
Greek = different houses. A heteroecious plant parasite is one that is compelled to parasitise two different species of host, often called the winter host, during which sexual recombination occurs, and the summer host, which involves asexual reproduction only, in order to complete its life cycle. A heteroecious pathosystem has two species of host.
In practice, the only heteroecious parasites of plants are species of aphids and rusts, but some of them are major pests and disease of crops. During the summer phase, these parasites are r-strategists and a system of biochemical locks and keys, derived from the vertical subsystem and the gene-for-gene relationship, can be shown to have a remarkable evolutionary survival advantage.
Heterogeneous
Of different descent (c.f., heterogenous = of different composition). Pronounce it heterogeneous (c.f., heterogenous). See also: homogeneous, homogenous.
Heterogenous
Of different composition (c.f., heterogeneous = different descent). Pronounce it heterogenous (c.f., heterogeneous). See also: homogeneous, homogenous.
Heterosis
The hybrid vigour that is exhibited by the progeny of two inbred (i.e., homozygous) but different parents. This vigour persists for only one generation, and it is the basis of hybrid varieties.
Heterozygous
This term refers to a plant whose two parents were genetically different. In plants, the term may refer to a single gene, or to the entire genetic makeup of the individual plant. Heterozygous plants do not ‘breed true to type’. See also: homozygous.
Hevea brasiliensis
Para rubber. Note that the name comes from the State of Para in Brazil, and that each ‘a’ is pronounced long, as in ‘art’. The name should not be pronounced with a short ‘a’ as in ‘parachute’.
Para rubber is a deciduous tree native to the Amazon Valley. It was taken to the Far East by the British, and this area became the main producer because it was free of the native pests and diseases. The Brazilian complaint that rubber was stolen from them is not justified in view of their enormous use of Old World crops such as coffee, sugarcane, and soybean.
In any event, plantation rubber does not thrive in Brazil, but old plantations, laid out by the Ford Motor Company in the early part of the twentieth century, have survivors that are resistant to disease and these merit screening for vegetative propagation.
Hexaploid
A cell or a plant with six sets of chromosomes. Diploid is the normal state in most plants and animals. See also: doubled monoploid, haploid, tetraploid, triploid.
Hibiscus cannabis
Kenaf, bimli jute, or Deccan hemp. A jute substitute that probably originated in Africa.
Hibiscus esculentis
See: Abelmoschus esculentus.
Hippomane manchinella
This is the plant from which arrowhead poisons are extracted in South America. It is of interest in that arrowroot got its name from being a supposed antidote to these poisons.
Holistic approach
A systems term meaning that systems analysis, or systems management, is being conducted at the highest feasible systems level. The converse, in which the system is studied at the lower systems levels, is called the merological approach. The holistic approach is essential if suboptimisation is to be avoided.
Homeostasis
The ability of a system to maintain an optimum in all its variables, and to recover from swings away from this optimum, at any systems level. The recovery is the result of negative feedback. For example, if people get too hot, they sweat, and the evaporation of the sweat cools them down. Conversely, if they get too cold, they shiver, and this unconscious exercise warms them up. See also: Genetic homeostasis.
Homogeneous
Of the same descent (c.f., homogenous = of same composition). Pronounce it homogeneous (c.f., homogenous, as in homogenised milk). See also: heterogeneous, heterogenous.
Homogenous
Of the same composition (c.f., homogeneous = same descent). Pronounce it homogenous, as in homogenised milk, (c.f., homogeneous). See also: heterogeneous, heterogenous.
Homologous evolution
Evolution in which similar features have a common origin (e.g., all the plants in one family have a common ancestor). This is in contrast to analgous evolution, in which similar features have different origins (e.g., the wings of birds, insects, and bats represent analogous evolution).
Homozygous
In plants, this term may refer to the alleles of a single gene, or to the entire genetic makeup of an individual plant. In the former situation, the two parents each had the same allele of that gene. In the latter situation, the two parents were genetically identical in all respects. A population of plants that are homozygous in their entire genetic makeup is called a pure line, and these plants ‘breed true to type’. (See also: heterozygous).
Hops
See: Humulus lupulus
Hordeum vulgare
Barley. A crop that is as old as wheat, dating from about nine thousand years ago. It is salt-tolerant, and it often substituted for wheat, in ancient times, in soils that had become salty from inappropriate irrigation. However, it is little used in human nutrition today. The main use is for animal feed, and for malting to make beer. There is plenty of scope for developing horizontal resistance, and the crop is probably amenable to the use of male gametocides.
Horizontal
In a plant epidemiological context, this term is entirely abstract, and it means that a gene-for-gene relationship is absent. Horizontal resistance and horizontal parasitic ability are both defined by the absence of a gene-for-gene relationship. A horizontal subsystem of a pathosystem is also defined by the absence of a gene-for-gene relationship.
See also: Horizontal parasitic ability, horizontal resistance, Horizontal pathotype, and Horizontal pathodeme.
Horizontal parasitic ability
Parasitic ability that does not result from a gene-for-gene relationship. Horizontal parasitic ability is the parasitic ability of the Biometricians, and its inheritance is usually controlled by many polygenes. Although it has been very little studied, it seems always to vary quantitatively. It is the parasitic ability that enables a parasite to obtain nutrients from its host after the vertical resistance has been matched, and in spite of the horizontal resistance. (See also: vertical parasitic ability).
Horizontal pathodeme
A population of a host in which all individuals have the same horizontal resistance. Many different cultivar with the same horizontal resistance, but with differing agronomic characteristics, all belong to the same horizontal pathodeme.
Horizontal pathotype
A population of a parasite in which all individuals have the same horizontal parasitic ability. The various members of a horizontal pathotype may differ in other respects, such as pesticide resistance.
Horizontal resistance
Resistance that does not result from a gene-for-gene relationship. Horizontal resistance is the resistance of the Biometricians; its inheritance is normally controlled by polygenes.
It results from many different resistance mechanisms; it is quantitative in both its inheritance and its effects; it controls all the consequences of a matching infection including Auto-infection; it also controls allo-infections in a continuous pathosystem that lacks a gene-for-gene relationship; and it is durable resistance.
The level of horizontal resistance can be at any degree of difference between the minimum and the maximum. The minimum level of horizontal resistance usually means that there is a total loss of crop in the absence of crop protection chemicals. Conversely, the maximum level of horizontal resistance usually means that there is a negligible loss of crop in the absence of crop protection chemicals.
In plants that don't have a gene-for-gene relationship, horizontal resistance is the sole protection, and the only resistance. In plants that have a gene-for-gene relationship, the function of horizontal resistance is to control all the consequences of a matching allo-infection, including all auto-infection. So horizontal resistance occurs in every plant against every parasite of that plant.
Horizontal resistance requires population breeding and recurrent mass selection. For this reason, Mendelian breeders do not like it, and it has been seriously neglected during the twentieth century. However, these population breeding techniques are so easy to use in most crops that amateur breeders can breed for horizontal resistance, especially with the support of OPBF or another plant breeders association.
Once adequate horizontal resistance is accumulated, the environmental and human hazards, as well as the labour and costs of applying crop protection chemicals are eliminated.
Because a good horizontally resistant cultivar never needs to be replaced, except with a better cultivar, breeding for horizontal resistance is cumulative and progressive.
As horizontal resistance is accumulated, the crop losses from pests and diseases decline, and the biological anarchy that was induced by the use of crop protection chemicals also declines, as biological control agents return and increase in numbers.
The improved biological control enhances the effects of the horizontal resistance. The two factors are mutually reinforcing.
See also: Horizontal parasitic ability, vertical resistance, Comprehensive horizontal resistance, Laboratorymeasurements, Relative measurements, Partial resistance, Field resistance, Race-non-specific resistance.
Horizontal resistance, comprehensive
The horizontal resistance to one species of parasite does not normally function against any other species of parasite. Comprehensive horizontal resistance means that a cultivar has high levels of horizontal resistance to all the locally important species of parasite. This is achieved during breeding by selecting for the one character of ‘good health’ (i.e., the holistic approach).
Because the epidemiological competence of parasites varies considerably between agro-ecosystems, the horizontal resistances that are comprehensive in one agro-ecosystem may be too high, or too low, in another agro-ecosystem. This is why on-site selection is important.
Horizontal resistance, examples
There are numerous examples of deliberate and successful breeding for horizontal resistance. The best summary is by N.W. Simmonds, entitled Genetics of Horizontal Resistance to Diseases in Crops, and published in Biol. Rev. 66: 189-241. See also: Phaseolus vulgaris.
Horizontal resistance, laboratory measurements
Horizontal resistance can be measured in the laboratory using plant growth chambers. But these measurements are expensive and difficult, and they do not necessarily correspond to field performance. They are not recommended for amateur breeders.
Horizontal resistance, opposition to
The scientific opposition to horizontal resistance during the twentieth century was apparently due to the fact that plant breeding was dominated by Mendelian breeders who (1) disliked working with polygenes and population breeding methods, and (2) favoured work with single-gene resistances, in spite of the ephemeral nature of vertical resistance. This attitude, which still endures, has led to a serious vertifolia effect in many crops, and it does much to explain why we now use crop protection chemicals in such enormous quantities. See also: Mindset.
Horizontal resistance, relative measurements
Horizontal resistance is difficult to measure and it has no exact scale of measurement comparable, say, to the Celsius scale of temperature measurement. In practice, the only feasible measurements of horizontal resistance are field measurements that are also relative measurements. That is, a cultivar is described as being either more or less resistant to a specified parasite, than another cultivar of known performance.
Horizontal subsystem
The subsystem of a pathosystem that is controlled by horizontal resistance and horizontal parasitic ability.
Hormone
A substance that regulates the behaviour of specific cells or tissues. Hormones can be natural or synthetic.
Hormone mimic
Some synthetic chemicals, such as insecticides, can mimic hormones and, at extremely low concentrations, they can damage an unborn human foetus, or an actively growing young child. The safest course for an expecting mother, or for young children, is to eat only organic foods.
Horse bean
See: Vicia faba.
Horseradish
See: Armoracia rusticana
Horticulture
That branch of crop husbandry that involves fruits, vegetables, and ornamentals.
Host
A species, or an individual organism, that harbours parasites, and supplies those parasites with nutrients.
Host range
The range of different species of host that a parasite is able to exploit.
Host-parasite relationship
The category of parasitism in which there is a high frequency of parasitism, but a low injury from parasitism. For example, fleas parasitise zebras. They parasitise every zebra in the herd, so the frequency of parasitism is maximal. But they do very little harm to each individual zebra, so the injury from parasitism is minimal. See also: Predator-prey relationship.
Hot water treatment
A treatment for seeds that are infected. With careful control of the temperature, it is possible to kill the pathogen without killing the seed. Loose smuts of cereals can be controlled in this way.
Houseflies
These flies were the first known insects to develop resistance to a synthetic insecticide, which was DDT in Naples, during World War II, thus demonstrating the previously unknown possibility of unstable insecticides.
Humulus lupulus
Hops, which are now used almost exclusively for brewing beer. The plant is a perennial vine which dies back to ground level each fall. The above-ground parts thus have a discontinuous pathosystem and they have vertical resistance.
Hops is a long-day plant and it is dioecious. It is propagated vegetatively, and only about eight clones dominated world production until quite recently. These include ‘Fuggle’ and ‘Golding’ in Britain, ‘Hallertaur’ in Bavaria, and ‘Saaz’ in Czechoslovakia. These clones are mostly ancient, and they demonstrate the utility and durability of horizontal resistance.
A breeding program started in Germany in 1922 accumulated polygenic resistance to downy mildew by breeding within the European population. This was one of the earliest examples of horizontal resistance being chosen over vertical resistance. This is not a crop for amateur breeders.
Humus
The decomposed organic matter in soils. Humus is a source of plant nutrients, and it is necessary for microbiological activity. It also contributes to soil structure and drainage.
Hundred seed weight
This measurement indicates the average seed weight in a crop such as wheat. Breeders who aim exclusively at total yield may end up with very many, very small grains; while breeders who aim exclusively at a high weight of individual seeds may end up with low yields. amateur breeders should be aware of this laboratory measurement when selecting parents in a program of recurrent mass selection.
Hungry Forties
The period during the 1840s when blight was destroying the potato crops of Europe. The famine was at its worst in Ireland where one million people died of starvation, and one and a half million emigrated, mainly to North America. This reduced the population of Ireland by about one third. Eastern Germany, Poland, and western Russia suffered similar famines.
Hunter-gatherers
People who are pre-herders and pre-agriculturalists. Hunter-gatherers still exist in areas where herding and agriculture are not possible (e.g., Kalahari desert). The early vegetarian hominids became tool-users, employing naturally shaped stones to break open large bones abandoned by carnivores. Later they became tool-makers, and this initiated a period of about two million years of hunter-gathering. Herding started only twenty thousand years ago, and agriculture began a mere nine thousand years ago.
Hyacinth bean
See: Lablab niger.
Hyaline
A mycological term that means a tissue which is lacking pigments and is almost transparent.
Hybrid
The offspring of a cross between two different genera, species, or varieties. Note the specialised meaning of hybrid variety.
Hybrid seed
See: Hybrid variety.
Hybrid swarm
A population, usually of an open-pollinated plant, that shows very great genetic diversity because it is derived from a cross between two or more different species. The tea crop is a typical example. See also: cline.
Hybrid variety
A cultivar of an open-pollinated species (e.g., maize, cucumber, onion) which has been produced by crossing two inbred lines. The resulting seed then produces plants that exhibit hybrid vigour, or heterosis. A hybrid variety can be used only once, because the hybrid vigour is largely lost in the second generation. This means that the seed of hybrid varieties is expensive, but the expense is more than justified by the increased yields. Hybrid varieties do not normally need the protection of breeders’ rights because the breeder has complete control of the inbred lines.
Hybrid vigour
Also known as heterosis, this is the increased vigour that is exhibited by an interspecific cross (e.g., mules, which are sterile hybrids of a horse and donkey), or by a cross between two inbred lines of a single species, particularly an open-pollinated species of plant. See also: Hybrid varieties.
Hybridisation
In plants, the cross-pollination that produces a hybrid.
Hydroponics
The cultivation of plants in a nutrient solution instead of in soil. This technique is used mainly in greenhouses, and it is particularly useful for single seed descent. The plant roots can be suspended directly in the solution, or in inert gravel wetted with the solution, or inside flattened, plastic, tubular, film that is lying on the ground. In the last case, the plant grows through a small hole in the film, and nutrient solution is pumped continuously through the tube.
The advantages of hydroponics are (i) a high density of plants using less greenhouse space, (ii) rapid growth and maturation leading to a shortened breeding cycle, (iii) general freedom from pests and diseases, and (iv) labour-saving.
While the vast majority of hydroponics use synthetic fertilizers, it is possible to use organic nutrients. Whether the food produced can be classed as organic depends on local regulations.
Hymenoptera
The Order of insects that includes bees, wasps, ants, and Ichneumons.
Hyperparasite
A parasite of a parasite. One of the principle agents of biological control. For example, rust is a parasite of coffee leaves, and it has a hyper-parasitic grub that eats its spores. If coffee trees are sprayed with insecticides, the effects of this hyper-parasitism are lost. See also: Predator.
Hypersensitive fleck
A small necrotic speck, just visible to the naked eye, which indicates a hypersensitive reaction of a gene-for-gene relationship to a non-matching allo-infection.
Hypersensitivity
The process in which a group of cells surrounding an infection site dies very rapidly, and the infecting parasite dies with them. The infection then fails. This is a common mechanism of vertical resistance against allo-infections in leaves, but note that not all vertical resistance is due to hypersensitivity (e.g., Fusarium and Verticillium wilts), and not all hypersensitivity is due to vertical resistance. See also: Hypersensitive fleck.
Hypha
A single strand of microscopic fungal mycelium.
Hypocotyl
The stem of a germinating seedling that is below the cotyledons.