by Colin Tudge
CIRCADIAN RHYTHM Regular rhythm of growth and activity over a roughly twenty-four-hour period.
CLADE “Clade” derives from the Greek clados, meaning “branch.” In biology a clade is a taxon of all the creatures that are descended from a common ancestor, plus the common ancestor itself. In modern taxonomy, no group is admitted to be a “true” taxon unless it is a true clade, as defined here. Small clades nest within bigger clades: species are contained within genera, which are contained within families, and so on, all the way up to domain; and each rank of taxon is itself a clade (provided the taxonomists have done their work properly).
CLADISTICS The set of techniques that are intended to help taxonomists decide whether the creatures they are attempting to classify do or do not form a true clade, and also to show how different clades relate to each other.
CLASS A class is a large taxon (clade) between phylum (for animals) or division (for plants) and order (see Linnean classification).
CLONE As a noun: a group of genetically identical cells or individuals. The term is also applied to each of the individuals. It may also be used as a verb (so that taking cuttings is an exercise in cloning).
COMMUNITY All the organisms that share a particular environment and interact with one another. Different members of the community may be of many different species.
CONIFER A cone-bearing tree.
CONVERGENCE (CONVERGENT EVOLUTION) Frequently, species from different lineages of creatures adapt to their surroundings in very similar ways, and so come to resemble each other, wholly or in part. This is convergence.
CORK Tissue with polygonal cells that are infused with suberin, which is a waxy material. When they are mature, the cork cells are dead—but in plants, and especially in trees, dead cells often contribute a great deal. Cork in general is protective. The waxiness repels and excludes water, but the gaps between the cells allow the passage of air.
COTYLEDON The seed leaf; the first leaves of the embryo. Typically though not invariably, dicots have two cotyledons, and monocots have one.
CULTIVAR A domestic variety of plant: a variety produced in cultivation, and generally maintained only in cultivation. Many garden trees are cultivars.
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DAY-NEUTRAL PLANTS Plants that flower without regard to day length.
DECIDUOUS Trees (and other plants) that shed their leaves periodically are said to be deciduous. Many temperate and boreal trees shed their leaves in autumn, and some tropical trees shed their leaves before the dry season.
DICOTYLEDON (DICOT) Basically, dicots are flowering plants whose embryos have two cotyledons. Traditionally, flowering plants were divided into two classes, dicots and monocots. But the dicotyledonous condition is now known to be primitive, so the “dicots” as originally defined do not form a true clade. (This is explained at length in the text.)
DIFFERENTIATION The process by which embryonic (“stem”) cells or tissues become specialized for particular functions. Differentiation is generally associated with loss of totipotency.
DIOECIOUS Unisexual. In dioecious trees and other plants, individuals contain either male flowers or female flowers but not both. Holly is an example of a dioecious tree.
DIPLOID, DIPLOIDY A cell with two sets of chromosomes is said to be diploid. The adjective also applies to an organism that has diploid cells. Diploidy is the abstract noun, referring to the state of being diploid.
DIVISION In botany, “division” is a large taxon, smaller than a kingdom but bigger than a class. Thus it is equivalent to the “phylum” of zoology (see Linnean classification).
DNA (DEOXYRIBONUCLEIC ACID) The stuff of which genes are made. DNA provides the code for proteins.
DORMANCY A special condition in which seeds or buds (or, in principle, any organ) enter a state of arrested development. Growth is actively suppressed (typically by hormones) and will not resume until the plant has been subjected to particular environmental signals. For instance, the seeds of many temperate trees and other plants need to be subjected to bursts of cold (sometimes extreme cold) before they will germinate. Such seeds may not germinate at all if the winters are too warm—which may happen because of global warming.
DOUBLE FERTILIZATION A strange characteristic of angiosperms. A male sex cell from the pollen fuses with a female sex cell from the ovule to form an embryo, as is normal in sexual reproduction. But at the same time, in angiosperms, a subsidiary cell in the pollen fuses with another cell from the ovule. The subsidiary male cell is haploid, while the subsidiary ovule cell is diploid, so the result is a triploid cell. This then divides to form the endosperm of the seed, which serves as a food store for the developing embryo. Extraordinary.
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ECOLOGY The study of all the interactions between different creatures, often of many different species, that share the same environment and between those creatures and the physical environment as a whole. The word derives from the Greek oikos, meaning household, which is also the root of the word “economy.”
ECOSYSTEM The sum total of an environment and all the creatures within it.
ENZYME A protein that serves as a catalyst, regulating the individual reactions that constitute the metabolism.
EPIPHYTE A plant that grows on another plant but is not necessarily a parasite. Trees all over the world are commonly festooned with epiphytes of all kinds—mosses, ferns, orchids, bromeliads. There are even some epiphytic cacti, such as zygocactus.
ETHYLENE A simple organic gas that is a major hormone in trees and other plants and may also serve as a pheromone.
EUKARYOTE, EUKARYOTIC Literally, “good cell.” In a eukaryotic cell the DNA is contained within a specialized region called the nucleus, surrounded by a protective, discriminating membrane. An organism that possesses eukaryotic cells is a eukaryote. Plants, animals, fungi, seaweeds, protozoans, and so on are eukaryotes. Bacteria and archaea are “prokaryotes.”
EVOLUTION The process by which organisms change over time, from generation to generation; Darwin spoke of “descent with modification.” He proposed that evolutionary change is brought about largely or mainly by natural selection, which leads to adaptation. Other mechanisms of a nonadaptive kind also play a large part, however, including “genetic drift.”
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FAMILY A taxon of middling size, smaller than an order but bigger than a genus (see Linnean classification).
FERTILIZATION In reproduction, fertilization is fusion of two gametes to form a diploid zygote. In plant nutrition, fertilization means increasing the nutrient content of the soil (and the term is also sometimes applied to improvement of soil texture).
FLOWER The reproductive structure of angiosperms. The whole “complete” structure consists of four whorls: the outer calyx, made up of sepals; the corolla, with the petals; the male stamens; and the female carpels. Many flowers are “incomplete,” however, and lack one or more of the whorls.
FREELOADER In ecology: a creature that cashes in on other creatures’ mutualistic relationships—taking what’s on offer but giving nothing back.
FRUIT The term should belong exclusively to angiosperms. Fruits may be fleshy or hard or papery, but in any case they are formed from the ovary, plus any other surrounding structures that may become incorporated. Reproductive structures of other plants—or even other nonplants, such as fungi—are sometimes called “fruiting bodies.”
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GAME THEORY A body of mathematical analyses that attempts to quantify the outcome of any encounter between two or more different game players—or two or more wild creatures. Through game theory military strategists and ecologists attempt to define the strategies that are most likely to succeed in any one circumstance.
GAMETE A haploid sex cell that fuses with another haploid sex cell to form a diploid zygote. In some primitive organisms all individuals produce gametes of the same size. But in organisms that traditionally were said to be “higher,” males produce very small, motile (mobile) gametes known as spermatozoa (or sperm), and females pro
duce large gametes, sometimes enlarged even further with considerable quantities of nutritious yolk, known as eggs. This is true of animals and of plants including cycads and ginkgoes. In conifers and angiosperms, however, the male sex cell is contained within a multicelled structure known as “pollen,” and the female sex cell is contained within the multicelled ovule. The male sex cell is then conveyed to the female sex cell via a “pollen tube.”
GAMETOPHYTE The generation of plants that produces gametes. In mosses, the predominant generation is the gametophyte. In ferns, the gametophyte is generally small. In angiosperms and conifers, the gametophyte is subsumed within the pollen and ovule.
GENE The unit of heredity. Genes are constructed of DNA.
GENE POOL The total catalog of all the alleles (genetic variants) within a population of creatures that are interbreeding sexually.
GENETIC DRIFT The processes by which some alleles are lost from the gene pool by means other than those of natural (or artificial) selection. Most notably: any one individual passes on only half of its genes to each of its offspring. It is possible, therefore, that some genes are not passed on at all. Particularly in small populations, and particularly in K-strategists (which have only a few offspring), it becomes quite likely that some of the rare genes (alleles) will be lost from the population entirely. Loss of genetic variation by genetic drift leads to evolutionary change, often significant evolutionary change, that is not primarily adaptive—and may indeed lead to the decline of the population and the extinction of the species.
GENOME The total apportionment of genes within any one organism.
GENOTYPE Related organisms with roughly similar genes are said to be of the same genotype.
GENUS A small taxon, smaller than a family but bigger than a species. The adjective from “genus” is “generic” (see Linnean classification).
GRADE Taxonomists speak of “clades” (defined above) and “grades.” “Grade” is a descriptive term that refers to the general level of organization of a creature: how complex it is, structurally and physiologically. Thus mosses, liverworts, and hornworts have much in common, both in what they possess and in what they lack: they are all small, green plants that practice very clear alternation of generations, with the main generation being the gametophyte; and they lack specialized conducting tissues (phloem and xylem). They may not be closely related to one another, and so do not seem to belong to the same clade. But in general form and way of life they are much of a muchness, and so can be said to be of the same “grade”—the grade that is commonly called “bryophyte.” Similarly, in zoology, the many various creatures commonly referred to as “reptiles” do not form a single, coherent clade. Tortoises have very different origins from snakes. But again they have much in common—leathery skin, relatively simple brains—and it makes sense to think of them together and give them the common name of “reptile.” But “reptile,” like “bryophyte,” is the name of a grade.
GYMNOSPERM A seed plant whose seeds are not fully enclosed within an ovary. The living gymnosperms are the cycads, ginkgoes and conifers.
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HABITAT The place and environment where creatures live.
HAPLOID A cell with only one set of chromosomes is said to be haploid. Gametes are haploid (at least when produced by diploid organisms). So are the body cells of gametophytes, as in mosses.
HARDWOOD The forester’s term for the timber of broad-leaved (dicotyledonous) trees.
HEARTWOOD The central core of the trunk of a mature tree, consisting of dead xylem tissue and ray tissue, often impregnated with tannins or other materials. Heartwood forms the greater part of timber and generally by far the most valuable part.
HERBARIUM A central repository where plant material is stored (most typically dried) and can be clearly identified, studied, and referred to.
HEXAPLOID A cell that contains six sets of chromosomes (or an individual composed of such cells).
HOMOLOGOUS, HOMOLOGY Organs of different creatures that have the same evolutionary and embryonic origin are said to be homologous, whether or not they have the same function. Thus, the wing of a bird is homologous with the arm of a human being (but not with the wing of a fly). The state of being homologous is homology.
HOMOLOGOUS CHROMOSOMES In a diploid cell (or organism), one of the two sets of chromosomes is derived from the mother, and the other set is from the father. Being of the same species, the two haploid sets are very similar; and each chromosome in each set has a corresponding partner in the other set. The sets of partners are said to be “homologous.”
HORMONE A chemical agent produced in one cell or tissue that affects the physiology or behavior of another cell or tissue (or, indeed, affects the physiology or behavior of the whole organism).
HOST An organism on which a parasite or epiphyte lives.
HYBRID Offspring of two genetically distinct parents. A hybrid between individuals from different genera is said to be “intergeneric”; a hybrid between individuals from different species is “interspecific”; a hybrid between different varieties from the same species is “intraspecific.” Many but by no means all hybrids are sexually sterile. Many otherwise sterile hybrids become sexually fertile by becoming polyploid.
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INBREEDING Breeding between two closely related organisms, such as siblings, or parents and offspring. Plants sometimes inbreed by self-pollination.
INFLORESCENCE A flower cluster. The form of the inflorescence is characteristic of each species. In the Asteraceae (Compositae) the arrangement of individual flowers is so tight that the whole inflorescence (as in a daisy) resembles a single flower (each individual flower within the inflorescence is then called a “floret”).
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KINGDOM The largest taxon recognized by Linnaeus (who proposed only two kingdoms: Plantae and Animalia). Nowadays, however, kingdoms are grouped within even larger “domains” and are divided into divisions (for plants) or phyla (for animals). (See Linnean classification.)
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LEGUME The name colloquially applied to members of the family formerly known as the Leguminosae but now properly called Fabaceae.
LENTICEL Holes in the surface tissues of stems or roots, loosely packed with cork cells, that allow the free exchange of gases between the inside of the plant and the outside. Common in many plants but of special significance in the roots of mangroves.
LIANA A large woody vine that climbs on other plants (and sometimes weighs them down).
LIGNIN A polymer containing nitrogen that binds cellulose fibers together and so provides enormous strength. Wood is basically cellulose toughened with lignin.
LINNEAN CLASSIFICATION The hierarchical system of taxonomy devised by the Swedish biologist Carolus Linnaeus in the middle decades of the eighteenth century. First, he made formal the “binomial” system of naming living creatures, which had been unfolding over the previous few centuries. In this system, each creature is given two names: the first is “generic”—that is, the name of the genus or “kind”; the second name is “specific”—the name of the particular species. Thus the common oak of Britain is Quercus robur—Quercus being generic and referring to all oaks (all 450 species of them), and robur denoting the particular English kind in question. (In this system, humans are Homo sapiens.)
Second, Linnaeus proposed a hierarchy of “taxa” (groups), in which smaller ones nest within larger ones, and so on. Linnaeus proposed five ranks. The biggest in his system was the kingdom, which was divided into classes, which were further subdivided into orders, which were then divided into genera and finally into species.
More ranks have been added since Linnaeus’s day, and the modern “Linnean” classification should really be called “neo-Linnean.” The complete modern sequence runs: domain, kingdom, phylum (for animals) or division (for plants), class, order, family, genus, species. Species may be further subdivided into subspecies or, less formally, into races. Races of plants are also sometimes called “varieties.” But varieties of plants produ
ced by artificial breeding in captivity are called “cultivars.” Varieties of animals produced by artificial breeding are called “breeds.” Varieties of animals or plants that are produced by informal selection on traditional farms are called “landraces.”
LONG-DAY PLANT A plant that will not flower unless first exposed to a minimum number of hours of daylight (although, in fact, long-day plants respond to short nights rather than to long days; see Chapter 11).
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MACRONUTRIENT An inorganic nutrient that a plant requires in large amounts, such as nitrogen, phosphorus, sulfur, and potassium.
MEIOSIS The form of cell division in which a diploid sex cell divides to form two haploid gametes.
MERISTEM Undifferentiated plant tissue from which new cells arise. The “apical meristem” is the growing tip.
METABOLISM The sum of all chemical processes occurring in a living cell or organism.
MICRONUTRIENT An inorganic chemical element that is essential to the growth of the organism but is required only in very small amounts. Also known as “trace element.” Among the essential micronutrients for trees are chlorine, iron, copper, manganese, zinc, molybdenum, and boron.
MINERAL A general term for any element or naturally occurring nonorganic compound.
MITOCHONDRION Mitochondria are organelles within a eukaryotic cell where most of the reactions of respiration are carried out. Sometimes colloquially called “the powerhouses of the cell.”
MITOSIS The process by which a diploid (or polyploid) cell divides to form two “daughter” cells containing exact copies of all its chromosomes.
MONOCOTYLEDON (MONOCOT) Basically, an angiosperm with only one cotyledon in the seed. The monocots all descend from a common ancestor and so form a true clade within the Angiospermae.