The Tree (56 page)

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.

H

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.

I

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”).

K

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.)

L

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 produced 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).

M

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.

MONOECIOUS
Refers to a plant with single-sex flowers, but in which both sexes occur on the same tree. Pines and oaks are among the many monoecious trees.

MUTUALISM
The form of symbiosis in which both partners in the relationship gain net benefit.

MYCELIUM
The total of all the hyphae in a fungus. The mycelium of a single fungus may extend over many acres, and form mycorrhizal relationships with hundreds of trees.

MYCORRHIZA (
pl.
MYCORRHIZAE)
A symbiotic relationship between fungi and the roots of plants. Many trees, from pines to oaks to acacias (and a great many others) rely heavily or absolutely upon their fungal associates for optimal growth or even for survival.

N

NATURAL SELECTION
The process that Charles Darwin proposed is the chief adaptive force in evolution. The basic idea is that all creatures have the potential to produce more offspring than the environment can support, and so there is
competition,
which he also called a “struggle for existence”; that among these offspring there is
variation;
that, inevitably, some of the variants will be more closely
adapted
(or “fitted,” as the Victorians tended to say) to the prevailing conditions than others, and so be more likely to survive and leave offspring of their own. Thus as the generations pass, the lineage becomes better and better adapted to the prevailing conditions (until and unless the conditions change).

NEO-LINNEAN CLASSIFICATION
See
Linnean classification.

NICHE
A particular conceptual space within a habitat, offering opportunity for exploitation by specialist organisms.

NITROGEN FIXATION
The process by which certain bacteria convert atmospheric nitrogen into soluble ions, notably ammonium, which is further converted to nitrate in the soil and can be used by plants as a macronutrient. Many plants, including many trees, harbor nitrogen-fixing bacteria within special nodules in their roots.

NUCLEUS
The special region of the cell, surrounded by a specialized double membrane, within which the chromosomes (which contain the DNA) reside.

NUT
A fruit that is dry, hard, and indehiscent (meaning it does not split naturally to release the seed inside but must be actively prized open).

O

ORDER
A taxon smaller than a class and bigger than a family (
see
Linnean classification).

ORGANELLE
A discrete, specialized structure within a cell, such as a nucleus, chloroplast, or mitochondrion.

ORGANIC
Chemists use the term “organic” to mean any compound containing carbon (or containing carbon, hydrogen, and oxygen, with carbon as the principal component). More generally the term applies to any living material (or at least to material that was formerly alive).

OSMOSIS
The net diffusion of water from a region where the concentration of dissolved materials is low to where the concentration is high.

OUTCROSSING
Fertilization effected between different individuals (as opposed to inbreeding).

OVARY
The enlarged base of the carpel (or the fused bases of adjacent carpels) that forms a chamber that contains the ovule or ovules.

OVULE
The structure within the carpel of a seed plant that contains the female gamete (egg cell) and that matures after fertilization to become the seed.

P

PALEOBOTANY
The study of ancient plants, generally conducted through the study of plant fossils (including fossil pollen).

PARALLEL EVOLUTION
Sometimes two separate lineages of creatures that live in similar habitats evolve over time in similar ways, so that at any one time in their history each resembles the other. This is parallel evolution.

PARALLEL VENATION
The condition characteristic of monocot plants in which the principal veins in the leaf run roughly in parallel from the leaf base to the tip.

PARASITE
An organism that lives on or within another organism, usually of a different species, and derives nutrient from it. Parasitic relationships always benefit the parasite and do varying degrees of harm to the host. When the host also benefits from the presence of the parasite, the relationship is said to be “mutualistic.” Thus mycorrhizal fungi might be said to be parasitic on the roots of plants, but they also bring great benefit to the plant.

PATHOGEN
Any organism that causes disease in another.

PHENOTYPE
The overall form of an organism. Two or more organisms of similar genotype may nonetheless look or behave differently—which means that although they are genetically similar, they have different phenotypes.

PHEROMONE
A chemical agent that passes from one organism to another and influences the physiology or the behavior of the recipient. Essentially, an airborne (or waterborne) hormone.

PHLOEM
The specialized tissue outside the cambium that contains elongated cells that transfer foods, particularly organic foods such as sugars, around the plant. The phloem forms the inner (living) part of the bark.

PHOTOPERIODISM
The mechanism by which plants respond to day length and so adjust their life cycle to the seasons.

PHOTOSYNTHESIS
The process, mediated by chlorophyll, whereby plants harness the energy of the sun to split molecules of water into hydrogen and oxygen, and then attach the hydrogen to carbon dioxide (from the atmosphere) to form organic materials.