The Secret Life of Trees

by Colin Tudge

  hardwood The forester’s term for the timber of broadleaved (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 behaviour of another cell or tissue (or indeed affects the physiology or behaviour 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, K

  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 (and each individual flower within the inflorescence is then called a ‘floret’).

  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 (plants) or phyla (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 fibres 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’; and 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. (Humans on the same system are Homo sapiens.)

  Secondly, 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 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 ‘neolinnean’ (though so far as I am aware, I am the only person so far to adopt this term). The complete modern sequence runs: domain; kingdom; phylum (animals) or division (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 text).

  M

  macronutrient An inorganic nutrient that a plant requires in large amounts, such as nitrogen, phosphorus, sulphur 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 non-organic compound.

  mitochondrion Mitochondria are organelles within a eukaryotic cell where most of reactions of respiration are carried out. Sometimes colloquially called ‘the power houses 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 hectares, 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 are more likely to survive and leave offspring of their own; and so as the generations pass the lineage becomes better and better adapted to the prevailing conditions (until and unless the conditions change).

  neolinnean 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 harbour nitrogen-fixing bacteria within special nodules in their roots.

  nucleic acid see DNA and RNA.

  nucleus The special region of the cell, surrounded by a specialized double membrane, within which the chromosomes (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 prised open).

  O

  order A taxon smaller than a class and bigger than a family; see Linnean classification.

  organdie 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 at least, 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) which 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 which matures after fertilization to become the seed.

  P

  palaeobotany 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 behaviour of the recipient. Essentially, an airborne (or water-borne) hormone.

  phloem The specialized tissue outside the cambium which 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.

  phototropism The tendency of plants to grow towards or away from light. In general, stems grow towards the light (positive phototropism), and roots grow away from it (negative phototropism).

  phyllode A flat, expanded area of petiole (leaf stacks) or stem that carries out photosynthesis. In some trees, including the coniferous celery pines, leaves have been abandoned in favour of phyllodes.

  phylogeny The term derives from the Greek phylos meaning tribe; so ‘phylogeny’ literally refers to the origin of different taxa. In practice phylogeny has come to refer to the true evolutionary relationships between different taxa. The entire methodology of cladistics in particular is designed to identify these true relationships. Modern, neolinnean classification, guided by cladistic principles, is intended to produce a taxonomy firmly rooted in true phylogeny.

  phylum A large taxon of animals, positioned between kingdom and class, and equivalent to ‘division’ in botany; see Linnean classification.

  physiology The study of all the functions and metabolic processes of living organisms.

  phytochrome A plant pigment (or collection of pigments) that absorbs red and infra-red light, and is heavily involved in the mechanism that controls photoperiodism.

  pigment Any chemical that absorbs light of particular wavelengths, and reflects what it does not absorb, so that it appears coloured. In general, all the many responses of plants to light are mediated by pigments of various kinds.

  pollen A small structure produced by conifers and angiosperms, containing the male sex cell.

  pollen tube When an individual pollen grain lands on the stigma of a flower it germinates to produce a pollen tube that grows down through the tissues of the style to reach the ovule beneath.

  pollination The process by which pollen is transferred from the anther to the stigma.

  polyploid A cell that contains more than two sets of chromosomes; or an organism that is compounded from such cells.

  population The term is used in various ways. It is best used to describe a group of individuals from the same species, living in the same place at the same time, who interbreed (or may be considered capable of doing so when circumstances permit).

  predator A creature that preys on another creature. The term is commonly taken to refer to carnivores, which prey on other animals. But herbivores such as giraffes, koalas and many a caterpillar may be seen as predators of trees.

  prokaryote An organism in which the DNA is not sequestered within a cell nucleus. There are two domains of prokaryotes: the Bacteria, and the Archaea. Plants, including trees of course, belong to the third domain, the Eucaryota, which have eukaryotic cells.

  R

  radicle The root of the plant embryo.

  rank, ranking In Linnean (or neolinnean) classification, ‘genus’ is a higher rank than ‘species’, ‘family’ is higher than ‘genus’, and so on all the way up to ‘domain’, the highest rank (or ranking) of all.

  reaction wood Wood with an abnormal structure that develops in response to particular stresses and strains. It is manifest in trees that lean, and as ‘compression wood’ beneath the boughs of conifers, and as ‘tension wood’ above the boughs of angiospermous trees.

  rhizome A more or less horizontal underground stem, frequently swollen and serving as a storage organ.

  RNA (ribonucleic acid) DNA, ensconced within the nucleus in a eukaryotic cell, provides the code for the construction of proteins. RNA in various forms ferries the information out from the nucleus and into the cytoplasm where the proteins are put together.

  S

  sap The fluid within the xylem, which is released when the stem is cut. ‘Cell sap’ is the fluid within individual cells.

  sapwood Outer part of the stem or trunk in which the xylem conducting tissue is still alive, and flowing with sap. Sapwood is commonly lighter in colour than heartwood, sometimes strikingly so as in the yew tree and some species of ebony.

  savannah Grassland with scattered trees.

  secondary growth Growth arising from division of cells of the cambium, which increases the girth of the trunk.

  short-day plant A plant that will not flower unless first exposed to days that are shorter than some critical day-length (or, more accurately, is exposed to nights that are longer than some critical length).

  shrub A perennial woody plant with several stems arising at o
r near the ground. The distinction between large shrubs and trees is arbitrary.

  softwood A forester’s name for the timber of conifers,

  species The basic ranking of living creatures; see –Linnean classification, and discussion in text,

  spore A cell (usually just a single cell) that is typically diploid, is released from the parent plant, and can then grow directly into a new plant. Thus spores serve as agents of asexual reproduction.

  sporophyte The generation of the plant that produces spores. In fact, although seed plants are properly called ‘sporophytes’, they do not produce free-living spores. They produce seeds instead. See text.

  strobilus A reproductive structure constructed from a number of leaves or scales, commonly arranged in a spiral, as in a cone.

  subspecies A subdivision of a species: generally synonymous with ‘race’; see Linnean classification.

  substrate Whatever the plant is growing on.

  succession In ecology, the sequence of species over time as new land is colonized.

  succulent Refers to plants that have fleshy leaves or stems, swollen with water.

  sucker A sprout arising from the roots that can give rise to a new plant – as in aspens and many willows.

  syconium Broadly speaking, the fruit of the fig tree. This is not quite accurate, however, since a fruit only becomes a fruit after the ovules within are fertilized. But the term ‘syconium’ also refers to the fleshy inflorescence, even before fertilization.

  symbiosis Literally means ‘together life’. Refers to the often close association of different organisms generally of different species. Although the term is commonly taken to imply mutual benefit, technically it can include parasitism, in which the host organism suffers. Symbiotic relationships from which both (or all) partners benefit are properly called ‘mutualistic’.

  systematics Essentially synonymous with ‘taxonomy’.

  T

  taxon, taxonomy Taxon literally simply means ‘group’; and taxonomy is the craft and science of placing creatures into groups – that is, of classification. A taxon (pl. taxa) may be of any ‘grade’: so ‘species’ is a taxon; ‘family’ is a taxon; ‘order’ is a taxon, and so on all the way up to domain. Ever since Darwin, however, mainstream taxonomists have insisted that a taxon cannot be formally acknowledged as such unless all the creatures within it are literally related to each other – meaning that they all share a common ancestor. Cladists have refined the idea so that no group can nowadays be admitted as a ‘true’ taxon unless it includes all the descendants of a common ancestor, and the ancestor itself, but includes no other creatures that are not part of the lineage.