by Colin Tudge
Sunlight contains both red and far-red, so Pr and Pfr are normally in equilibrium. At noon, with the sun at its brightest, about 60 per cent of the phytochrome is in the form of Pfr. But in the dark of night, as the hours pass, Pfr steadily spontaneously degrades to become Pr: the active form decays into inactivity. Yet one brief flash will reconvert the accumulating Pr back into the active Pfr. In a long-day (short-night) plant, the burst of Pfr induces flowering. In a short-day (long-night) plant, it suppresses flowering.
12 WHICH TREES LIVE WHERE, AND WHY
Robin L. Chazdon and T. C. Whitmore (eds), Foundations of Tropical Forest Biology (University of Chicago Press, Chicago, 2002).
Verna R. Johnston, California Forests and Woodlands (University of California Press, 1994).
Plant Phylogeny and the Origin of Major Biomes. Discussion meeting at the Royal Society, London, March 2004, organized by Toby Pennington, Quentin Cronk and James Richardson. Proceedings published by the Royal Society in October 2004.
1. Recorded in J David Henry, Canada’s Boreal Forest (Smithsonian Institution Press, Washington, 2002).
13 THE SOCIAL LIFE OF TREES
Egbert Giles Leigh, Jr., Tropical Forest Ecology: A view from Barro Colorado Island (Oxford University Press, Oxford, 1999).
Ghillean T. Prance, ‘The Pollination of Amazonian Plants’ in G. T. Prance and T. E. Lovejoy (eds), Key Environments, Amazonia (Pergamon Press, Oxford, 1985).
1. S. Patino, E. A. Herre and M. T. Tyree, ‘Physiological determinants of Ficus fruit temperature and implications for survival of pollinator wasp species’, Oecologia 100, 13-20. Other key sources for their researches on figs and fig-wasps are:
Edward Allen Herre and Stuart A. West, ‘Conflict of interest in a Mutualism: Documenting the elusive fig-wasp–seed trade-off, Proceedings of the Royal Society, Series B, 267 (1997), 1,501–7.
Edward Allen Herre, ‘Population Structure and the Evolution of Virulence in Nematode Parasites of Fig Wasps’, Science 259 (1993), 1,442–5.
Carlos A. Machado, Emmanuelle Jousselin, Finn Kjellberg, Stephen G. Comp- ton and Edward Allen Herre, ‘Phylogenetic Relationships, Historical Bio- geography and Character Evolution of Fig Pollinating Wasps’, Proceedings of the Royal Society, Series B, 268 (2001), 685–94.
E. A. Herre, N. Knowlton, U. G. Mueller and S. A. Rehner, ‘The evolution of Mutualisms: Exploring the paths between conflict and cooperation’, Trends in Ecology and Evolution 14 (1999), 49–53.
2. Stanley A. Temple, ‘Plant–Animal Mutualism: Coevolution with dodo leads to near extinction of plant’, Science 197 (1977), 885–6,. See, too, the protest in Science 203, 1,364 from A. W. Owadally of the Forestry Service, Mauritius, and Dr Temple’s reply in the same issue.
14 THE FUTURE WITH TREES
Global Environment Outlook 3 (Earthscan/UNEP, 2002).
M. Ibrajim and J. Beer (eds), Agroforestry Prototypes for Belize (CATIE, Turrialba, Costa Rica, 1998).
Wangari Maathai’s Nobel Acceptance Speech and other articles about her work can be found on the website of the Green Belt Movement of North America: www.gbmna.org
Andrew W. Mitchell, Katherine Secoy and Tobias Jackson (eds), The Global Canopy Handbook (Global Canopy Programme, Oxford, 2002).
L. Szott, M. Ibrahim and J. Beer, The Hamburger Connection Hangover (CATIE, Turialba, Costa Rica, 2000).
Glossary
A
allele Many genes are ‘polymorphic’, meaning they may take more than one form. An allele is any one of the possible variants.
alternation of generations All land plants practise alternation of generations, in which a diploid generation (the sporophyte) gives rise to a haploid generation (the gametophyte) which in turn gives rise to another sporophyte generation, and so on. (Some animals also exhibit alternation of generations including the Cnidarians, which include jellyfish and anemones. But the basis of this is quite different.)
analogous Applied to structures that are similar in function, but originate in different ways. Thus the wings of a fly are merely analogous to the wings of a bird. Many plants, including some acacias and conifers, have phyllodes instead of leaves: they do the same job, but originate differently.
angiosperm Technically, the term refers to plants whose seeds are completely enclosed within an ovary. More casually (but accurately) angiosperm simply means ‘flowering plant’.
aril A covering around the seed that is often formed by outgrowth from the base of the ovule. Arils are often brightly coloured, and lure animals that disperse the seeds. Yew ‘berries’ are arils, and so is mace, the aromatic lacy covering of the cinnamon seed.
B
broadleaf The term colloquially applied to a dicot tree.
bryophyte A primitive land plant that lacks specialized conducting tissue (‘tracheary elements’) for internal transport of water and nutrients. The most conspicuous generation is the gametophyte. The living examples are hornworts, liverworts and mosses. In earlier classifications these three were grouped together in the formal taxon ‘Bryophyta’, spelt with a capital ‘B’. But the three do not necessarily share a specific common ancestor, and so do not form a true clade, and so should not be presented as a formal group. But they are all of the same ‘grade’, which can be denoted by the informal ‘bryophyte’, spelt with a small ‘b’.
C
cambium A meristem that gives rise to parallel rows of cells. The cambium of coniferous or angiospermous trees is responsible for ‘secondary thickening’, producing xylem tissue on the inside and phloem tissue on the outside.
carbon fixation The process by which hydrogen is combined with carbon dioxide from the atmosphere to produce organic molecules.
catkin An inflorescence of single-sexed flowers arranged as on a spike. Catkins are found primarily in woody plants, including trees such as willows and oaks.
chlorophyll The green pigment that mediates photosynthesis.
chloroplast The organelle that contains the chlorophyll
chromosome Chromosomes are long, thin structures that carry the genes. During most of the life of the cell, each chromosome is spread throughout the nucleus, and in this relaxed form they cannot be seen under the light microscope. But during cell division (mitosis and meiosis) the chromosomes contract to form short rods which, when suitably stained, can be seen clearly. In this visible, contracted form, each chromosome has its own characteristic size and shape; and each organism has its own characteristic number of chromosomes, each with its own characteristic size and shape. In truth, each chromosome consists of one enormously long molecule (or rather ‘macromolecule’) of DNA.
circadian rhythm Regular rhythms of growth and activity over a roughly 24-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: so 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 (animals) or division (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 (con
vergent 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.
D
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, 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 (deoxyribonudeic 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.
E
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 non-adaptive kind also play a large part, however, including ‘genetic drift’.
F
family A taxon of middling size, smaller than an order but bigger than a genus (see Linnean classification).
fertilization (1) In reproduction, fertilization is fusion of two gametes to form a diploid zygote. (2) 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 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 ‘Fruit’ is a term that 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 non-plants such as fungi – are sometimes called ‘fruiting bodies’.
G
game theory A body of mathematical analyses that attempts to quantify the outcome of any encounter between two or more different games 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 which 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 produce 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 multi-celled structure known as ‘pollen’, and the female sex cell is contained within the multi-celled 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 catalogue 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 ev
olutionary 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 practise very clear alternation of generations, with the main generation being the gameto-phyte; and they lack specialized conducting tissues (phloem and xylem). They may not be closely related to each other, 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.