The Book of Leaves: Leaf-by-Leaf Guide to Six Hundred of the World's Great Trees
658
The Book of Leaves: Leaf-by-Leaf Guide to Six Hundred of the World's Great Trees
658eBook
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Overview
Of all our childhood memories, few are quite as thrilling, or as tactile, as those of climbing trees. Scampering up the rough trunk, spying on the world from the cool green shelter of the canopy, lying on a limb and looking up through the leaves at the summer sun almost made it seem as if we were made for trees, and trees for us.Even in adulthood, trees retain their power, from the refreshing way their waves of green break the monotony of a cityscape to the way their autumn transformations take our breath away.
In this lavishly illustrated volume, the trees that have enriched our lives finally get their full due, through a focus on the humble leaves that serve, in a sense, as their public face. The Book of Leaves offers a visually stunning and scientifically engaging guide to six hundred of the most impressive and beautiful leaves from around the world. Each leaf is reproduced here at its actual size, in full color, and is accompanied by an explanation of the range, distribution, abundance, and habitat of the tree on which it’s found. Brief scientific and historical accounts of each tree and related species include fun-filled facts and anecdotes that broaden its portrait.
The Henry’s Maple, for instance, found in China and named for an Irish doctor who collected leaves there, bears little initial resemblance to the statuesque maples of North America, from its diminutive stature to its unusual trifoliolate leaves. Or the Mediterranean Olive, which has been known to live for more than 1,500 years and whose short, narrow leaves only fall after two or three years, pushed out in stages by the emergence of younger leaves.
From the familiar friends of our backyards to the giants of deep woods, The Book of Leaves brings the forest to life—and to our living rooms—as never before.
Product Details
| ISBN-13: | 9780226176864 |
|---|---|
| Publisher: | University of Chicago Press |
| Publication date: | 12/10/2014 |
| Sold by: | Barnes & Noble |
| Format: | eBook |
| Pages: | 658 |
| File size: | 159 MB |
| Note: | This product may take a few minutes to download. |
About the Author
Allen J. Coombes is botanist at the Sir Harold Hillier Gardens and Arboretum in Hampshire, England, and the author of many books about plants and trees. Zsolt Debreczy, is research Director of the International Dendrological Research Institute in Boston.
Read an Excerpt
The Book of Leaves
A Leaf-by-Leaf Guide to Six Hundred of the World's Great Trees
By Allen J. Coombes, Zsolt Debreczy, Adam Hook, Coral Mula, István Rácz
The University of Chicago Press
Copyright © 2010 The Ivy Press LimitedAll rights reserved.
ISBN: 978-0-226-17686-4
CHAPTER 1
WHAT IS A TREE?
We all recognize a tree when we see one, and there are plenty of characteristics that allow us to distinguish an oak from a beech, or an apple tree from a pear tree. But it is surprisingly difficult to pin down the defining characteristics of trees in general.
When trying to define a tree, we can begin by saying that it is a perennial (that is, long-lived), woody plant with normally a single, clear stem that reaches at least 10 or 13 ft (3 or 4 m) tall. This distinguishes it from a shrub (also a woody perennial), which has several to many stems from the base and is shorter. However, there is no clear distinction between trees and shrubs at the point at which they meet. A tree can have more than one stem, and sometimes several. Moreover, many species featured in this book can grow as either a shrub or a tree. For example, many trees, while they make magnificent specimens in the lowlands, will be reduced to stunted shrubs when growing on mountains or in other exposed positions.
A tree, then, is not a botanically defined group of plants, as are the oaks or beeches. It is better to think of it as a form of growth, often subjectively defined, which some plants may or may not assume. A young oak seedling cannot be described as a tree, although it has the potential to become one.
THE FIRST TREES
Trees are land-based plants and, like land-based animals, they evolved from organisms that lived in the seas. The earliest plants grew in water and had no need of vascular tissue (for transport of water and nutrients, and also support). Only when these had been developed to supply water and nutrients to aerial parts could plants start to reach any size.
The earliest trees (strictly speaking, treelike plants since they did not possess wood) were very different from those we are familiar with today. They were the now-extinct relatives of modern club mosses (Lycopodium) and horsetails (Equisetum), which often reached a large size. The later ferns also had tree-sized members but these were restricted to moist habitats as fertilization occurred outside of the plant and needed a moist environment.
The first true trees, and the first of the seed-producing plants, were the gymnosperms (plants with naked seeds), in which fertilization occurs internally and is not reliant on an aqueous environment. This, together with the fact that they are wind-pollinated, enabled them to spread more widely into drier habitats. Today they are mainly represented by the conifers, but primitive gymnosperms such as Ginkgo biloba (the Maidenhair Tree) and the cycads still exist, though with much reduced distributions.
THE BROAD-LEAVES
The final group of plants (and trees) to emerge were the angiosperms (broad-leaves, or plants with seeds enclosed in an ovary). These often have colorful or scented flowers that enable them to be pollinated by insects. They have developed a wide range of different flowers, often adapted to attract different pollinators. Insect pollination is an advantage because it reduces the amount of nutrient-rich pollen that a plant needs to produce. Moreover, the seeds of angiosperms are enclosed in a range of different fruits, enabling them to be dispersed widely by animals. These adaptations make the angiosperms the most widely spread and successful group of living plants. All the leaves in this book are from trees in this group.
CHAPTER 2WHAT IS A LEAF?
A leaf can be defined as a vegetative outgrowth from the stem of a plant. The leaves of temperate-zone broad-leaved trees—the leaves that are discussed in this book—mostly have a flattened shape and are green in color. However, that broad similarity encompasses a vast evolutionary diversity of form.
A leaf typically consists of the blade (lamina), and stalk (petiole). In the axil of the leaf, where the leaf joins the stem, there is an axillary bud. Some species also bear structures known as stipules. These are small and often leaflike and are borne at the base of the leaf stalk. They are often small and inconspicuous, falling early, but sometimes they are large and useful for identification. In Sorbus sargentiana, for example, the stipules are very conspicuous. In other trees, for example Robinia pseudoacacia, the stipules are transformed into a pair of spines.
LEAF TYPES
While a simple leaf is undivided, although it may be lobed or toothed, a compound leaf is one that is divided into a number of leaflets. It may be palmately compound (divided in the manner of a hand) or pinnately compound (divided like a feather). In a compound leaf, the leaflets are always separate from each other and attach directly to the leaf stalk in a palmate leaf, or to the rachis (the extension of the leaf stalk in a pinnate leaf to which the leaflets are attached).
In some cases simple leaves on a shoot can be mistaken for the leaflets of a pinnate leaf, especially if the leaves are opposite. The easiest way to tell the difference is to look at the point where the leaf (or leaflet) is attached. A simple leaf on a shoot will have a bud in its axil where it joins the shoot. This is the point from which a shoot may develop in the next year.
Tree leaves are generally flat; they can also become reduced to tiny scales or spines. In some cases, the true leaf blade is absent. In many species of Acacia, for example, young plants have leaves divided into leaflets, but in mature plants the function of the leaf is taken over by a structure known as a phyllode, which develops from the expanded leaf rachis. There are also intermediate leaves with an expanded rachis as well as some leaflets.
HOW LEAVES DEVELOPED
Although leaves are one of the most conspicuous features of trees, the earliest land plants were leafless. Living in an atmosphere with high carbon dioxide levels, plants were able to obtain carbon dioxide through small pores called stomata on their stems. Some early plants had tiny outgrowths on the stem—these were the earliest leaves. But large-leaved plants, with few stomata, could not have survived in the relatively high temperatures that were then prevalent, since they would have been unable to cool themselves adequately. It is thought that leaves as we know them today developed as a response to a drastic reduction in carbon dioxide levels, and the resulting reduction of temperature. As carbon dioxide levels decreased it became an advantage, and a necessity, to have a greater surface area bearing stomata. It was only with the temperature decrease that such structures could survive.
The leaves of the earliest plants are considered to have been simple (that is, not divided), untoothed, and arranged alternately on the shoot. As flowering plants diversified (trees included), different kinds of leaves evolved. Variations, such as opposite and compound leaves, toothed and lobed leaves, probably evolved many times independently. This is the process that resulted in the great diversity of leaf form we see today.
CHAPTER 3LEAF FUNCTION
The basic function of a leaf is to perform photosynthesis. This means that it uses energy from sunlight to convert carbon dioxide from the atmosphere and water into glucose and oxygen. All green parts of a plant carry out photosynthesis, but in most plants the majority of the process occurs in the leaves. The green parts of plants contain chlorophyll, the pigment that is essential for photosynthesis. Chlorophyll absorbs energy from the blue and red wavelengths of sunlight. The green wavelengths are not absorbed and are reflected, which is why leaves appear green.
The carbohydrates produced during photosynthesis are used by the plant in a process called respiration. This is the conversion of carbohydrate to energy in the leaf and other structures. One result of this process is the production of water and carbon dioxide. While photosynthesis only occurs in sunlight, respiration occurs both day and night.
In order to carry out these two processes—photosynthesis and respiration—leaves need to perform gas exchange: that is, they need to absorb carbon dioxide from the atmosphere, and to release oxygen. Gas exchange is done through tiny pores (stomata) on the leaf surface that can be opened and closed.
LEAF COLOR
Leaves contain other pigments, but these are mostly masked by the intense green of chlorophyll. They can often be seen, however, at the beginning and end of the seasons. Many trees have leaves that are bronze or red when they first emerge in spring or summer, and many are well-known for their striking fall colors of yellow, red, or purple.
These other pigments have several functions. In young foliage they may protect developing chloroplasts (structures that contain chlorophyll and where photosynthesis occurs) from intense radiation, which can damage them, as well as making the foliage less attractive to predators. In mature leaves, although the pigments may not be visible they can absorb excess radiation, which might otherwise overload the photosynthetic system.
The other leaf pigments usually break down after chlorophyll, so the green color of the leaf disappears leaving its fall color, which is determined by the remaining pigments. When leaves of deciduous trees turn color and fall in the fall, they are not simply dying; they are being recycled by the tree as leaves contain a sizeable supply of nutrients, not least in the form of chlorophyll, which is rich in magnesium and nitrogen. That this is an active process can easily be seen by observing the fate of a branch broken on a tree in summer. In the fall most of the tree's leaves will change color and drop while those on the broken branch remain attached and brown. Leaf shedding requires a considerable input of energy from the plant; it is thought that leaf pigments can protect chlorophyll from intense radiation, enabling it to remain active for as long as possible before the leaves are shed.
LEAFLIKE BRACTS
There are leaflike structures known as bracts, which are associated with flowers or groups of flowers. Bracts can have several functions, such as protecting the flower, or if green, acting as a source of nutrients to the flower and developing fruit. Some bracts, when associated with flowers that are relatively insignificant, can take on the role of petals and act to attract pollinating insects. An example of this is the well-known poinsettia (Euphorbia pulcherrima), widely grown for its striking red bracts. Trees with showy bracts include the flowering dogwoods, such as Cornus florida and C. kousa, and the Dove Tree (Davidia involucrata). In the Dove Tree, the bracts are green at first and can photosynthesize, but as the flowers open they turn white. Not only do they attract insects to the flowers, they also protect the flowers from rain as the pollen loses viability when it is exposed to water.
CHAPTER 4THE VARIETY OF LEAVES
One of the most obvious differences in the types of leaf found on temperate-zone broadleaved trees are those that remain on the tree during winter and those that are shed in fall. The evergreens originated in moist and humid tropical regions and keep their leaves for more than one year; deciduous trees evolved to survive in extreme climates, and shed their leaves in the unfavorable season.
The first trees were evergreens. Modern evergreen species have the advantage that they are able to photosynthesize after deciduous trees have lost their leaves. They have tougher leaves that are more resistant to predators and as they replace a smaller proportion of them annually, tend to perform better on poorer soils. Broad-leaved evergreens tend to favor regions where the climate does not fluctuate excessively without long periods of cold or drought.
Deciduous trees, on the other hand, have the advantage that they are dormant in the unfavorable season. This means that they do not have to produce leaves that can survive very cold weather, and they do not have to supply them with water when the ground may be frozen. Not all deciduous trees lose their leaves because of cold winters. Many tropical species lose them in the dry season to avoid excessive water loss. Deciduous trees tend to be prevalent in regions with extremes of climate, either with long, cold winters, or with long dry seasons.
As evergreen leaves represent a greater investment for a plant, they need to be designed to last longer. So they are usually thicker, and many evergreens have spiny leaves to protect them from predators. Examples of trees with spiny leaves include the European Holly (Ilex aquifolium) and the Holm Oak (Quercus ilex) as well as other species of Ilex and Quercus. Both of these have leaves that are edged with spines so as to deter herbivores from eating the foliage. Such leaves grow low in the canopy; higher on the tree, where herbivores cannot reach, the leaves tend to have fewer spines or are spineless.
LEAF ADAPTATIONS
Trees that grow in places where moisture is not always available face the constant danger of desiccation (drying out). Leaves need to be flat to ensure maximum exposure to sunlight, and they need to open their stomata in order for photosynthesis to occur, but this results in the loss of water from the leaf. Plants have adopted a number of strategies to minimize water loss while enabling gas exchange to take place.
Leaf structure is part of the solution. To reduce water loss, most stomata are found on the lower leaf surface and normally close at night when there is no need to take in carbon dioxide, or during periods of water stress. In some plants, the stomata only open during the night and early morning, and any carbon dioxide that is absorbed during that time is stored in the leaf until it can be used in photosynthesis in the presence of sunlight. This process (known as crassulacean acid metabolism or CAM) occurs in many plants from dry regions, in particular succulents and also in the Hop Bush (Dodonaea viscosa).
As transpiration (the movement of water through the plant and into the atmosphere) can also take place through the leaf epidermis, this is covered by a waxy cuticle that cuts down water loss. The cuticle is particularly thick on the upper side of the leaf in evergreen plants and in those that grow in dry areas, and it is thicker on leaves that grow in sun than those in shade.
Under normal conditions, when a leaf has its stomata open, a layer of humid air builds up on the underside of the leaf. This layer decreases the amount of water lost from the leaf, but any movement of air will remove the layer and increase transpiration and water loss. Plants therefore have a number of ways to reduce the movement of air around the leaf and so reduce transpiration. An effective way to do this is to cover the stomata. In the Mediterranean Olive (Olea europaea) for example, the lower surface of the leaf is covered with tiny, wax-covered scales.
HAIRY LEAVES
Many trees have their leaves covered with hairs (technically called trichomes), particularly on the underside. These too disrupt the flow of air over the leaf surface reducing transpiration—but hairs can also have other functions. They are often present on the upper side of young leaves, to reduce the damaging effects of strong sunlight on the young tissue; they are usually lost as the leaf ages. Sticky, glandular hairs can deter insects that might feed on the young growths. Insect damage can also be reduced by the presence of extra-floral nectaries (organs that secrete sugar-rich nectar) on leaves such as in the Elder (Sambucus nigra). These provide food for certain insects that can then defend the plant from other, harmful insects.
(Continues...)
Excerpted from The Book of Leaves by Allen J. Coombes, Zsolt Debreczy, Adam Hook, Coral Mula, István Rácz. Copyright © 2010 The Ivy Press Limited. Excerpted by permission of The University of Chicago Press.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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Table of Contents
Contents
Foreword,Introduction,
What is a tree?,
What is a leaf?,
Leaf function,
The variety of leaves,
Leaf shape and arrangement,
Leaf anatomy,
Understanding plant names,
Identifying tree leaves,
The leaves,
Appendices,
Glossary,
Resources,
Index by common name,
Index by scientific name,
Acknowledgments,