Science and the Garden

The Scientific Basis of Horticultural Practice

David S. Ingram, Daphne Vince-Prue, Peter J. Gregory, David S. Ingram, Daphne Vince-Prue, Peter J. Gregory

• Publisher: Wiley-Blackwell
• Year: 2015
• ISBN:
  9781118778395
• Edition: 3
• Topic: Scienze biologiche
• Subtopic: Orticoltura

Most conventional gardening books concentrate on how and when to carry out horticultural tasks such as pruning, seed sowing and taking cuttings. Science and the Garden, Third Edition is unique in explaining in straightforward terms some of the science that underlies these practices. It is principally a book of 'Why' – Why are plants green? Why do some plants only flower in the autumn? Why do lateral buds begin to grow when the terminal bud is removed by pruning? Why are some plants successful as weeds? Why does climate variability and change mean change for gardeners? But it also goes on to deal with the 'How', providing rationale behind the practical advice.

The coverage is wide-ranging and comprehensive and includes: the diversity, structure, functioning and reproduction of garden plants; nomenclature and classification; genetics and plant breeding; soil properties and soil management; environmental factors affecting growth and development; methods of propagation; size and form; colour, scent and sound; climate; environmental change; protected cultivation; pest, disease and weed diversity and control; post-harvest management and storage; garden ecology and conservation; sustainable horticulture; gardens and human health and wellbeing; and gardens for science.

This expanded and fully updated Third Edition of Science and the Garden includes two completely new chapters on important topics:

• Climate and Other Environmental Changes
• Health, Wellbeing and Socio-cultural Benefits

Many of the other chapters have been completely re-written or extensively revised and expanded, often with new authors and/or illustrators, and the remainder have all been carefully updated and re-edited. Published in collaboration with the Royal Horticultural Society, reproduced in full colour throughout, carefully edited and beautifully produced, this new edition remains a key text for students of horticulture and will also appeal to amateur and professional gardeners wishing to know more about the fascinating science behind the plants and practices that are the everyday currency of gardening.

CHAPTER 1
Diversity in the plant world

Summary

In this Chapter the stages in evolution of the diversity of plant life on earth are outlined and the essential characteristics of the most successful land plants summarised, as an introduction to Chapters 2 and 3. The characteristics, origins and occurrence in the garden of ‘primitive plants’ are then summarised. Finally, flowering land-plant forms, their occurrence in the wild and their value in the garden are presented in tabular form.

Introduction

The most remarkable thing about plants is that they are green (Fig. 1.1), a property that makes it possible for them to generate the energy required to sustain almost the entire living world. To appreciate the significance of this it is necessary first to consider what happens to the average motor car if, like the one in Fig. 1.2, it is neglected for long enough: the bodywork rusts and the non-ferrous components disintegrate and decay. Indeed, it is the usual experience that all inanimate things, left to themselves, eventually reach a state of disorder: buildings crumble, books turn to dust and machines rust. This general tendency is expressed in the second law of thermodynamics, which states, in essence, that in an isolated system the degree of disorder and chaos – the entropy – can only increase.

Figure 1.1 ‘The most remarkable thing about plants is that they are green.’ Entrance to the ‘Professor’s Garden’ at Brantwood, Coniston, Cumbria. Photograph by David S. Ingram.

Figure 1.2 It is the usual experience that inanimate things, left to themselves, like this VW Beetle, eventually reach a state of disorder. In contrast, living things, like the plants of oilseed rape that surround it, are able to create order out of disorder, assembling atoms and molecules to form tissues and bodies of great complexity and sophistication. Photograph by David S. Ingram.

Creating order out of disorder

When one thinks about living things, however, it is immediately apparent that they are able to create order out of disorder, assembling atoms and molecules to form tissues and bodies of great complexity and sophistication (Fig. 1.1). How is this creation of order out of disorder thermodynamically possible, given that living things, just like inanimate objects, operate according to the laws of physics and chemistry? The answer is that the cells of living things are not isolated systems in a thermodynamic sense, as a motor car is, for they are constantly deriving energy from another, external, source, the sun. It is necessary to go back in time to find out how this came about.

The earth first condensed from dust and ashes about 4540 million years ago, and life must have appeared some time during the first thousand million years of the planet’s existence. The molecules that made life possible may have arrived from another planet in, for example, a comet, but current theories suggest they were probably generated here on earth. The earliest life forms were heterotrophic, deriving their organic molecules (those containing carbon) from their surroundings, a legacy from the prebiotic ‘soup’ of chemicals that was left on the cooling earth after its genesis. These would have provided them with energy and the building blocks for making cells. But as these natural resources were exhausted, a key event was the evolution of the process called photosynthesis, whereby sunlight is harnessed to provide an alternative, external, source of energy (see Chapters 2 and 8).

The study of very old rock formations in Australia has suggested that this may have occurred more than 3600 million years ago, for by that time there were present on the planet simple organisms consisting of single cells or chains of cells that resemble the blue-green Cyanobacteria (‘blue-greens’) that grow in shallow, stagnant water or as a greenish slime on the surface of marshy soils and wet lawns even today (Fig. 1.3). These primitive organisms were so successful that they have remained virtually unchanged throughout almost the entire course of evolutionary time. The Cyanobacteria possess the ability to capture the electromagnetic radiation of the sun and incorporate it into a chemical energy source. This is made possible by the presence of light-absorbing pigments that give the blue-greens their characteristic colour, the most significant being the green pigment chlorophyll a (see Chapter 2). For the first time in evolutionary history there had appeared on earth autotrophic organisms, which were able to make their own food. The great diversity of plants alive today sprang from these humble beginnings.

Figure 1.3 A chain of cells of the cyanobacterium (blue-green) Anabaena. Light microscope photograph by Patrick Echlin.

The evolution of photosynthesis had another significant consequence. As the number of photosynthetic organisms increased they altered the earth’s atmosphere. This is because the most efficient form of photosynthesis, the one employed by most primitive plants, involves the splitting of water molecules (H₂O) to release oxygen (O₂) (see Chapter 2). This increased the oxygen level in the atmosphere, which had two important effects. First, some of the ‘new’ oxygen in the outer layer of the atmosphere was converted to ozone (O₃), a gas that absorbs the ultraviolet (UV) radiation from sunlight, which is very damaging to living organisms. This meant that organisms could survive in the surface layers of water, and even on land. The current depletion of the ozone layer as a result of human activity is a serious reversal of a critical stage in the evolution of life on earth. Secondly, the increase in the level of oxygen made possible the process of aerobic respiration, whereby carbon molecules formed by photosynthesis are broken down to release energy required for building bodies in far greater quantities than are released by anaerobic respiration, which occurs in the absence of oxygen. It may be speculated that when photosynthesis first began to occur on a very large scale about 2400 million years ago, the resulting substantial quantities of oxygen in the atmosphere and the concomitant increase in the number of aerobic organisms may have caused great extinction event, the massive decline of anaerobic life ...