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 forms. It must be emphasised, however, that no evidence has yet been found in the fossil record to support such a claim (see also Box 1.1).

Before the atmosphere became enriched by oxygen the only organisms that existed were prokaryotes (Table 1.1), comprising simple cells lacking a nucleus defined by a membrane (see Chapter 2). These first prokaryotes, called Archaea, which translates as 'ancient ones', may still be found in places as diverse as hot springs and the human navel. Bacteria, which appeared soon after, are also prokaryotes, and some of these and some of the Archaea are capable of photosynthesis.

BOX 1.1

Five major extinctions, in each of which more than 50% of all species died out, are known from the earth's fossil record. The most catastrophic of these occurred c. 252 million years ago at the end of the Permian period. The causes of this so-called great dying or more correctly, the Permian-Triassic Extinction Event, in which as many as 97% of all genera of living organisms may have been lost, were probably complex. However, one factor is thought to have been the massive proliferation of photosynthetic land plants, leading to a build-up of very substantial quantities of free oxygen in the atmosphere, which in turn triggered catastrophic global environmental change.

Table 1.1 The main groups of photosynthetic organisms mentioned in Chapter 1. There is currently no single agreed system of higher level classification, but a series of competing ones. This table is broadly based on the five-kingdom system; the gymnosperms based on Christenhusz et al.a, and the flowering plant classification follows the Angiosperm Phylogeny group III systemb

*Frequently referred to collectively as the gymnosperms.

**Informal names for groups recognised that have no formal botanical rank.

ASource: Christenhusz, M.J.M., Reveal, J.L., Farjon, A., Gardner, M.F., Mill, R.R. & Chase, M.W. (2011) A new classification and linear sequence of extant gymnosperms. Phytotaxa, 19, 55-70.

bSource: Angiosperm Phylogeny Group (2009) An update of the Angiosperm Phylogeny Group classification for the orders and families of flowering plants: APG III. Botanical Journal of the Linnean Society, 161(2), 105-21.

The presence of oxygen in the atmosphere also led about 2100-1600 million years ago to the gradual evolution, by natural selection, of eukaryotes (see Chapter 2), whose cells had a clearly defined, membrane-bound nucleus, complex chromosomes and membrane-bound organelles. The latter are subcellular structures with specialised functions. They include mitochondria, where respiration occurs and, in plants, chloroplasts (Fig. 1.4; see also Chapter 2), where photosynthesis occurs. It is probable that organelles such as chloroplasts and mitochondria, which...