In general, the variation given in the descriptions is somewhat wider than that presented in the key, and many characters used in the latter have had to be omitted. In using the descriptions, the following features must be assumed for most species of a family, unless otherwise stated: milky sap absent, habit not succulent, parts of the flower free from each other, stamens not antepetalous, and anthers opening by longitudinal slits. The ptyxis, as far as it is known, is given for each family: this refers to leaves if they are undivided, to leaflets if the leaves are divided.

The families are listed in the xorder of the Melchior system (see p. 3). No attempt has been made to group them into suprafamilial units (‘orders’), as these have no significance for practical identification.

The following points concerned with presentation should be noted.

Morphology

The oblique stroke (/) is used instead of ‘or’; the letter ‘n’ is used instead of ‘many’ or ‘numerous’ (i.e. more than 10 or 12). Abbreviations: K, calyx-segments or sepals; C, corolla-segments or petals; P, perianth-segments when these are undifferentiated; A, stamens; G, carpels. These letters are also used in the collective sense; for example, ‘A antepetalous’ means stamens antepetalous. Brackets are used to indicate that the segments of any particular whorl are united to each other; for example, C(5) means a corolla of 5 lobes united below into a cup or tube.

The identification of plants is carried out on the basis of the information available about the plant in each particular case. In most situations this information will be derived from a specimen of the plant itself, either whole (if the plant is small, or if it is being examined in situ while growing) or a part (generally a stem or twig, with or without flowers or fruits), and consists of the structure displayed by the specimen (its morphology) together with other information that might be available (e.g. where the plant came from originally). On the basis of this information one can make use of the keys in this book to obtain an accurate identification of the family to which the specimen belongs. In order to do this, the specimen has to be observed carefully, so that the structure it displays, and the terminology needed to describe it, are properly understood. The rest of this chapter provides a very brief survey of flowering plant morphology in so far as it is needed for family identification. Each new term is italicised at its first appearance, and appears in the Glossary (p. 269). Further information can be found in textbooks of botany, in Bell, A. D., Plant Form, Oxford (1991), which is extremely well illustrated with fine photographs, and in other glossaries, such as Hickey and King, The Cambridge Illustrated Glossary of Botanical Terms, No. 19 in the annotated bibliography (p. 264).

The keys in this book are of the bracketed type and are dichotomous throughout, i.e. at every stage a choice must be made between two (and only two) contrasting alternatives (leads), which together make up a couplet. To facilitate reference to particular leads, each couplet is numbered and each lead is given a distinguishing letter (a or b). As the main key allows for the identification of over 320 families, it has been arranged into groups, with a key to the groups at the beginning.

To find the family to which a specimen belongs, one starts with the key to groups and compares the specimen with the two leads of the couplet numbered 1. If the specimen agrees with 1a, one proceeds to the lead with the number that is the same as that appearing at the right-hand end of lead 1a (in this case, 2); if, however, the plant agrees with lead 1b, then one proceeds to the couplet numbered 14. This process is repeated for subsequent couplets until, instead of a number at the right-hand end of a lead, a group is reached. Throughout this process, it is very important that the whole of each couplet be carefully read and understood before making a decision as to which lead to follow.

Exploiting plants

Human beings use thousands of species of plants for food, either as food or flavourings, for fuel, for construction materials and as sources of chemicals (oils, resins, gums, dyes, medicines and poisons). Almost all our calories and protein come either directly from plants or indirectly from plants used as food for our domesticated animals (the remainder comes from algae and fungi). All parts of plants have been directly exploited. Food has been obtained from the root (root-tubers, tap-roots), stem (tubers, rhizomes, and canes), leaf, flower (nectar and pollen in honey), seed and fruit. Wood, timber, fibres and other materials such as resins and latex have been obtained from roots, stems and leaves.

Humanity has always exploited plants but perhaps only for the past 10,000 years or 1% of human history have we cultivated them. Almost uniquely, human kind is a gardener, a cultivator. The first gardeners, in hunter–gatherer societies, were likely to have been women. They selected favoured plants, helped their cultivation and, in doing so, unconsciously changed the plants. The first cultivation of plants may have occurred in the Mesolithic period some time after 15,000 years ago. It was a time when the climate fluctuated rapidly.

Plant development

Plants undergo an orderly succession of developmental changes (ontogeny) starting with the simple structure of the embryo and ending with the highly complex organisation of the mature plant, senescence and death. At least, this is the zoocentric view of the plant life cycle. In reality it is more complex than that because many plants, by shedding their parts, actually may be said to be constantly dying while they are living. Programmed cell death (PCD), called apoptosis in animals, occurs in the normal life cycle of all plants, for example, during maturation and senescence of leaves, flowers and fruits, and abscission in the regular seasonal cycle of temperate plants during leaf-fall, and as the result of stress. In aquatic and semi-aquatic plants cell death creates air channels that aerate the submerged tissues. In addition, the great propensity for vegetative reproduction gives plants almost immortality. Such serial changes contribute to the unfolding development of the plant as a whole, but they also occur at all levels of organisation, from cells and tissues to organs.

The emergence of scientific botany

The study of plants must be one of the oldest occupations of humans who, even in their most primitive state, required a wide knowledge of the plants that provided food or remedies for illness. By trial and error they knew which plants were poisonous and which were edible. This expertise led to the first sowing of wild seeds, the start of agriculture and therefore the beginning of civilisation. The earliest classification systems were utilitarian ‘common-sense’ classifications but could be extremely sophisticated. The Mayan folk classification of plants, for example, is no less systematic than the latest scientific classifications based largely on analyses of DNA sequences.

The long history of botany is a record of our attempts to describe and understand plants. This is not as straightforward as it might seem. Even a simple term such as ‘leaf’ can be interpreted in several ways and its meaning depends upon the context of its use. A concept such as species is more complex.

It is common for new botany students to complain about the number of terms, names and concepts they have to learn. Botany uses language in which the things are, in a sense, ‘created’ by the words we use to describe them.

The phylogeny of plants

One of the best ways to understand variation is by comparison among related groups. Perhaps the greatest early success in this approach was that of Hoffmeister in the nineteenth century when he realised that the evolution of ovules and seeds could be best understood by understanding the variations of heterosporous and endosporic nonseed plants. The availability of an independently-derived phylogeny, from DNA sequence data, has vastly increased the power of this comparative approach.

Looking at phylogeny it is clear that particular forms have evolved repeatedly. Time and again similar morphologies and anatomies have evolved separately in distinct lineages. These examples provide a key to understanding the evolution of plants not just in terms of adaptation, say in understanding a convergent feature as one that has evolved to fit a similar function, but perhaps more importantly in understanding the shared environmental and developmental processes that have constrained or permitted certain evolutionary pathways.

The paths of diversity

Imagine the map of diversity as if it were a city plan. There are the city blocks, at different longitudes and latitudes of morphology, anatomy, physiology and chemistry. These are the archetypes. They are connected by the paths that represent the developmental pathways between them.

The yin and yang of reproduction

The trio of sex, multiplication and dispersal are the pillars of the evolution of life on Earth. Multiplication comes through the processes of sex and dispersal but requires neither. Many organisms reproduce only, or mainly, asexually and have no special mechanisms for dispersal. For organisms living in water, dispersal was never much of a problem. Water currents dispersed them haphazardly. But dispersal on the land is a formidable challenge, a cliff that the first land colonists had to scale. The landscape was only patchily friendly and even if a foothold could be established the leap from one damp patch to another was a huge challenge. The plants that first met this challenge, and the structures they used to do it, their spores, provide the first record of complex terrestrial life. In colonising the land, plants transformed it making it an easier place for other colonists.

The living response

The plant in its world: macrocosm and microcosm

The environment of plants exists on vastly different scales. Plants are the primary producers and are basal to almost all food chains except marine ones where they are replaced by the algae, and a few others such as some deep-sea hydrothermal vents where chemoautotrophic organisms live. They play a vital role in the flow of energy through all ecological cycles. The whole system of life rests solidly on their industry without which the evolution of many other organisms could not have occurred. Vegetation forms the macrocosm of life on Earth, yet the relationships of individual plants to their environment operate on a microcosmic scale. Many of the adaptations of plants to life on land have involved internalising the external, creating their own atmosphere in the spaces between their cells in their leaves and stems or garnering moisture and nutrients by colonising the soil in the finest possible way.

Plant diversity around the world

A complete treatment of vegetation around the world would be impossible in a whole book let alone a single chapter. Instead, we concentrate on plants that inhabit different environmental extremes. We bring into focus the biotic relations of plants and, in addition, we consider some aspects of plant evolution in relation to the Earth's history and climate change, by looking at plants of islands.

The greatest omission this chapter is an account of the forests of the world. Every botanist should visit a tropical rainforest at least once. No vegetation formation on Earth can compare to tropical rainforest in its staggering wealth of life forms, its diversity of species. It is the ‘Ultima Thule’ of the botanical world, after which everything else falls into perspective.

Living at the edge of chaos

This chapter provides a short history of the pre-biotic Earth and of organisms in the early stages of the evolution of life. It covers the origins of photosynthetic organisms, the setting of the stage for the evolution of plants and terrestrial ecosystems, and for the subsequent diversification of plants from the Silurian Period onwards. Key early events are the evolution of metabolism, including photosynthesis, of mechanisms of heredity and of cells. Later symbiotic associations between cells provide a much broader canvas for life-forms to diverge. Other important stages in the evolution of plants were the origin of multicellularity and subsequently the functional specialisation of cell types in the multicellular organism.

Process, form and pattern are three primary features of living systems. In this section we focus individually on each of these primary criteria of life. Process first, concentrating on the origin of the processes fundamental to life, and particularly to plants – photosynthesis.