Newton's rejection of myriad Cartesian and mechanist views was a revolutionary development in late seventeenth-century natural philosophy. He reconfigured the relationship between physics and metaphysics, transforming crucial metaphysical questions into empirical issues while granting others a basic autonomy from the development of empirical science. This left Newton's physics with a parallel autonomy from the metaphysical presuppositions insisted upon in various ways by the Cartesians and the mechanists. Newton's conception of natural philosophy was novel in its emphasis on employing mathematical, especially geometrical, techniques to understand how motions of various kinds arise from forces of various kinds. But his consistent contention that the study of the divine represents a proper part of natural philosophy was more commonplace. And this vision of divine metaphysics has a traditional cast to it: like many figures in his day, Newton understands the world as consisting of substances that can act only where and when they are present. Among all of those substances, only one is infinite and necessary; the rest are finite and contingent. Yet even Newton's use of traditional metaphysical categories is modified by his controversial insistence that all substances, even God, are spatiotemporal entities whose action is limited to their location within space and time. These form essential components of Newton's limited but deeply held Weltanschauung.

The basic elements in that Weltanschauung were articulated especially through Newton's dissatisfaction with Cartesian metaphysics.

What does it mean to treat Newton as a philosopher? We cannot identify any overarching philosophical position with Newton, as we identify dualism with Descartes, monism with Spinoza, or even classical empiricism with Locke. Newton never wrote a systematic philosophical treatise of the order of the Meditations, the Ethics, or the Essay. This was a significant choice, for he was perfectly familiar with such treatises, having already analyzed some in his youth. Even in his voluminous correspondence, and in his most important unpublished manuscripts, we do not find a systematic engagement with metaphysical issues akin to Descartes's. Newton positioned himself as a strong critic of Cartesianism, but his response to Descartes is significant as much for its lacunae as for its central claims. Thus these canonical philosophical figures, with their canonical texts, cannot serve as a model here.

To treat Newton as a philosopher might simply be to avoid an anachronistic characterization of his intellectual milieu. As scholars of the early modern period regularly note, the intellectual categories and disciplines of Newton's day – which ranges, roughly, from 1660 until 1730 – differ radically from our own. What we would consider to be separate fields of study – for instance, aspects of what we categorize as philosophy, especially metaphysics and epistemology, the physical sciences, and even theology – were interwoven into one overarching field called natural philosophy.

We have seen that gravity is a physical quantity, although Newton remains ignorant of how it fits into the ontology that most of his interlocutors would accept. He is ignorant of whether gravity is a property, a mode, or something else entirely. Many of Newton's interlocutors – Leibniz most prominent among them – balked at this profession of ignorance, arguing that at best, he tacitly treats gravity as a property of all material bodies. From their point of view, Newton had thereby revived an aspect of late Scholastic natural philosophy that the mechanists had attempted to bury, the doctrine of occult qualities. Newton and the editor of the Principia's second edition, Roger Cotes, an astronomy professor at Trinity College Cambridge, attempted to rebut this charge, adding an extensive preface by Cotes in which he ridicules Newton's critics. However, determining the proper response to this criticism broke the ranks of the Newtonians. Cotes insisted that gravity was a primary quality like any other, not occult but manifest. Newton resisted that response, avoiding the contention that gravity was any type of quality.

Newton's response to Leibniz's criticism required a delicate balancing act: he proclaimed explicitly that all bodies in the universe “gravitate” towards one another, even while denying that he took gravity to be a quality of bodies. As we saw briefly in the last chapter, we can understand this subtle view only if we recognize that mass, rather than gravity, is the new quality discovered in the Principia.

John Keats could not see the flowers that surrounded him as he listened ‘darkling’ to his Hampstead nightingale, although from their scent he guessed them to be hawthorn, sweetbriar, sweet violets and ‘the coming musk rose’. A few miles away, in Epping Forest, but two decades later in time, John Clare was to listen to another nightingale singing, as the species mostly does, by daylight – the light of a long May evening in which the poet's eye would distinguish, below the intense green of young beech and hawthorn leaves, a ground layer of bracken, orchids and foxgloves, ‘where mugwort grows like mignonette’. Mugwort was familiar to Keats from his medical studies. But it is difficult to imagine him giving it a place in his verse; whereas the flicker of self-mockery in ‘like mignonette’ suggests that Clare knows Artemesia vulgaris to be as much at home in his poetry under its common name as sweetbriar is in Keats's ode under its pastoral name of eglantine. As honest, sturdy mugwort, it takes its due place, alongside ragwort, fleabane and sow thistle, among the 370 plants that Clare actually names in his poetry and prose.

This is an astonishing tally. Indeed the list is so comprehensive that when Clare fails to mention a common wild flower we find ourselves asking why.

A flower less like the primrose than the canna lily is hard to imagine. With its shield of high-held leaves and casque of huge blossoms, Canna indica is a piece of conspicuous expenditure in municipal gardens, suggestive of well-tended greenhouses and an army of bedders-out. Today we think of it as a typically Victorian plant; most at home, perhaps, in Mumbai or Calcutta, where ordered ranks of it form an imperial guard before the monuments of the Raj. Yet it reached Europe from the other – the West – Indies at the time of the Discoveries. It soon acclimatised in southern Europe, and sometimes shines out from the pages of a seventeenth-century florilegium. But Gerard could not coax it to produce its ‘fantastic’ flowers in his London garden, where he found it ‘very impatient to endure the injury of our cold climate’.

Cannas would have been even more impatient of the Swedish climate. Carolus Linnaeus had not seen one in flower at the time he devised his classification of plants according to what he called the sexual system: one based, that is, on the number and arrangement of stamens. Linnaeus was not the discoverer of plant sex. Gardeners had always known that, to obtain fruit from certain plants, pollen had to be transferred from individuals carrying it in their flowers to the stigmas of other individuals of which the flowers had no pollen.

17 June 1844 started badly for John Ruskin. ‘Set off about eight, to show my father and mother the fall of the eau de Bérard, but they were neither of them well, lost their tempers and spoiled all.’ His diary entry does not however begin with this family spat, but with ‘Today high up above the val d'Entragues, Cout[t]et pointed out to me a flower of the Soldanella which had actually pierced the snow, and was budding in the midst of it.’ For later the same day, Ruskin had been able to escape to clearer air. The Swiss guide showed concern for his safety on the snow-covered slopes, but whereas the protectiveness of his parents was irksome, that of Joseph Couttet, henceforth often to serve as a surrogate father (pro-papa is Ruskin's term), was fortifying. Moreover, he knew all about alpine plants. Two weeks previously, he had identified a small purple flower that still reposes in Ruskin's album of dried flowers, and told him that it was the first species to pierce the snow in spring. Now, above the Val d'Entragues, it was doing just that, breaking free in a thrust of self-affirmation that aroused in Ruskin a deeply empathic response.

Soldanella, the alpine snowbell, reappeared at another time of family tension four years later. Ruskin and his bride of two months had joined his parents for an intended tour of English cathedrals.

For a natural philosopher of the eighteenth century, botany could be an exotic and exciting pursuit. There was often the thrill of knowing that the seeds he propagated in his hothouse had been brought from the other side of the world, and the plants they grew into might prove spectacular in a number of ways. Or he might find himself nursing the inflorescence of a male palm throughout a nine-day journey from Leipzig to Berlin, in order to pollinate a female palm that had languished unfertilised for eighty years. But for the general run of English botanists at the beginning of the next century, many of them clergymen on modest stipends, the science was an altogether tamer business of distinguishing, identifying and recording the species and subspecies native to a small locality. The only excitement such a field naturalist could hope for was that, as he went about his parish duties, his eye might light upon some hitherto unrecorded grass or moss.

George Crabbe, like Erasmus Darwin a botanist and a poet, at first sight appears to conform to this more homely model. In fact, he is no more to be stereotyped as a botanist than, as poet, he is to be fitted into any of the traditional categories of literary history. Although younger than Darwin by a generation, Crabbe began to publish verse before the elder poet did, winning Dr Johnson's approbation for The Village as early as 1783.

Primroses grow in some surprising places, thanks to the mice and ants that pick up and then drop their seeds. Soon after Wordsworth and his sister settled in Grasmere, they noticed one on top of a boulder where it (or its progeny) was to remain safely out of human reach for the next thirty years. And as in life, so in literature. In the third part of Modern Painters (1856) a primrose makes a sudden appearance that is inexplicable till we realise where it has come from. The passage affords a good starting point for this study because in it Ruskin is trying to distinguish between the ways in which poets see plants.

When D. H. Lawrence and Earl Brewster met in an olive grove, in a drift of poppies, the American thought Lawrence ‘looked like a man who lived devoted to the study of such life – a botanist’. This was remarkably perceptive. Lawrence had a lifelong interest in plants, and he is unique among major English writers in having studied botany as a university subject. In another way, though, Brewster was wide of the mark. What Lawrence devoted his writing life to was ‘the problem of today, the establishment of a new relation, or the re-adjustment of the old one, between men and women’. Every reader knows that he writes wonderfully about flowers, whether in his novels – the moonlit lilies in Sons and Lovers perhaps come first to mind – or in essays such as ‘Flowery Tuscany’, or in many poems of which ‘Bavarian Gentians’ is the most anthologised. But when Connie Chatterley and her lover deck each other's body hair with forget-me-nots, it is an odd reader (and a southerner into the bargain) who wonders how they have come by forget-me-nots in the middle of a wood – before remembering that there is a woodland species, Myosotis sylvatica, and that it grows abundantly in Derbyshire.

Is, then, the floweriness of Lawrence's fictional worlds – the 145 named species in his first novel, for instance – to be regarded simply as by-work? He did not himself think of it in this way.

This is a book about the ways in which poets see plants. More exactly it is an attempt to record something of poets' perceptions of the richness and intricacy and life-giving importance of the plant world as these have been revealed in the generations since Linnaeus made botany big science. At least, that is its field, or scope. But like a plant, a book requires a supply of energy, a driving force; and to define that force I have to have recourse to a fragment of autobiography.

Many years ago, I came round from an emergency operation to find myself in a tropical hospital. Around me were all the mysterious comings and goings that belong to such an occasion. Then, as the bustle died down, one stranger more (the matron, I was later to discover) placed a small bowl of white flowers by my bed. ‘Just watch those’, she said before she, too, disappeared. So I lay and watched as the flowers changed – for they were Hibiscus mutabilis, blushing hibiscus – from white to shell pink, thence to a deeper pink, and finally, as night fell, to red. I meanwhile, transfused with fresh blood as they with pigment, came off the danger list. Next morning the bowl had gone, leaving my mind, now re-activated, busy with two questions. What was this empathy that during those fragile hours had caused me to feel I was being brought back to life by the flowers? And what made the blushing hibiscus blush?

Any reader of poetry who lived through most of the 1900s was bound from time to time to feel that the close relationship between poetry and plant science, which had held ever since Linnaeus, was in danger of being severed. To begin with, the poets who dominated the earlier part of the century declined to write about the green world. Next, botany as the study of whole organisms began to lose its attraction for scientists and their backers. Then, in the century's closing decades, the very plants themselves began to vanish.

Fortunately, disappearance does not always imply extinction. One of England's rarest plants is called the ghost orchid, not just because it has the palest of flowers and no green parts, but also because of the spook-like way it vanishes for years from a particular spot and then re-appears 100 metres away. Cold and dry weather may discourage buds from developing, but the coral-like root can throw out long underground runners in search of better conditions and in the hope of better springs. It had not however surfaced in England for some years, and was giving cause for concern, when Michael Longley sought to conserve it in a terse lyric: