Chemical and Mechanical Explanation of Physiological Processes
The seventeenth-century upheaval in natural philosophy meant that savants were embroiled in countless debates. They disputed theories and challenged evidence, and they disagreed about the order in which investigations should proceed. Although in principle academicians argued for completing natural historical research before addressing natural philosophical questions, in practice they could not wait. In botany, they sought principles that would account for the life cycle of a plant and turned to three forms of explanation: chemical, mechanical, and biological, with the biological model itself depending on chemical and mechanistic explanation. Chemistry was pivotal because it was susceptible to both mechanistic and vitalist interpretations; like botany it was in the throes of redefinition. Academicians were uncertain just how far they could reduce chemical processes to mechanical causes, and their eclectic accounts of plant physiology reflect their quandary.
Generation and Reproduction
Perhaps the most perplexing aspect of the life sciences during the seventeenth century was generation. The respective roles of female and male, the nature and function of eggs and spermatozoa, and the sexuality of plants were all at issue. When academicians considered the generation of plants, they focused primarily on determining which of two incompatible mechanistic theories — spontaneous generation or preformation — accounted
more plausibly for certain phenomena. According to the theory of spontaneous generation, animals and plants could be generated from soil, air, or corpuscles directly, without the need for progenitors of their own type. Preformationists held that living creatures carried within themselves perfectly formed descendants in miniature, so that the process of generation involved little more than giving birth to these entities. New experiments, observations with the latest instruments, and theological objections undermined both theories, however, and at the same time clarified the role of seeds in plants.
Perrault opened the Academy's discussion of germination and reproduction with his paper of January 1667; academicians subsequently studied seeds, earths, and the conditions for germination, examined vegetative reproduction, and debated preformationism. Always interested in testing the claims of the ancients, Perrault and Dodart were skeptical about Theophrastus's assertion that plants grew from their saps, and also about the contemporary chemical notion that salts extracted from plants acted as seeds. Other academicians emphasized reproduction by seeds and by vegetative propagation. Here the natural history complemented their natural philosophical research. The Marchants described the location and appearance of the seed, Bourdelin analyzed seeds chemically, and Homberg discussed their properties.
Most academicians were dubious about spontaneous generation. In 1667 Perrault suggested testing the theory by observing earth taken from so deep in the ground that seeds could not have penetrated there; this would determine whether plants could grow without seeds. In the 1690s Tournefort showed that it was best to assume plants had seeds; if seeds had not been seen, it was probably because they were small enough to elude observation. He identified them in several plants previously said to have been seedless, especially ferns. Examining with a microscope the dust-like dots on ferns, he saw that they were "tiny sacks, each of which contains a large quantity of seed." In one capsule Tournefort counted more than three hundred individual seeds, and he grew a fern from some of them. He opened capsules previously thought to be the seeds of Lunaria and Polypode and, using a microscope, showed that they too contained numerous tiny seeds. Where even he could not find seeds, Tournefort argued analogically: if a plant whose seed was unknown passed through the same stages of growth as a plant known to grow from seeds, then it was proper to infer that the first plant also grew from seeds. Thus Tournefort claimed that the maidenhair fern of Montpellier (capillaire de Montpellier ) grew from a seed,
because its shoots consisted of a leaf and a thin root, like those of other plants.
Although Tournefort described his own observations in the context of Grew's, Ray's, and Morison's discoveries, he criticized his English counterparts for stopping short of full discovery. He contradicted Morison, for example, who believed that mushrooms sprang directly from the earth. In a large fungus taken from the woodwork in the abbey house of Saint Germain, Tournefort identified as seeds some fine dust attached by delicate threads to the pores of the fungus. He then looked for their source, what he called the ovary of a plant, in the rough crust on the back of the mushroom but concluded that this could not be the ovary since there was no seed there. Since he believed that similar natural objects should display similar natural processes, Tournefort disagreed with Morison's explanation of why the fungus Erysimum was more common after the London fire of 1666. Morison claimed that the mushroom had grown spontaneously from soil that was altered by the fire, but Tournefort replied that changes in the soil simply encouraged the seeds to germinate.
Both Mariotte and Tournefort examined the mechanisms of seed dispersal, especially in plants like wood sorrel, dittany, wild cucumber, and others that threw their seeds great distances. Their shapes and appendages seemed to enhance propagation. Mariotte observed that the seed of a moss called rampion was slim and thus could slip through the dense growth to the ground. Tournefort believed the spiral shape of dittany seeds helped them spring away from the plant. Seeds were known to travel great distances, sometimes because their hooks and hairs attached to animals. Mariotte also described tip-layering and recalled that one of the Marchants had shown him a clover growing in the Jardin royal "whose flower, when it began to dry, curved and grew into the soil, so that the seed formed there and the clover in effect planted itself."
Driven by a Baconian quest for data and inspired by zoological models, academicians observed vegetative reproduction and checked their hypotheses experimentally. Above all, they used their findings to adjudicate between the theories of spontaneous generation and preformation, with the mechanists Mariotte and Tournefort on opposite sides.
The theory of spontaneous generation had a long pedigree stretching back to the ancient atomists. Although rejection of the theory has been heralded as a seventeenth-century contribution to modern science, some of the best minds continued to accept that plants and animals could be generated spontaneously. Mariotte, one of the principal experimentalists
of the Academy, argued that plants could be propagated when two or three corpuscles hooked together in the air, water, or earth. Such a union could "give the first impulse to [a plant's] growth," as for example when grass grew on the site of a dried pond where no seed could have fallen. Unlike Tournefort, who used circumstantial evidence to subsume the exceptional under a general inductive rule, Mariotte invoked a corpuscularian theory of matter to perpetuate the exceptional cases.
The theory of spontaneous generation waned in popularity during the seventeenth century, albeit more slowly among botanists than among zoologists. Thus discoveries concerning generation often became grist for the preformationist mill. Savants cited animalcules to refute spontaneous generation and to support preformation. Plants had long seemed to offer particularly strong evidence in favor of preformation, and here, exceptionally, an analogy from botany influenced the development of theoretical zoology.
Opinion in the Academy was split. At the end of the century, Tournefort and Dodart spoke for preformation. But Mariotte had earlier rejected it altogether after he, Dodart, and Jean Marchant studied bulbs of tulips, lilies, and narcissi in 1677 and 1678 without finding even one entire, mature plant in miniature. Mariotte concluded that the mature plant did not exist in either bulbs or seeds. Other evidence also seemed to disprove the theory: the knots on a rosebush produced flowers in the spring but branches and leaves in the autumn, and grafts might take three years to flower. Nor could preformationism account for variations within a type of plant: apple trees, pear trees, and melons were so varied that to accept the preformationist view — that one seed could contain an infinite number of identical plants — entailed making unpalatable assumptions, such as that one plant produced varied seeds or that plant families did not exist.
Mariotte repudiated preformation and also the related theory of emboîtement , which maintained that the parent organism contained its descendants, but that these were not perfectly formed. He argued instead that plants developed their parts and properties gradually, as the result of an interaction between the plant and its sap. Seeds contained "only the principal parts of plants" and all other parts were developed "in succession as a result of the way the first parts affected the sap." Mariotte concluded:
But it is not believable that this small composite of corpuscles contains all the branches of this Plant, its leaves, its fruits, and its seeds; and even less that in these seeds there could be contained in miniature all the branches, leaves, flowers, etc., of the Plants which will be produced ad infinitum after this first germination.
Like his contemporaries, Mariotte could not adopt both spontaneous generation and preformation (or emboîtement ); unlike most of them, his corpuscularian theory of matter predisposed him toward spontaneous generation.
Mariotte was the exception to a seventeenth-century trend. Most savants cited microscopic research to disestablish corpuscularian theories of generation in favor of preformation. Here observations, not alternative theories, were decisive. With the microscope, savants saw a new world of seeds, spermatozoa, ova, cells, and other objects previously unimagined. Finding seeds where earlier they had gone unsuspected helped dethrone spontaneous generation. But for Mariotte, corpuscularianism was too useful a theory to abandon. It even seemed to be confirmed by the microscope, which showed tiny particles, invisible to the naked eye, that might well be corpuscles. Because of his theoretical bias, Mariotte's microscope became a weapon not against spontaneous generation but against preformation. The microscope was important but not necessarily decisive. A savant assessed experimental evidence within a framework of theological or philosophical assumptions before choosing between theories. In Mariotte's case, theological objections to spontaneous generation were less persuasive than the philosophical merits of corpuscularianism.
Germination, Maturation, and the Role of External Factors
Zoology also inspired the Academy's experiments and theories about germination. Perrault recommended microscopic research to discover just how plants grew from seeds, and Dodart compared the Academy's studies of germination to studies of chicks in the egg. Although academicians never rivaled the detail of Grew's or even Samuel Foley's studies of the anatomy or development of germinating seeds, they claimed interesting analogies between plant and animal parts. Working with a germinating white bean squash and other seeds, Mariotte and Perrault described the filament connecting lobes to leaves as an umbilical cord. Mariotte also thought that seed lobes of beans, pumpkins, cucumbers, and melons resembled the yolk of an egg or the liver in being a source of food for the embryo plants.
Mariotte explained the process of germination mechanically:
It is thus probable that the principal parts of the germination of Plants are contained in their seeds, and that they are predisposed to form fibers and pores suitable for the filtration and the union of certain principles that pass there, as
if through channels or molds; from which the other parts are formed, such as fruits, seeds, and the beginnings of the second germination.
His mechanistic and analogical explanation challenged traditional views of savants like Duclos. Where Mariotte dismissed a vegetative soul in plants on the ground that no one had ever found one, Duclos preferred vegetative souls to analogies between the parts of plants and the organs of animals. Duclos thought plants and soil had similar natures; unlike his colleagues he did not differentiate the various parts of plants by their function, saying rather that all parts of plants (and of soils) contributed equally to germination and maturation.
Any account of germination had to weigh the roles of the plant itself and of external conditions such as sun, soil, air, and water. Perrault and Mariotte stressed the sun. Perrault conjectured that it cooked nutritious minerals in the rainwater. Mariotte demonstrated that sunlight was essential to the growth of plants by comparing seedlings grown under earthenware pots with seedlings grown under glass domes. He proposed two theories, one mechanical, the other chemical, to explain the sun's effect: either sunlight encouraged water to rise in the plant, and then water turned the plant green, or sunlight affected the chemical nutrients in the water.
Soil, air, and water seemed to be the most influential external conditions for germination and maturation. Local conditions could encourage or inhibit growth, and plants either depended passively on what they took from the soil or transformed it to suit their needs. Mariotte and Perrault discussed the nutrients plants required from the soil. Tournefort thought that the earth's juices were the single most important stimulus of germination. They were prepared in the soil by agitation, moisture, heat, or cold, and shaped by the air and the pores of soil through which they passed. Tournefort emphasized mechanical processes — motion, air pressure, the sieving effect of pores — but he also considered the chemical composition of soils.
Academicians studied earths systematically in the 1670s. In 1675 Dodart proposed several experiments. He planned to extract "the salts, and if possible, some other substances from the different kinds of earths." He hoped to distinguish soils chemically, to differentiate among the salts in soils, to discover a connection between salts found in the earth and the salts of plants that grew there, and to identify the different proportions of salts in the same soil under various conditions.
For Dodart and Borelly the purpose of research on earths was to understand what made soil fertile. But they could not persuade Bourdelin
to study soils according to the methods they preferred. From the end of 1675 until the end of 1677, Bourdelin analyzed marls, clays, and other kinds of soils, distilling them as usual in the retort, some with and others without the fixed salt of saltpeter. He calcinated the testes-mortes and then tested the liquids extracted for their reactions to chemical solutions, just as he did with the products of distilled plants. Bourdelin found that earths released liquid with a sulphurous odor, vitriolated salts, volatile salt, and oil; the teste-morte of one earth was said to taste like common salt.
Borelly promoted solvent analysis over distillation and objected to using the fixed salt of saltpeter in distilling earths. He and Dodart preferred lixiviating earths "in order to extract all the salt and all the various substances together in their chaos"; afterwards the earths could be rectified and purified so that their separate parts might be identified. Salts obtained thus could be used in numerous experiments. Any earth remaining after lixiviation could be tested further by solvents.
After Borelly criticized him, however, Bourdelin simply distilled fewer earths, while never adopting Borelly's method. Like the dispute over the relative merits of distillation and solvent analysis of plants, so the disagreement over the proper way of studying soils found Bourdelin and Borelly in opposition. Both disputes were resolved in Bourdelin's favor by his intransigence; Bourdelin would not abandon his preferred technique, despite the pressure exerted by his colleagues.
How Plants Grow and How They Are Nourished
Mariotte and Tournefort explained plant growth mechanically, Homberg chemically. Mariotte thought that as water evaporated, various "earthy, salty, and oily parts" remained to mix and unite with the plant, creating "the hardness and solidity of the branches." He argued that although plants could not select what they took from the soil, they could transform it and actually absorb more oil as they matured. For mechanists like Mariotte and Tournefort, the circulation of the sap caused growth by putting pressure on the extremities of the plant. Rising sap stretched plant cells, causing the cells and hence the plants to grow; when a cell could stretch no longer, the plant withered and died. Tournefort thought this theory also explained the maturation and release of seeds. Observing the organs of plants — the ovaries of hellebore, aconite, and crown imperial, the fruits of spiny poppy, false dittany, toothwort, and the pods of the plant Caspar Bauhin named Lathyrus latifolius — Tournefort found that plants
released their seeds when the fibers in the ovaries dried and contracted, and he argued that when the ovaries opened, air entered and helped the seeds ripen.
Homberg, however, used Bourdelin's analyses to develop chemical explanations of how seeds and plants grew. Homberg concentrated on the maturation of seeds. Noting that unripe seeds yielded a lot of water, less oil, but more fixed salt than ripe ones, Homberg gave this account of their development:
… the organs of the young seeds contain only a watery and very fluid sap, which is not yet well digested; after these salty, earthy, and watery parts have mixed together more perfectly over time, they thicken and create this oil that forms little by little.
To strengthen his claim, Homberg compared young seeds with ripe fruits, nuts, and olives; stored in a dry place for three or four months they too yielded more, and thicker, oil than when they were freshly plucked. Homberg reasoned that the young seed resembled ripe fruit and became oilier as it matured. He also distilled fetid oil with quicklime; this diminished the oil, changed its color, and produced a lot of water. Homberg argued that the phlegm, salt, and earthy matter of young seeds "together create over time the quantity of oil that is found in ripe seeds." He believed he had separated the oily compound into the simple substances out of which nature had formed it. When rectifying oil with quicklime, for example, the quicklime separated fixed salt and earth from the oil; thus, Homberg reasoned, the oil must consist of salt, water, and earth.
All academicians agreed that only chemical analysis could explain the origin and nature of plant nutrients. Boyle, Helmont, and others stressed water and deprecated soil as the source of nourishment, and academicians investigated the role of rainwater and dew in the growth of plants. But savants who did not accept Helmont's view that water was the ultimate source of matter had to identify the origin of the nutriments found in water. Academicians debated whether these came from the atmosphere or from the earth, a question that turned on salts.
For Mariotte and Perrault, nutrients came intermediately from the earth but ultimately from the atmosphere when sulphur, saltpeter, and volatile salts fell in rainwater to the ground. Duclos disagreed with his colleagues. In his view, the fertile "fatty and sulfurous salts" formed in the soil, not in the air. As proof he cited his analysis of the waters that condensed inside and outside a concave vessel placed on the ground: dew had more volatile salt than either the rainwater or the air vapors that collected on the outside. His
arguments convinced Perrault, who conceded that the nutrients originated in the soil from living or decayed plants and animals or from whatever produced mineral salts in well water. Perrault thought such nutrients were cooked by the sun when they rose in vapors.
Applying chemical analysis to a concept of the food chain, Homberg developed a theory about the different origins of the different salts. He noticed that most plants were composed of three salts: fixed lixivial salt, volatile urinous salt, and volatile acid salt. Bourdelin's analyses showed that fixed lixivial salts and volatile urinous salts occurred only in the distillants of plants, of herbivorous animals, or of animals that ate herbivorous animals. The third substance, volatile acid salt, was found in soils, including ones that had no vegetation, and in plants that grew in all kinds of soil. Volatile acid salts thus came from the earth. The other two salts — fixed lixivial and urinous volatile — were therefore manufactured in the plant.
The Academy was divided on whether seeds and plants were active or passive. Did they simply absorb juices already prepared in the earth or did they transform what they took from the earth? Tournefort took the passive view. He believed that juices altered in the earth and became more or less suitable to various plants. Mariotte, Perrault, Duclos, and Homberg took the contemporary view that seeds and plants changed what they took from the soil, but they disagreed about how this happened. Mariotte believed the liquid was simply a vehicle for minerals and that plants used the mineral residue only after the liquid had evaporated. Perrault and Duclos believed the plant transformed both liquid and minerals, and Duclos thought it did so by coagulation. Homberg believed plants imbibed one kind of salt from the earth but created two additional salts themselves.
Seventeenth-century botany was eclectic. Zoological anatomy and physiology lent it a vocabulary, an experimental repertoire, and some explanatory theories about physiological processes. Chemistry and mechanics accounted for generation and reproduction, germination and growth, and nutrition. Some academicians took these explanations as complementary, especially when they answered related but different questions. Thus, they might explain chemically how a plant assimilates nutrients and mechanically how a plant gets larger or dies. Mariotte, for example, cited both mechanistic and chemical theories in his discussions of plant physiology. Homberg and Tournefort, however, did not try to combine the theories — Homberg's primarily chemical, Tournefort's predominantly
mechanistic — that they presented separately but contemporaneously at meetings of the Academy. Nor did the Academy as a whole encourage them to unify their hypotheses into a general account of plants. Academicians never tried to develop a treatise on plant physiology that would rival either the detailed and original works of Grew and Malpighi or the derivative, systematic essays of Régis. The natural philosophy of plants was too tentative a discipline to justify a collaborative project similar to the natural history of plants.
The Academy as an institution never encouraged members to develop a comprehensive theory of plants, to resolve their inconsistent interpretations, or to explore the implications of their piecemeal observations and conclusions. Nevertheless, it aided the development and expression of their theories in three ways. First, the Academy offered a forum for discussing and publishing these early studies of plant physiology. Second, the Academy's natural historical research supported its natural philosophical inquiries. Studies of seeds, the cultivation of plants, and chemical analysis — all ingredients of the natural historical project — fed theories about reproduction, the conditions of germination, and maturation. The official project provided not only data but also protection for the hesitant and unofficial natural philosophical studies. As a result, collaboration on the failed natural history of plants bore fruit in individual studies of plant physiology, with chemical analysis, more than any other tool of research, connecting the two kinds of inquiry.
Third, the Academy's interdisciplinary composition and interests encouraged members to study plants with the new instruments. As the next chapter will show, however, these instruments were theory-laden; they revealed novel details but also interposed a screen between observer and object. When academicians examined a plant with the microscope or the air pump, they saw not just a plant but also a link in an evidential chain that was not primarily botanical. Indeed, the cogency of their research depended on the theoretical certainty of the discipline that stimulated them to use the new apparatus. Thus, microscopic and pneumatic botany had different fates: the Academy used microscopes to support theories about physiology, but it used the air pump to clarify theories about air and the void.