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.