The Circulation of the Sap
In the summer of 1668 Claude Perrault and Edme Mariotte defended the hypothesis that sap circulates in plants as blood does in animals. They identified five ways in which plants resemble animals: plants have two sorts of vessels, corresponding to veins and arteries; there are two sorts of sap,
and these are the equivalents of venous and arterial blood; sap is nutritious for the plant, just as blood nourishes the animal; the root manufactures sap just as the liver produces blood; and sap circulates frequently and quickly through the plant, replenishing itself with water from the leaves and being recooked in the root, just as blood circulates and is refreshed during its circuit through the body. Like Harvey, they had a hierarchical concept of the organism and emphasized "the control and stewardship" of one part of the body over the others, and like Riolan they incorporated traditional physiology — for example, the idea that the blood was itself a nutriment — into their theory.
The debate that the Academy sponsored marks the first systematic effort to apply circulatory theory to plants. Similar ideas were current outside the Academy in the 166Os: Johann Daniel Major had suggested the analogy, Timothy Clark had written about a circulation of the liquid in sensitive plants and had searched with a microscope for structural equivalents of valves, and Nicaise Le Febvre had compared the functions of sap and blood. In the 1670s and 1680s Nehemiah Grew and Marcello Malpighi impressed the botanical world with their systematic studies of plant anatomy and physiology. But it was Mariotte and Perrault who first pushed the analogy between blood and sap to its limits.
The debate of 1668 represents one of the Academy's most productive efforts at refereeing research. It began with the conflicting claims of Mariotte and Perrault for priority, and ended amicably by recognizing that their independent judgments had coincided. The exchange of evidence and opinions in 1668 influenced not only the two protagonists but also Duclos. Originally drawn into the debate to review the evidence, Duclos opposed the theory during the summer of 1668 but supported it in 1680. Finally, Perrault, Mariotte, and Duclos published their views about the theory.
The essential scientific traits of Perrault and Mariotte are exemplified by their work on the circulation of sap. Perrault was theoretical. He conjectured, offered plausible arguments, and identified the need for experimental support. In citing experiments, however, he rarely used the first person, and all the experiments cited in his 1680 book were actually performed by Mariotte, Huygens, Duclos, and Bourdelin at the Academy, not by Perrault himself. In contrast, Mariotte was emphatically experimental, and even his initial inspiration that sap circulates was prompted by an experiment.
Proving the circulatory hypothesis required academicians to address the three issues that Hesse has identified as crucial. Academicians had to establish the pretheoretic similarities between plants and animals that would make the analogy materially plausible; here they relied on lazy
analogies and on a functional resemblance. Next they had to push the analogy to its limits, testing for traits in plants that would correspond to those in animals; here academicians identified crucial dissimilarities that ruled out the relations of causality they originally anticipated. Finally, they were faced with the problem of crucial dissimilarities: plants were not comparable to animals in several significant respects, and in particular they lacked any internal motive force that could pump the sap as the heart pumped the blood. Because this latter dissimilarity could not be resolved, the analogy as a whole failed, and Mariotte and Perrault were left with a most difficult and important botanical question: how does sap rise in the first place? For an answer they turned ultimately to disciplines other than botany or zoology.