Pushing the Analogy to Its Limits
Academicians sought to prove that sap circulated, first, by showing that their general theory of growth necessitated circulation and, second, by developing experimental evidence that demonstrated the descent of sap. Scientists at the end of the seventeenth century explained growth mechanically as resulting from the pressure of blood or sap against vessels in the extremities of animals and plants. That is why, explained Perrault, the French use the word "pousser" to speak of growth. Since all parts of plants and animals grow larger, all must be subject to this pressure. From a mechanistic theory of growth, it followed that sap must push downward as well as upward, because roots and the tops of plants grew, downwards and upwards respectively, in proportion to one another.[11]
Experimental evidence for circulation was sought in several ways. Everyone assumed that sap rose from the root to the top of the plant. The
novelty was in showing that it descended again to the root. In 1668 Mariotte had already performed most of the experiments that proved descent of sap from the tops of plants toward the roots. He cut stems and observed that sap flowed in both directions. When he planted seeds upside-down, or placed them with the leaf-end in water and the root-end exposed above the water, the seeds grew. When he cut the filaments on roots, they bled. Uprooted chives, placed in water with only the shoots immersed, survived and grew for a fortnight. Most of the early experiments immersed seeds, plants, or parts of plants upside-down in water. Usually the plant survived and grew for several days. From these experiments, Mariotte and Perrault concluded that sap did move toward the root, that they had found two kinds of sap (yellowish and whitish in color, or thick or thin in consistency) in most plants, and that leaves could absorb water.[12] Proving experimentally that sap descends to the root hence led to a promising observation, that there were two kinds of sap, and to an unforeseen consequence, that leaves had an important role in the nutrition of plants.
If leaves could absorb food, they endangered the analogy with animals, because an animal ingested food through a single mouth. Mariotte tested leaves in water and observed the sap in their branches. He concluded that leaves not only absorbed water but also carried more water than did the root to the vessels containing yellow sap. He also noticed that drops of water formed on the leaves of plants under a glass bell. Assuming that these were dewdrops, seeing that the plants remained healthy, and believing that no other water was available to the plants, Mariotte concluded that the leaves absorbed enough water to sustain the plant.[13] Mariotte, the experimentalist, showed that plants, unlike animals, could take their food through two orifices. Perrault, the theoretician, tried to reconcile the new evidence to the analogy with animals. Here Harvey's own theory offered an explanatory model, for he had noted that medicaments applied externally to one part of the body entered the bloodstream and traveled to the entire body. Perrault, therefore, suggested that leaves absorbed liquids the way the skin of a dog absorbs the heat of a fire, or the skin of a butcher the fat of the meats he is handling. Mariotte affirmed the consequences of the experiments, while Perrault drew on folk wisdom and lazy analogy to reconcile the behavior of plants and animals.[14]
If the descent of sap proved the circulation of sap, the existence of two kinds of sap in a plant provided another likeness between blood and sap. Harvey had shown that there were two kinds of blood: one, going out to the body from the heart, that was "hotter, perfect, vaporous, spirituous and, so to speak, nutritious," and one whose nourishment and heat were exhausted
and which returned to the heart "cooled, coagulated, and … figuratively worn out."[15] For sap to be comparable to blood, therefore, rising sap should nourish and falling sap be weak and useless. Perrault asked whether there was a thick sap equivalent to arterial blood and a watery sap equivalent to venous blood, and Mariotte found plants that contained two different saps, yellow or white, thick or thin.[16] The difficulty was to show that one was more nutritious than the other. Citing trees tapped for their sap in the spring, Perrault argued that falling sap cannot be nutritious, or trees would die from the loss of so much of it. Academicians analyzed the saps chemically but could not agree on the results.[17] Although academicians compared the two sorts of sap with venous and arterial blood, they asserted resemblances more effectively than they proved them.
The analogy also constrained academicians to find separate vessels for carrying sap up and down the plant. In 1668 Mariotte could not determine whether there were two kinds of vessels. Perhaps mindful of Harvey's insistence to Riolan that one vessel could not accommodate simultaneously the flow of two liquids in opposite directions, Mariotte inferred from the two kinds of sap that there must be two sorts of vessels. Perrault, on the contrary, defended Riolan's stance, and tried to show by analogy that a vessel might permit both upward and downward flow, if the two liquids were sufficiently different.[18] By 1679, Mariotte had studied the anatomy of plants more systematically and with a magnifying glass. He described the appearance of stems and the arrangement of fibers, channels, and spongy matter in them.[19] But he still could not positively identify different vessels for different saps. In 1680, Perrault argued from Mariotte's 1668 experiments and from general information about trees that some plants had separate vessels and were therefore like "perfect" or higher animals. But he cautioned that the absence of separate vessels could not disprove circulation. Some plants were like insects, which do not have separate vessels. Perrault argued that the parts of plants must be able to differentiate cooked from raw sap, perhaps because of the disposition of their pores; in other plants the double bark or the bark and the pith might serve as separate conduits for different saps.[20]
Equally serious was the lack of a mechanism for controlling the direction of flow. Mariotte and Perrault were uncertain whether they could identify valves or equivalents that prevented the rising sap from falling prematurely.[21] Among academicians, La Hire agreed with Robert Hooke's sentiment that valves seemed "very necessary for conveying the juice of trees up to the height of sometimes 200, 300, and more feet; which he saw not how it was possible to be performed without valves as well as motion." La
Hire claimed to have found valves in canes and reeds.[22] Allowing expectation to prejudice observation, he argued from similarity of function to resemblance of structure and insisted that he had found valves where none existed.
Skepticism about the existence of valves or separate conduits required academicians to consider whether sap flowed in different directions in the same vessel. Grew believed this was so.[23] Perrault tried to explain that it was possible by using an analogy with water vapor, which can rise through oiled paper but cannot penetrate it again once the vapor has condensed. He offered a second analogy, with two sponges, one soaked in water and the other in oil, each of which absorbed liquid of its own type when placed in a mixture of oil and water. Both arguments presumed that the parts of plants were designed to accept one kind of sap and reject the other.
Each structural comparison endangered the analogy. The closest resemblance was between blood and sap, liquids that were of two kinds and that made a complete circuit of the body in question. Organs and blood vessels, however, challenged the ingenuity of savants. Academicians tried to compare the root to the heart, the soil in which a plant stood to the intestines, but even with a microscope they could not identify two distinct sets of vessels. The most serious difficulty, however, was that since plants lacked a heart, they had no pump to drive the sap.