Explaining the Rise of Sap
The concept of capillary action was regarded as novel during the 1660s. Robert Hooke believed the phenomenon had first been observed by French scientists:
An Eminent mathematician told me one day, that some inquisitive Frenchmen (whose Names I know not) had observed, that in case one end of a slender and perforated Pipe of Glass, … be dipt in water, … the liquor will ascend to some height in the Pipe … tho held perpendicular to the plain of water. And to satisfie me, that he mis-related not the Experiment he soon after brought two or three small Pipes of Glass, which gave me the opportunity of trying it.
Robert Boyle experimented with capillary tubes in 1660, discussed capillary action in 1671 and 1676, and tried to measure the force of capillary imbibition in a seed. Huygens knew of his work, saw demonstrations of capillary action at Rohault's house in December 1660, and attended a meeting at Gresham College in April 1661 where the phenomenon was discussed; he also owned a copy of "Boyle's article on the rise of water in small tubes and on other phenomena which we call capillary."
Capillary action seemed to seventeenth-century scientists to explain several natural phenomena. Hooke listed these effects of capillary action:
… the Rising of Liquors in a Filtre, the rising of Spirit of Wine, Oyl, Melted Tallow, & c. in the Weake of a Lamp (tho made of small wire, threeds of Asbestus, Strings of Glass, or the like) the Rising of Liquors in a Spunge, piece of Bread, perhaps also the ascending of Sap in Trees and Plants, through their small, and some of them imperceptible Pores, (of which perhaps I may say more on another occasion) at least the passing of it out of the earth into their roots.
He believed that capillary action was the result of unequal air pressure, with the air pressing heavier on the reservoir of water surrounding the thin glass tube than on that within the tube itself. Hooke reiterated the view in 1665 and stated that air pressure caused sap to rise in plants, basing his explanation on the analogy with capillary action. This confusion between capillary action and the effects of air pressure persisted throughout the century, despite G. A. Borelli's argument in 1670 that capillary action was not due to air pressure.
Because capillary tubes resembled the stems and vessels of plants, they were an obvious model. In the late 1670s Mariotte and La Hire used the analogy between glass tubes and the vessels of plants to explain how sap rose. Mariotte limited the effect of capillary action, however, to "the first entry of the water into the roots." This, he said "occurs by a law of nature similar to the movement of union of which I have already spoken; since whenever very narrow tubes touch water, it enters, and it even rises despite its natural tendency to descend." For capillary action to work in plants, three conditions had to exist: the water in the soil had to touch the plant, it had to have access to that part of the plant in which it could rise, and something had to cause the liquid to enter and rise in the plant. Mere contiguity seemed to be insufficient because if a glass tube were dirty or rubbed with tallow, water would not rise in it. Furthermore, the pores of plants had to be properly "disposed to allow the subtle parts of other bodies to enter." Finally, these subtle parts had to "be pushed by some principle of motion." This principle Mariotte referred to as "a movement of union" and as an effect "that is popularly called attraction"; he did not cite the pressure of air as Hooke had done.
But there were problems inherent in comparing the rise of sap in plants to the rise of liquids in capillary tubes: the cause of capillary action was disputed and liquids did not rise so high in capillary tubes as they did in plants. Thus, whether or not capillary action was cited, savants turned to air pressure in order to explain the rise of sap in taller plants.
Huygens initially favored the view that sap rose because of air pressure, and in 1668 Perrault propounded an explanation that depended on multiple causes, including air pressure. The theory that air pressure causes sap to rise was known to be defective, however, by 1679. Pierre Perrault and Huygens debated whether sap rose because of air pressure, as Huygens maintained, or because of attraction and nature's abhorrence of a vacuum, Perrault's view. Perrault cited the following as a decisive argument against Huygens's theory:
How can we understand the sap that rises in trees? Can one say that air pressure causes it to rise between the bark and the wood, as in a pump? For that it would be necessary for the foot of the tree to rest in a reservoir of sap. Even if that were the case, this sap could rise only thirty-two feet, but there are trees that are more than one hundred and twenty feet tall.
Pierre Perrault concluded that sap rose as a result of "attraction due to abhorrence of the void," a phrase that he and Mariotte used apologetically
and as a manner of speaking not intended to impute emotions to inanimate or mechanical objects.
Since neither capillarity nor air pressure seemed sufficient to raise sap higher than thirty feet, some savants adduced chemical reactions inside the plant. Claude Perrault had argued all along that sap rose for many reasons, including the existence of appropriate passages in the plant, air pressure, external propulsion from the wind, and coction of sap. Influenced by his brother's objections, however, he modified this view when he published his Circulation in 1680. There he cited both air pressure and attraction due to fear of the void, but he also elaborated in Cartesian fashion his earlier chemical explanation: when sap was prepared, fermentation and effervescence reduced its concentration so that more sap rushed in to fill the potential void. To show that sap would flow into an area of diminished pressure within a plant, Perrault cited the effects of an air pump: "plants that are full of sap let it run when the air is being evacuated, and when the pressure of the air is diminished, the sap dilates and becomes less condensed than it was." Mariotte developed a slightly different theory in 1681: sunlight evaporated sap in the upper regions of the plant, creating an area of lower pressure into which sap rose. Such formulations anticipated Hales's conjecture that the loss of liquid due to transpiration pulled sap upward, continuing a process begun by capillary action.
Tournefort presented a different eclectic theory in 1691. He believed he had found two different systems in plants, one in which sap rose by absorption and another in which it rose by capillary action. The vessels in most plants, he said, were soft, spongy, and composed of many small, empty bladders or pouches that were connected so that sap passed through them. He compared them to felt strips or cotton that filtered and conducted liquids. Not all vessels were spongy, however; the stems of water-plants were like cylinders pierced longitudinally with holes. These tubes carried sap, and Tournefort thought they resembled capillary tubes: "this structure seems to favor the sentiment of some natural philosophers who believe that the sap ascends in plants for the same reason that water rises in very narrow glass tubes." Tournefort followed G. A. Borelli, who claimed that the dilation and condensation of air enclosed in plants caused sap to rise and that the spongy matter in plants facilitated that rise. La Hire, however, challenged the Borelli-Tournefort hypothesis after observing that water did not rise significantly in absorbent materials such as sponges inserted in glass tubes or paper strips. The best result was a height of 225 lignes over a period of more than eighty-four days. La Hire concluded that neither absorbent matter nor capillary action could account for the rise of sap.
Instead, he maintained that only hollow tubes in plants could transport sap, and he claimed to have found hinged, woody valves in them that enabled sap to rise.
The problem of how sap rose elicited varied responses from academicians. Huygens proposed air pressure, while Perrault listed several interrelated causes, such as air pressure, wind, fermentation, and the different weights of raw and cooked sap. Once air pressure was ruled out as a sufficient cause, Mariotte and La Hire focused on capillary action, while Tournefort combined capillary action with a Borellian theory about spongy matter in plants. Later, La Hire adopted the view that the vessels of plants were valved. In each case, academicians used analogies. Since the biological model of valves was inapplicable, they drew mostly on chemical or physical explanations. But the limitations of all models forced most academicians to develop theories based on multiple causation.