Plants and the Air Pump
The air pump was one of the most controversial inventions of the seventeenth century, because the evidence produced with it was cited by both sides in the debate over the existence of a vacuum. To determine whether a void could exist, natural philosophers placed small animals,
their own arms, lighted candles, bells, and magnets into glass receivers, from which they evacuated air. After observing the asphyxiation of mice and birds, the rise of blood to the surface of an arm, the extinction of a flame, and so on, savants acknowledged that an evacuated receiver held little or no air. But they could not agree on whether or not another substance took the place of air, thereby preventing the formation of a vacuum.
It was natural that plants also be tested. Air pumps excited the experimental impulse in their early users, who compared the behavior of as many different objects as possible in their machines. But experiments on plants were not popular, for the results were never spectacular. Juice running out of a pricked fruit was less impressive than nearly suffocating a canary only to revive it by readmitting air to the evacuated chamber. Waiting several days to ascertain whether a plant would die in an evacuated bell jar was tedious by comparison with testing whether a ringing bell sounded in an airless environment. Where plants were simply part of a comparative analysis of the effects of airlessness, they were among the less interesting subjects of study.
Some savants, however, thought the air pump could shed light on botany. Sir Kenelm Digby, for one, had already suggested that air might be important to plants, and in the 1660s and 1670s Hooke tested seeds with an air pump for the Royal Society, after a request from Boyle. In the Academy, Borelly once suggested testing plant distillants and earth in an evacuated receiver, and Huygens and Homberg examined the growth of plants in a vacuum. Academicians narrowed the question: instead of asking whether air was somehow important to plants, they inquired more specifically whether the presence of air was a necessary condition for the germination of seeds.
Huygens was a pioneer in the development of the air pump and was the first academician to use it. He recommended experiments with it in a 1666 memorandum to Colbert and brought his own machine to the Bibliothèque du roi when he moved in that year. In the spring of 1668 he introduced the air pump to his colleagues, starting with simple demonstrations calculated to interest his audience and drawn from his private research of 1662. He pricked the skin of an apple and placed the apple in the receiver: juice ran out of the fruit as air was removed from the jar. He made spirit of wine bubble by evacuating a receiver. Having demonstrated what the machine could do, Huygens designed an experiment that he characterized as "more important," because it would ascertain whether seeds and plants could grow in a vacuum.
With this experiment, Huygens introduced the Academy to the use of the air pump in botanical research. All subsequent tests followed the same pattern: plants, seeds, branches, and soil were placed in the receiver, or bell jar, which was evacuated and removed from the pump; its contents were observed over a period of several days. Huygens evacuated the bell jar until his measuring device indicated there was no more air inside. Then he removed the bell jar from the pump and observed it for about eight days.
One of the risks to botanists was that even when the initial pretext of these experiments was botanical, their ultimate interest often lay in unexpected side-effects that were inconsistent with the known properties of air. Huygens's test set the pattern in this respect as well, for he was surprised to discover a puddle of water on the floor of the bell jar, and this deflected his attention from the plants. Huygens explained the puddle by saying that the soil had exhaled some vapors that condensed on and ran down the walls of the bell jar, but he was puzzled that vapors could rise in a vacuum when feathers fell like lead. As the experiment continued, Huygens observed additional phenomena that seemed to confirm that vapors rose, and he concluded that the vapors were being converted into air. As for the plants and seeds, while they did not grow or flower more, neither did they die as expected in an airless atmosphere.
Huygens later tried to re-examine how lack of air affected plants, using not an evacuated receiver but a double bottle. In May 1672 he sealed the bottle, which was one-quarter full of earth; when he brought it to a meeting in August 1675, more than three years later, academicians observed that a large amount of grass and some moss had grown in it, even though air had not entered the bottle. Both of Huygens's experiments puzzled academicians, for they seemed to show that air was not necessary for the growth of plants.
The Academy dismissed such issues from its collective mind until Homberg revived them two decades later. Like Huygens, Homberg perceived experiments on germination in a vacuum primarily as tests of the nature and functions of air. He performed only two such experiments with plants. The first was brief and improved on Huygens's test of 1668 by including a control, while the second was more elaborate and more carefully considered for its implications about germination.
In the first, Homberg sowed seeds in two boxes, placing one under a glass dome and the other in a receiver, which he evacuated and then placed beside the glass dome in a window with a southern exposure. On the first evening, he noticed that the earth in the vacuum had split in several places and that drops of water covered the sides of the receiver; on the base of the
receiver was more water, which seemed to be the liquid used to moisten the soil at the start of the experiment. In the control box the soil was not so cracked and the glass dome was covered with less water; some water had fallen onto the stone base, but because the base had cracked and water had escaped, Homberg could not measure it. He attributed the excess liquid in both containers to evaporation and condensation, and he thought there had been less evaporation under the glass dome than in the evacuated receiver. He did not try to explain why the earth cracked, nor did he continue the experiment long enough to see whether seeds germinated in one container or the other. Homberg explained evaporation and condensation in a vacuum, which had troubled Huygens in 1668, as the effect of "ethereal matter," which forced vapor against the sides of the receiver, where it became water and ran to the bottom.
Homberg's second experiment lasted about six weeks, from 1 May through 12 June 1693, and like the first had been designed with a control. He sowed the seeds of five plants — purslane, cress, lettuce, chervil, and parsley — in two small boxes. One box he placed in a receiver, the other he left in the open air. He evacuated the receiver every morning and watered each box every third day.
Homberg recorded the weather and the daily appearance of the shoots, noting withering and death, the sizes of the shoots, and any continued growth. In the evacuated receiver, chervil and parsley never germinated, and purslane, cress, and lettuce began to grow much later than did their counterparts in the open air; purslane died the day after its shoot appeared in both boxes; cress in the receiver died five days after it first appeared; lettuce grew very tall in the receiver, but its leaves were small and, after an initial rapid growth, it stopped growing entirely.
Homberg decided to test whether seeds that had not germinated in the vacuum would do so when exposed to air. On 25 May he admitted air to the receiver and then closed the stopcock to prohibit air from passing into or out of the vessel. Chervil, purslane, cress, and parsley germinated, but shoots that had developed in the vacuum did not change. On 7 June Homberg removed the box from the receiver, but by 12 June all had died.
Because some seeds had germinated in the evacuated receiver, Homberg decided that neither the weight nor the elasticity of air could be the principal cause of germination. But fewer seeds germinated in the receiver than in the open air, and none of the seedlings in the evacuated bell jar grew normally or survived more than a few days. Homberg therefore concluded that air was "at least an accidental cause" of germination.
Trying to explain how a vacuum could injure seeds, he argued that the
seeds contained air which "dilates because of its spring rather more easily in the void, where nothing impedes it, than in the air where it is pressed on all sides." Seeds exposed to a vacuum were, therefore, damaged by the expansion of the air they contained, and "since the organs that serve to carry and to distribute nourishment are ruptured, germination cannot take place." A damaged seedling could grow but would not develop into a healthy plant.
Homberg noted three additional phenomena in the evacuated receiver: the soil swelled, a fungus grew, and drops of water formed at the tops of the seedlings. He believed the soil changed because of moisture and air. Moisture penetrated and broke down small masses of earth; it filled the small spaces between clumps of soil, making the earth seem oily, soft, and slimy. Assuming that air was "mixed with the new water" used to moisten the soil, and that air expands in a vacuum, Homberg argued that its "effort" to escape accounted for the "swelling and bubbling" he had noticed in the soil.
The second striking phenomenon was the appearance of what was probably the fungus pythium on the surface of the soil. Homberg first noticed this on the eighth day of the experiment, when he thought that "the earth enclosed in the vacuum had changed color" and seemed from certain angles to be "grayish and shining." Examining the surface of the soil with a microscope, he observed "a lot of small transparent and grayish filaments that looked like a spider's web":
Some of these filaments were straight, others rested on the soil and were attached to small protrusions of earth; since they were crisscrossed, they formed a sort of fabric so strong and tight that the water with which we wetted the soil was held there and formed drops as big as beans, without moistening the soil.
He tasted them, expecting a flavor of saltpeter because they resembled the mildew on the walls of cellars. But he noticed no taste at all and did not test the substance with any of the chemical reagents commonly used in the Academy's laboratory.
Finally, Homberg observed guttation induced by the vacuum: at the top of each seedling in the void there was always a drop of water; when it ran down the stem, a new drop formed immediately. Homberg claimed that "this water did not come out of the pores of these sprouts," but attributed it instead to vapors formed in the void by the action of ethereal matter on damp earth. He believed these droplets were the equivalent of the drops of water he had observed on the glass walls of the receiver in his previous experiment. Homberg was perplexed and wondered, just as Huygens had
twenty-five years earlier, how drops of water could rise in a vacuum, when light objects fell.
Homberg believed that an airy matter was present in substances such as water and seeds. This led him to expect air to reappear in an evacuated, well-sealed bell jar, if it held any substances containing airy matter. But he did not classify plants as air-producers, and neither he nor Huygens thought that plants could produce the drops of water observed on leaves. Analogies with dew or with the condensation of water on a glass dome were so persuasive that Huygens and Homberg did not suspect that plants might exude water, even though contemporaries like Mariotte had shown that leaves could absorb moisture, and John Wallis had demonstrated transpiration experimentally. But Homberg never drew on such notions to explain the phenomena he observed with the air pump, seeing his studies as useful to botany but not vice versa.
Some of the peculiarities of pneumatic research on plants were due to its having been borrowed: the air pump was initially developed in other countries, and those who performed experiments with it were trained in and inspired by other disciplines. The air pump never caught on as an experimental tool at the Academy, despite Huygens's enthusiasm, and such experiments as Huygens performed in Paris were often modeled on those of his English colleagues. Thus, Boyle influenced Huygens as much in his choice of experiments as in his development of the pump itself. Huygens was also familiar with Digby's discourses on vegetation, which, with their arguments that plants depend on a vital nutritious substance in the air, may well have stimulated Huygens's tests of 1668 and 1672.
Claude Perrault was unusual in connecting Huygens's studies of the vacuum to his own explanations of how sap rises. He also used pneumatic observations to refute Duclos's claim that an "expulsive faculty" in branches and trunks propelled sap into the roots. Perrault pointed out that "plants that are filled with a lot of sap" simply "let it flow," but when the receiver is evacuated, so that "the compression of the air is reduced, the sap dilates and becomes less condensed than it had been." He believed that this experiment, which was not one of those demonstrated by Huygens in 1668, showed that the weight of the air — and not any expulsive faculty — pushed juices from or through plants, thus causing sap to move.
Curiously, while animal physiology often influenced plant physiology, it had relatively little effect on how academicians interpreted vegetable phenomena in the vacuum, even though academicians also tested animals with air pumps. Perhaps Homberg derived his theory that a vacuum destroyed the internal organs of seeds from observing the damaged muscles and
organs of animals that had died in evacuated bell jars. Otherwise academicians explained the physiology of seeds with references to the passive, absorptive qualities of dead sponges. Thus, seeds were said to absorb water like sponges and to germinate by swelling, until they burst their shells and developed into new shapes; savants wanted to know whether moist seeds would inflate in a vacuum as sponges did. This question, not the example of small animals dying in evacuated receivers, motivated Homberg to study seeds.
Experiments on plants in an evacuated receiver held little interest for academicians, and Huygens's and Homberg's tests mostly went unnoticed. Since air pumps were used primarily to test the weight and elasticity of air, it was always these properties that academicians cited to explain what happened to seeds and plants. In any case, vegetable behavior seemed less interesting than the other phenomena associated with botanical pneumatics. Putting plants in the air pump represents not a successful interdisciplinary exchange but isolated studies whose anomalies sidetracked academicians from answering questions about plants.