Early Botanical Microscopy at the Academy
Microscopes had already been used to study plants by the time the Academy was founded, but only a few academicians were interested in botanical microscopy. As a result, microscopic observation affected the Academy's natural history of plants only peripherally. Descriptions were meant to distinguish plants from one another, but not to include their anatomies or to clarify vegetable physiology. The engravers found microscopes useful, because the Academy wanted the smallest external details of each part of the plant to appear. Otherwise, academicians only occasionally used microscopes to study plants.
Perrault recommended microscopy for research on germination. In 1667 he made these plans:
Experiments on how plants grow will be made by considering the roots and seeds and examining them diligently with the microscope, both before placing them in the ground, and by taking them out of the ground at various times in order to consider the different changes which occur with respect to size, or to shape of their pores, to their saps, weight, color, odor, taste, and so on. Then one will consider what happens to their sprouts when they begin to grow, especially to those which are enclosed within large seeds, such as acorns, where one notices the root, the trunk, and the branches of the entire tree, which seems already formed and distinct before emerging between the two sections into which the acorn normally separates.
Perrault hoped that microscopes would settle disputes over preformation and emboîtement and would ease comparisons between the growth of seeds and the development of the chick in an egg.
Although Perrault's suggestions were not officially adopted, some members studied plants with microscopes and magnifying glasses. Examining hemp thread (fil de chanvre ), La Hire saw filaments which he compared to capillary tubes and claimed that sap passed up them to nourish the plant. Dodart examined young shoots of wheat to find the tiny grains; he was testing preformationist theory. In the 1680s some academicians considered studying plants and their distillants with the microscope. Not institutional policy but individual interests incorporated microscopes into the arsenal of discovery and argument in botany.
Unlike most of his colleagues during the 1660s and 1670s, Mariotte routinely used hand lenses and compound microscopes. They were invaluable in his studies of vegetable physiology. His arguments against preformation and for the circulation of the sap depended on meticulous observations.
He examined leaves, bulbs, seeds, and cut stems; his descriptions of bark, skin, fibers, vessels, spongy matter, and saps are the verbal equivalent of Grew's illustrations. In shrubs he identified "canals or pores" in the marrow of the cutting; a microscope revealed these pores to be "several small oval cells [cellules ]" resembling honeycombs.
Without a lens Tournefort could not have found the "seeds" of ferns or examined the growth, desiccation, and contraction of seed cases, observations that he cited against spontaneous generation. When he examined plant vessels Tournefort found resemblances to bones and muscles, and he claimed that the very vessels that carried sap eventually dried and became fibrous, stiff, woody, and capable of supporting the plant. Microscopy provided evidence for Tournefort's theories about the motion of sap and the growth and reproduction of plants.
Huygens contributed incidentally to botanical microscopy when he brought a spherical microscope to the Academy in the summer of 1678. Although his primary interest was animalcules, his colleagues examined a section of a fir tree, some pollen from a lily, and the marrow of a fig tree. But Huygens's apparatus held little interest for the botanists, and besides him only the astronomers Picard and La Hire used it to study plants, or rather their pollen. Picard compared the shapes of pollens from different plants, without conjecturing about the nature or function of that "dust" or "flour." He merely commented on the shapes, colors, and structures of pollen. Huygens and his brother Constantyn went beyond Picard's simple comparisons of appearance. They considered internal structure and the connection between pollen and the activities of bees. Huygens observed that the "dust" of crocus flowers and the dust on bees' feet looked the same, and he argued that pollen adhered to the feet of bees, who made wax from it. When his brother expressed surprise that pollen stored for two months still contained a liquid, Huygens replied, "What you say of the liquid in yellow powder confirms again what I said, that it served to make wax." Although he hypothesized about accidental uses of pollen, Huygens never applied his observations of pollen to any theories about plant physiology. No one in the Academy took more than passing notice of his or Picard's findings.
Only one of the academicians who normally studied botany, La Hire, used Huygens's spherical microscope to examine plants. This surprising lack of interest among the Academy's botanists was due to the difficulty of using the spherical microscope and to its limited range of applications. Huygens noted that many of his colleagues saw animalcules only with great effort, while others never saw them at all. Proper use of the lens and
careful mounting of the object were crucial for success. Huygens, Roemer, and Hartsoeker developed a way of mounting several objects on a rotating wheel, an invention to which no Frenchman contributed, Huygens was quick to point out. But even when every precaution was taken, the instrument was of limited use for studying plants, because it was not yet possible to cut plant sections so thin as to be transparent or to fit between slides. Few parts of plants, therefore, lent themselves to study with Huygens's apparatus.
Despite their inherent defects and the difficulty of using them, spherical microscopes impressed Parisian scientific savants. Huygens was as gratified by the reception of his microscopes as by the inability of Parisians to make the lenses. He reported that "the curious" were "astonished by the great effect it makes"; Locke had heard of "the extraordinary goodness" of Huygens's microscope. Protestations of interest no doubt outnumbered clear sightings of animalcules, because the skepticism that had earlier greeted the telescope and air pump was now less tenable; by 1678 few amateurs would have chanced embarrassment by challenging Huygens on the basis of negative evidence. Instead, Huygens's enemies contented themselves with ghost-written articles disproving Huygens's claims of priority.
Microscopy was always ancillary to other ways of studying plants at the Academy. Academicians who habitually studied the natural philosophy of plants used a convex hand lens and sometimes a compound microscope to observe the details of plant anatomy and to support various hypotheses about plant physiology; they applied the readily available microscopes to subjects that had long interested them. Huygens and Picard, on the other hand, were more engrossed by the novelty of the instrument than by its botanical applications; they never pursued plant microscopy exhaustively because they were more interested in other subjects. Finally, academicians who studied the anatomy and physiology of plants preferred theoretical issues to exhaustive description. For them microscopy was one technique among many, and they used it along with naked-eye observation, chemical analysis, and analogical reasoning in order to support their hypotheses about germination, nutrition, and growth.