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long, at Camerarius’s own expense. As a private impression, the number of copies of the first
and only printing was very limited, and only six copies of the Epistola survive. Camerarius
probably sent a few copies to selected colleagues at various German universities, but inter-
national distribution was no doubt restricted by the war.^47 The only surviving copy at the
University of Tübingen library was bequeathed in the nineteenth century by the estate of
a Tübingen physician. A second copy, now missing, is believed to have been donated by
Camerarius himself.
As discussed earlier, the British botanist John Ray, in his Historia Plantarum (1686),
had endorsed Grew’s sexual theory, noting that it would explain the presence of two sepa-
rate “sexes” in various species. One of the dioecious trees not cited by Ray, but known to
seventeenth- century gardeners, was the mulberry tree (Morus nigra). Early in his Epistola,
Camerarius stated that plants fell into three categories based on the structure of their flow-
ers: Class I (hermaphrodite), Class II (monoecious), and Class III (dioecious).^48 Camerarius
was mulling over these categories when, in 1689, he noticed that the female mulberry tree
growing in the Hortus Medicus was bearing fruit in the absence of any known male trees in
the vicinity. Camerarius realized that if this were true it would contradict the sexual theory.
Boyle had written that it was the duty of the scientist to investigate the cause of all such
discrepancies between the predictions of a theory and actual observation. To investigate the
cause of the apparent discrepancy of the fruit- bearing female mulberry tree, Camerarius
dissected the individual fruits (drupelets) under a microscope and discovered that they con-
tained only aborted, empty seed cases— that is, seeds without embryos. He likened such
sterile fruits to the “wind eggs” that chickens lay in the absence of a rooster. What at first
appeared to be a discrepancy between the prediction of the sexual theory and observation,
upon further investigation turned out to support the theory after all. Camerarius thus dis-
closed a valuable principle: the presence of fruits alone was not a reliable indicator of fertil-
ity. The true test of fertility was the production of viable seeds.
Armed with this important insight, Camerarius set about designing his first systematic
experiments testing the sexual theory. According to John Ray, one of the species reputed
to consist of two sexes was the common European wildflower Mercurialis annuis (Dog ’s
Mercury). In May of 1691, Camerarius selected— “from an abundant growth of delicate
little plants that were pushing up their shoots in a carefree fashion”— two females and
transplanted them to a remote part of his garden— “far away from any other Mercurialis.”^49
Although the isolated female plants produced abundant fruits, these fruits abruptly with-
ered as soon as they had expanded to about half their normal size, and none produced viable
seeds. Camerarius published these results in the Ephemerides in 1691 as a short communica-
tion titled “Wind- eggs in Mercurialis.”
For the next three years, Camerarius relentlessly pursued his investigations into the sex-
ual theory. In addition to Mercurialis, he experimented with two other dioecious species,
Spinacia (spinach) and Cannabis (hemp), isolating the female plants from the males and
observing whether or not they produced viable seeds. As in the case of Mercurialis, all the
seeds produced by the isolated Spinacia females were sterile. In the case of Cannabis, how-
ever, the results were less clear- cut. Following a similar protocol to the one he had used with
Mercurialis and Spinacia, he transplanted three female Cannabis sativa plants at an early
stage of flowering to his garden, which did not contain any males. At the end of the flower-
ing season, Camerarius was gratified to find that, as expected, the three female Cannabis