For me the most interesting entries in Goethe's Dornburg diary are his observations on the begonia plant or, as he refers to it, Bignonia radicans. These begin on August 25: "Finished above-mentioned poem," that is, Byron's unfinished play Heaven and Earth, which he had been reading the day before. Then: "Reflected on the Bignonia radicans and its gland-like growths on the back of each node" (Reflectirte über die Bignonia radicans und über drüsenartige Auswüchse an der Rückseite jedes Knotens). The next day he seems to have begun thinking of writing on the purpose of these "gland-like growths," for, on September 2, he records: "I revisited again the remarkable suctorial organ [Saugorgan] of the Bignonia radicans." The next day, according to his diary, he finished writing an essay on the subject. It never appeared in his scientific writings, only in the Weimar edition (II, 6, 340-45).
In this essay he begins by mentioning the impression made on him by the begonias in the botanical gardens in Padua (as seen above) in September 1786: a wide, high wall was completely covered with an unspeakable abundance of the deep yellow, chalice-like crown-flowers (Kronen-Blüten). He then goes on to describe it as a creeping plant that seems to have the tendency to continue infinitely, lacking, however, the organs with which it would be able to nestle, cling, or hold on. Thus, he writes, we force the plant to climb with the help of lattices, but, as he has observed with a certain distaste, the stem then retreats behind the lattices. Because of this the blossoms are unable to hang down, thus denying the viewer the beautiful effect of the hanging blossoms. To figure out why this is the case he turns to the "gland-like growths," which in the essay he describes as resembling grapes. (These are evident in the illustrations here.) The essay elaborates on their function.
One notices Goethe's close observation of environment: on the stems of begonias climbing the sides of a building there is no trace of this organ; it appears only on plants growing in moist places without a lot of sunlight. He comes to the conclusion that the organ is brought forth by moisture, which it then transmits to the plant (das Organ wird durch die Feuchtigkeit hervorgerufen, die es der Pflanze mitteilen soll).
Thus, the begonia is a "creeping" (rankende), not an upward climbing (aufsteigende) plant. Rather it is a hanging (niederhängende) one, and we are mistaken in our handling of it when we force it to climb, since it is thereby deprived of its most essential nutrient. He differentiates the cultivation of begonias from the grape wine: since the latter can grab hold anywhere with its tentacles (Gabeln), we should let it creep and prevail, as it seems good and useful to do so (lasse man ranken und walten, wie es gut und nützlich zu sein scheint). But with such a strikingly beautiful plant as the Bignonia radicans, plant it above and let it hang down (pflanze man oben und lasse sie sich herabsenken). I wonder what Goethe would have thought of the Japanese practice of forcing plants into unusual shapes, as seen in this lovely painting by the Japanese artist Koson Ahara (1877-1945). It does seem that the upper branches of the begonia in this painting seem deprived of the organ Goethe describes.
This small essay indicates anew (as if we didn't know already!) Goethe's intense attention to the factness of the natural world. The essay is written in the precise, descriptive style that is characteristic of his late poetry. I suspect that this style -- somewhat like the genesis of the moisture glands on the back of the begonia flower -- was a product of his scientific writing that was then transferred to his literary works.
With thanks to "Never Yet Melted" for the Koson image.