In 1787 Sprengel had remarked that the bases of the petals of Geranium silvaticum are beset with long hairs. Persuaded that no natural structure can be devoid of meaning, Sprengel asked what purpose these hairs might serve. A honey-gland in their midst suggested that they might protect the honey by keeping off the rain, which easily enters this shallow flower. Other honey-secreting flowers were found to possess mechanisms adapted to the same end. His first question suggested a second: Why should flowers secrete honey? Malpighi had described the honey-glands of crown-imperial (1672), and had seen that the honey must be secreted by the petals, and not deposited from the atmosphere, according to the notion then current. Kölreuter (1761) had showed that insects may effect the pollination of flowers. Linnæus (1762) had given the name of _nectary_ to the honey-gland. He thought that the honey served to moisten the ovary, though he knew of staminate flowers furnished with nectaries. He also threw out the alternative conjecture that the honey is food for insects, which disperse the pollen by their wings. Sprengel improved upon all his predecessors, and made it clear that transference of pollen is the main purpose of the honey in flowers. He was put on the right track by the study of a forget-me-not flower. Here he found the honey protected from rain by the narrowness of the corolla-tube, whose entrance was almost closed by internal protuberances. The protuberances were distinguished by their yellow colour from the sky-blue corolla, and this conspicuous colouration led Sprengel to infer that insects might be thereby induced to seek for the store of honey within. He tested his conjecture by examining other honey-bearing flowers, and soon collected many instances of spots, lines, folds, and ridges, which might not only make insects aware of hidden stores of honey, but guide them to the exact place. Contrivances of the most diverse kinds, but all tending to invite the visits of insects and utilise them for the benefit of the plant, rewarded Sprengel's continued inquiries. He found that night-flowering plants, which could derive no advantage from coloured patterns, often have large white corollas, easily discerned in a faint light, and that these flowers give out an odour attractive to nocturnal insects. He found that the pollen-masses of an orchis are actually removed by large insects, though here no honey could be detected in the flower. Sprengel's fertility in probable conjecture is shown by his explanation of this puzzling case; he suggested that the orchis is a sham honey-bearer (Scheinsaftblume), which attracts insects by assuming the conspicuous size and coloration found in most honied flowers. Darwin suspected, and Herman Müller proved, that though the spur of the orchis-flower is empty, it yields when pierced a fluid attractive to bees and other insects. Sprengel discovered too how insects get imprisoned in the corolla of an Aristolochia, whose