Sprengel and the Fertilisation of Flowers.
Conrad Sprengel, an unsuccessful schoolmaster who lived in a Berlin attic and got his bread by teaching languages or whatever else his pupils wished to learn, wrote a book which marks an epoch in the study of adaptations. This was his Secret of Nature Discovered, which appeared in 1793. Half a century passed before its merit was recognised by any influential naturalist; even then the recognition was private, and never reached the author, who had died long before. There was no striking of medals, no jubilee-celebration, nothing more than this, that Robert Brown recommended the book to Charles Darwin, who found in it, as he says, "an immense body of truth."
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 reflexed hairs allow small flies to creep in, but effectually prevent their escape until they have fertilised the pistils, when the hairs relax. These are only specimens of a multitude of adaptations which fill the book.
Sprengel insists upon the study of flowers under natural conditions; he could never have made out by the examination of plucked flowers how Nigella is fertilised. Flies with attached pollen-masses, which he found in spiders' nests, gave him the hint as to the way in which the fertilisation of orchids is effected. Definite questions must be put if observation is to be profitable. What is the use of honey to the plant—of this coloured spot—of these hairs? He notes the peculiarities of wind-fertilised and insect-fertilised flowers, the relative abundance of the pollen, the form of the stigma, the presence or absence of honey, the size, colour, and scent of the corolla. Here is a pretty illustration from his pages. Pluck a branch of hazel, aspen, or alder, with unexpanded catkins, and also one from the male sallow; place them in water, and keep them in a sunny window until the anthers are ripe. A vigorous puff will then discharge a cloud of pollen from the wind-fertilized catkins, but none from the insect-fertilised catkin of the sallow. What Linnæus said about the flowers of trees appearing before the leaves, in order that the pollen may more easily reach the stigmas, holds good, Sprengel remarks, only of wind-fertilised trees. The lime, which is insect-fertilised, flowers in the height of summer, when all the branches are crowded with leaves.
Sprengel left it to later biologists to complete his discovery. "That wonderfully accurate observer, Sprengel," says Darwin,[32] "who first showed how important a part insects play in the fertilisation of flowers, called his book The Secret of Nature Displayed; yet he only occasionally saw that the object for which so many curious and beautiful adaptations have been acquired, was the cross-fertilisation of distinct plants; and he knew nothing of the benefits which the offspring thus receive in growth, vigour, and fertility." Not even Darwin could exhaust the inquiry. "The veil of secrecy," he goes on, "is as yet far from lifted."