This section is from the book "On British Wild Flowers Considered In Relation To Insects", by John Lubbock. Also available from Amazon: Nature Series On British Wild Flowers Considered In Relation To Insects.
An extensive order, ranging nearly over the whole world, but represented in Britain by only four genera, Saxifraga, Parnassia, Drosera, and Chrysosplenium.
The species of the genus Saxifraga are melliferous, and proterandrous. Bergenia (Saxifraga) crassi-folia, which, however, is not British, though frequently grown in gardens, is according to Engler, protero-gynous. In Chrysosplenium the anthers and stigma ripen simultaneously. Parnassia palustris, as its name indicates, inhabits wet and boggy places. It has ten stamens, of which however five only bear anthers, while the others secrete honey at the base, and terminate in from eight to seventeen beautiful yellow globular glands. These glands so closely resemble drops of honey that it is difficult to believe they are perfectly dry. They probably serve as sham drops of honey to attract flies. The five polliniferous anthers ripen, not simultaneously, but successively, and "as each ripens it places itself right on the top of the stigma, with its back to it, and the pollen is then discharged from the anther on the side away from the stigma, so that it is scarcely possible for any to fall on it; and this is done by each of the five stamens in succession" (Bennett, " How Flowers are Fertilised," 1873, p. 19). The flowers are much visited by insects, especially by flies.
Fig. 81 - Drosera rotundifolia.
In the cases we have hitherto considered, the relation between the flowers and insects is one of mutual advantage. The honey of the flowers affords to the insects a rich and nutritious food; and if the latter rob the flowers of some of their pollen, they make ample amends by carrying a portion of the remainder from one flower to another, and thus conferring on the plant the great advantage of cross-fertilisation. In Drosera (Fig. 81), on the contrary, we find a very different state of things, for the plant catches and devours insects. This genus, and the other plants which have this remarkable habit, have recently been the subject of an admirable memoir, by Dr. Hooker, read before the British Association (Nature, Sep. 3, 1874). The first observation on insect-eating flowers was made, about the year 1768, by our countryman Ellis, on Dionaea, a North American plant, the leaves of which have a joint in the middle, and thus close over (Fig. 82), kill, and actually digest any insect which may alight on them. The plant has recently been studied by an American botanist, Mr. Canby, and, says Dr. Hooker, "by feeding the leaves with small pieces of beef, he found, that these were completely dissolved and absorbed; the leaf opening again with a dry surface, and ready for another meal, though with an appetite somewhat jaded. He found that cheese disagrees horribly with the leaves, turning them black, and finally killing them. Finally, he details the useless struggles of a curculio (beetle) to escape, as establishing the fact that the fluid is secreted, and not the result of the decomposition of the substance which the leaf has seized. The curculio being of a resolute nature, attempted to eat his way out - 'when discovered he was still alive, and had made a small hole through the side of the leaf, but was evidently becoming very weak. On opening the leaf, the fluid was found in considerable quantity around him, and was without doubt gradually overcoming him. The leaf being again allowed to close upon him, he soon died.' " Prof. Burdon Sanderson has recently made some interesting observations on the electrical changes by which these movements are accompanied. (Brit. Ass. Report, 1873.)
Fig. 82. - Two leaves of Dionaea: one open, one closed upon a fly.
In the genus Drosera (Fig. 81), the hairs which cover the leaf, fold over and capture insects. This was first observed almost simultaneously by Mr. Whatel}' and Mr. Roth. The latter says, "I placed an ant upon the middle of the leaf of D. rotundifolia, but not so as to disturb the plant. The ant endeavoured to escape, but was held fast by the clammy juice at the points of the hairs, which was drawn out by its feet into fine threads. In some minutes the short hairs on the disc of the leaf began to bend, then the long hairs, and laid themselves upon the insect. After a while the leaf began to bend, and in some hours the end of the leaf was so bent inwards as to touch the base. The ant died in fifteen minutes, which was before all the hairs had been bent themselves." Mr. Darwin has recently shown that while the leaves will in this way close over, and actually digest pieces of meat or other animal matter, they take little notice of inorganic substances.
I cannot pass from this subject without mentioning another insectivorous plant, the genus Sarracenia, though it is not British, and does not belong to the present order. S. variolaris has some of the leaves in the form of a pitcher which secretes a fluid, and is lined internally with hairs pointing downwards. Ants, flies and other insects which fall into this pitcher cannot get out again, and are actually digested by the plant. Up the outside of the pitcher there is a line of honey glands, which lure the insects to their destruction. Bees, however, appear to be scarcely ever caught.