This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Many plants, whose root system is not well developed, or which live in swampy places, where they have difficulty in procuring a sufficiency of nitrogen in the usual way, have evolved some peculiar contrivances for eking out the supply. The Sundews (Drosera), Venus Fly-trap (Dionaea), Pitcher Plants (Nepenthes), (fig. 41). Butterworts (Pinguicula), and Bladderworts (Utricularia), belong to this class, and many of them are cultivated. By various means they manage to capture and detain insects and other small creatures, which they digest or dissolve, absorbing the nitrogen. The Sundew (fig. 11) develops on the upper surface of its leaves numerous tentacles, each terminated by a sticky gland. Flies alighting upon a leaf get held fast by the viscid matter, while the other tentacles close upon their victim. The protoplasm now forms a "ferment", and the liquid is spread over the fly till dissolved, when the juices are reabsorbed. A stone or other object would cause the infolding of the tentacles, but if such objects contain no nitrogen the tentacles soon unfold, without having produced any chemical changes in the protoplasm, thus proving that nitrogen was the element of food required.