Honey-dew. - The leaves, or other organs, of many plants are sticky in hot weather, owing to the excretion of a sweet liquid containing sugar, the consistency and colour of which vary according to circumstances. This honey-dew must not be confounded with the normal viscidity of certain insectivorous plants - e.g. Sundew - or with the sticky secretion on the internodes of species of Lychnis, etc., where it plays the part of a protection against minute creeping things.
Honey-dew is often met with on Lime trees, Roses, Hops, etc. In many of these cases the honey-dew is excreted by Aphides, which suck the juices of the leaves and pour out the saccharine liquid from their bodies. The sweet fluid is in its turn sought after by ants, and also serves as nutritive material for various epiphytic fungi - e.g. sooty mould, Capnodium, Fumago, and Antennaria - which give the leaves and honey-dew a brown or black colour. Certain Coccideae also excrete honey-dew, especially in the tropics.
At least one case is known where honey-dew is formed as the result of the parasitic action of a fungus, namely Claviceps purpurea in its conidial stage on the stigmas of cereals, and this may be compared with the sweet odorous fluid excreted by the spermogonia of certain Aecidia. In both cases the sweet fluid attracts insects which disperse the spores.
Honey-dew may also be formed without the agency of fungi or insects, when hot and dry days are followed by cool nights, with a saturated atmosphere, e.g. Caesalpinia, Calliandra and other trees in the tropics, which are called rain trees owing to the numerous drops of fluid which drip from the leaves under the abnormally turgescent conditions referred to.