Citrus Trees

Citrus trees are species of sub-tropical and tropical climates. They are intolerant of freezing, and their foliage and fruits will be damaged by even a relatively short exposure to freezing temperatures for just a few hours. Colder temperatures can kill the entire tree.

Species of citrus trees range in size from shrubs to trees. Most species have thorny twigs. The leaves are alternately arranged on the twigs, and the foliage is dark green, shiny, aromatic, leathery, and evergreen. The roots of citrus trees do not develop root hairs, and as a result citrus trees are highly dependent for their mineral nutrition on a mutualistic symbiosis with soil fungi called mycorrhizae.

Citrus trees have small white or purplish flowers, which are strongly scented and produce nectar. Both scent and nectar are adaptations for attracting insects, which are the pollinators of the flowers of citrus trees. Some species in the genus Citrus will easily hybridize with each other. This biological trait can make it easier for plant breeders to develop profitable agricultural varieties using controlled hybridization experiments to incorporate desirable traits from one species into another. However, the occurrence of hybrid Citrus plants makes it difficult for plant taxonomists to designate true species. As a result, there is some controversy over the validity of some Citrus species that have been named.

The ripe fruit of citrus trees is properly classified as a hesperidium, which is a type of berry, or a fleshy, multiseeded fruit. The fruits of citrus trees have a relatively leathery outer shell, with a more spongy rind on the inside. The rind of citrus fruits is very rich in glands containing aromatic oils, which can be clearly detected by smell when these fruits are being peeled. The interior of the fruit is divided into discrete segments that contain the seeds surrounded by a large-celled, juicy pulp. The seeds of citrus trees are sometimes called "pips." Edible fruits like those of citrus trees are an adaptation to achieve dispersal of their seeds. The attractive and nutritious fruits are sought out by many species of animals, who eat the pulp and seeds. However, the citrus seeds generally survive the passage through the gut of the animal, and are excreted with the feces. In the meantime, the animal has likely moved somewhere, and the seeds have been dispersed far from the parent tree, ready to germinate, and hopefully, develop into a new citrus plant.