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Spur (botany)

The term “Spur” is given to outgrowths of tissue on different plant organs. The most common usage of the term in botany refers to nectar spurs in flowers.

-         nectar spur (flower)

-         spur (branch)

Common in fruit trees such as Prunus, spurs are short stems that bear flowers and fruits on. The spurs have horticultural importance. For example, the number of spurs on an almond (Prunus dulcis) tree is highly connected to the overall almond yield.

-         spur (leaf)

In tropical pitcher plants of the genus Nepenthes, the spur is a small appendage at the base of the pitcher lid.

Nectar spur
A nectar spur is a hollow extension of a part of a flower. The spur may arise from various parts of the flower: the sepals, petals, or hypanthium. Spurs often contain tissues that secrete nectar (nectaries), but can also function as a receptacle for nectar drained from a nectary outside of the flower. Nectar spurs are associated with coevolution with pollinators such as hummmingbirds, moths, and bees.

Functional significance of nectar spurs
Spur length can be an important diagnostic character for taxonomy, but it can also play a functional role in the ecology of the flower. The length of nectar spurs varies, and is generally correlated with the lengths of the tongues of their major pollinators. can restrict access of pollinators to nectar, limiting the range of potential pollinators. In a famous historical story, Darwin predicted the that that Angraecum sesquipedale, an orchid with an extremely long spur, must be pollinated by a pollinator an equally long tongue. The pollinator, the sphinx moth Xanthopan morgani ssp. praedicta with a 22 cm tongue, was found 40 years after Darwin made his prediction.

The spur can also aid in consistent placement of pollen on the pollinator, functionally positioning the pollinator so that its contact with the anthers/stigmas is more controlled.

Nectar Spurs and Evolution
Nectar spurs have been cited as prime examples of “key innovations” that may promote diversification, and play a part in the adaptive radiation of clades. Columbines (Aquilegia) have been studied in depth for the link between their floral nectar spurs and their rapid evolutionary radiations. However, there has also been some refutation to this idea recently, suggesting that the adaptive radiation of Aquilegia may have been due more to climate and habit.

Underlying Development and Genetics
In terms of development, the varying lengths of nectar spurs has been found to be based solely on the anisotropic elongation of cells. For example, in

The genetic basis underlying the development of nectar spurs has been explored in several clades of plant families, such as Linaria and Aquilegia. Studies in model plant Antirrhinum identified KNOX genes in spur development, which gene expression studies confirmed in Linaria. However, the same genes were not expressed during spur development in Aquilegia, suggesting a case of convergent evolution, where the nectar spur has developed through different developmental pathways.

List of plants with nectar spurs:
Orchids: (Satyrium, Disa)

On petals: Aquilegia, Lentibulariaceae, Viola, Fumarioideae

On sepals: Delphinium, Impatiens

From hypanthium: Tropaeolum