A plant spur refers to two distinct, specialized structures: the floral spur and the fruiting spur. The floral spur is a tubular extension of a flower that guides pollinator interactions, representing a complex biological adaptation focused on reproduction through pollination. The fruiting spur is a short, perennial branch on woody plants dedicated to bearing fruit, primarily relevant to horticultural management. Understanding the function and morphology of both structures is fundamental to appreciating the diverse strategies plants employ for reproductive success.
Anatomy and Basic Function of the Floral Spur
The floral spur is a hollow, tube-like outgrowth extending from a flower, often representing a modification of sepals, petals, or the hypanthium. Internally, it contains specialized secretory tissues called nectaries that produce and store a sugary fluid reward for visiting animals. The spur’s form, which can be straight, curved, or coiled, controls access to this energy-rich reward.
Developmentally, the spur arises from cell division followed by asymmetrical cell elongation, giving it its distinct, elongated shape. The primary function of this reservoir is to physically restrict nectar access, ensuring only specific pollinators with suitably long mouthparts can reach the reward. By forcing the animal to probe deep, the plant precisely controls the contact point between the pollinator’s body and the flower’s reproductive organs, guaranteeing efficient pollen transfer.
Co-evolutionary Relationship with Pollinators
The morphology of the floral spur has been shaped by a reciprocal evolutionary process known as co-evolution, an interaction where the plant and its pollinator exert selective pressure on one another. The spur’s length and shape correlate directly with the length of a specific pollinator’s tongue, proboscis, or beak. This tight relationship means that a plant with a slightly longer spur gains a reproductive advantage, as the pollinator must push further into the flower to reach the hidden nectar, increasing the likelihood of pollen deposition.
In response, any variation in the pollinator population resulting in a slightly longer tongue is favored, as these individuals can more efficiently harvest the nectar reward. This dynamic creates an evolutionary “race,” where both the floral spur and the pollinator’s feeding apparatus gradually increase in length over successive generations. The result of this specialization is a high degree of reproductive fidelity, minimizing wasted pollen and maximizing the efficiency of transfer between members of the same plant species. This specialization limits the range of potential visitors, promoting reproductive isolation.
The Horticultural Definition: Fruiting Spurs
The term spur also defines a specialized branch found on many fruit trees, such as apples, pears, plums, and cherries. Fruiting spurs are short, slow-growing, perennial shoots that arise from older wood. These branches are stout, typically measuring less than six inches long, and are characterized by extremely short internodes.
Horticulturally, they function as the primary sites of fruit production. Unlike the fast-growing, vegetative branches that extend the tree’s canopy, fruiting spurs concentrate energy into forming flower buds and bearing fruit. A single spur can remain productive for many years before its productivity declines. The density of these spurs is a major factor in determining the yield and management strategies for commercial fruit orchards.
Diverse Examples Across Plant Families
The structure and function of the plant spur are best understood through specific examples.
Floral Spur Examples
In the genus Aquilegia (Columbine), all five petals are modified into long, tubular spurs. The extreme variation in spur length among different Aquilegia species is a textbook illustration of co-evolution, with lengths adapted to specific pollinators like hummingbirds or hawk moths. A dramatic example is the Malagasy star orchid, Angraecum sesquipedale, which has a remarkably long, foot-long floral spur. This structure famously led Charles Darwin to predict the existence of an equally long-tongued moth, a prediction confirmed decades later with the discovery of the hawk moth Xanthopan morganii praedicta. Floral spurs can also arise from other parts, such as the sepals in plants like Impatiens or the fused base of the floral parts (hypanthium) in Tropaeolum, demonstrating convergent evolution.
Fruiting Spur Examples
Apple trees provide a clear horticultural example. Specific strains, known as ‘spur-type’ cultivars, are genetically selected for their high density of short, fruit-bearing shoots, such as ‘Delicious’ and ‘McIntosh’.