Flower Structures Protect Pollen

Biological Strategy

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Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

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Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Modify Position

Many resources that living systems require for survival and reproduction constantly change in quantity, quality, and location. The same is true of the threats that face living systems. As a result, living systems have strategies to maintain access to shifting resources and to avoid changing threats by adjusting their location or orientation. Some living systems modify their position by moving from one location to another. For those that can’t change location, such as trees, they modify position by shifting in place. An example of an organism that does both is the chameleon. This creature can move from place to place to find food or escape predators. But it also can stay in one place and rotate its eyes to provide a 360-degree view so that it can hunt without frightening its prey.

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Adapt Phenotype

Living systems evolve over time in response to selective pressures. Selective pressures are environmental factors that reduce the reproductive success of some individuals in a population. Genetic makeup (genotype) is one way in which living systems mitigate selective pressures, but another is adaptable phenotypes. A phenotype is an observable characteristic that can be thought of as the expression of the genotype, combined with modifications caused by the environment or developmental conditions. Individuals, populations, or ecosystems that have phenotypes capable of reducing the effects of selective pressures can survive. Some plants, for example, modify their leaf shape in response to changing environmental conditions. A single olive tree has variable leaf shapes on sunny compared to shady areas of the tree, yet the next year, those same buds may develop differently shaped leaves.

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Structures of flowers protect their pollen from rain by various physical structures.

Yun-Yun Mao and Shuang-Quan Huang of Wuhan University in China studied the response to rain and water of 80 species of flowers. Their work revealed that many flowers have different shapes and structures to prevent their pollen from getting wet. Other flowers developed waterproof pollen instead.

Of the 80 species studied, 20 produce flowers that completely protect their pollen. Some plants shelter their pollen grains through a change in floral orientation or closing their corolla on rainy days. For example, tulip flowers close their petals rapidly when rains come. Some plants have flowers that droop downward, while others have outlets in the base of the flower that let water quickly drain away. But 44 of the 80 species expose their pollen completely, giving it no protection. Of these species, 13 produce pollen that is highly resistant to water, suggesting they have evolved an alternative way to deal with the rain.

Video courtesy Moving Art by Louie Schwartzberg

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Last Updated October 23, 2016

References

“We have demonstrated that flower structures can protect susceptible pollen from rain, indicating that rain could interact with other factors governing the evolution of floral traits…Understanding plant reproductive strategies for protecting pollen from rain may provide a clue to the diversity of plant–pollinator interactions. In high-rainfall places or seasons, a high proportion of species were observed with nodding, pollen-concealed flowers or upward-facing flowers capable of temporary closure through perianth movements to avoid pollen loss (Dafni, 1996; Bynum & Smith, 2001; Huang et al., 2002; Hase et al., 2006). In bowl-shaped flowers exposed to rain, the petals are often separate or fused only at the base, so that raindrops flow quickly away between the petals, comprising a water drainage system. Sympetalous flowers are often pendulous and rarely erect. Erect sympetalous flowers usually have a narrow corolla. Morning glory flowers (Convolvulaceae) seem exceptional, with a wide open corolla, but they are generally ephemeral and secund. We observed that the corolla of Calystegia sepium closed on rainy days. Such floral changes may function to shelter pollen from rain damage before pollinators arrive. Extended pollen presentation duration in rain and the restoration of flower performance in fine weather involve reproductive costs that could otherwise have been invested in seed production. If we recognize that rain affects floral evolution, it is apparent that increased variability of precipitation may have potential effects on plant diversity and distribution in a given area.” (Mao and Huang 2009:898)

Journal article

Pollen resistance to water in 80 angiosperm species: flower structures protect rain-susceptible pollen

New Phytologist | 26/06/2009 | Yun-Yun Mao, Shuang-Quan Huang

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