Seeds Float to the Best Conditions

Biological Strategy

Red mangrove

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Disperse Seeds

Plants disperse their seeds using a variety of mechanisms, including wind, rain, and attachment to animals. Seed dispersal can be important if plants growing too close together will compete for resources, such as nutrients and sunlight. Since plants are literally rooted in place, seed dispersal is also an important mechanism for expanding their range or reaching new environments.

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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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Modify Buoyancy

Buoyancy is an upward force exerted by air or liquid on a solid object that works against the object’s weight. A hawk glides through air and a duck floats on water due to buoyancy. Some living systems (such as fish eggs) can remain sedentary and therefore maintain the same buoyancy at all times. But most must adjust their buoyancy because they can’t survive long unless they change position. These living systems require strategies to not only be buoyant, but also adjust buoyancy level. They often modify buoyancy by adding or decreasing lift, or by changing their weight. For example, birds like condors that soar for a long time shift the directional orientation of their wingtip feathers to manage buoyancy. Fish alter the amount of air in their swim bladders to increase or decrease their weight, thus altering their buoyancy.

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Regulate Reproduction or Growth

Reproduction and growth are two physiological processes that occur in all living systems. There are situations when conditions are right for both, and other situations when continuing either harms the living system because both have a very high energy cost. Reproduction and growth are unique in that both can stop until conditions improve, although stopping either for an extended time can cause problems. An example of regulating reproduction is a process called delayed implantation or embryonic diapause found in some mammals, such as otters. An otter’s embryos sometimes temporarily cease developing and won’t develop further until the female senses that conditions are suitable.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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Seeds of mangroves find optimal conditions by reacting to time passage and light conditions.

“All mangroves disperse their offspring by water. A distinctive feature of the majority of mangrove species is that they produce unusually large propagating structures or propagules…The long, pointed appearance of Rhizophora propagules hanging on the parent tree has led to the belief that they plummet like darts into the mud below and so immediately establish themselves…The reality is more complex. Rhizophora propagules generally float for some time before rooting themselves. Initially, floating is horizontal. Over a period of a a month or so they shift to a vertical position. This makes it more likely that the tip will drag in the mud surface and result in the propagule stranding when the tide recedes. Roots first appear after 10 days or so, and many of the propagules lose bouyancy [sic] and sink. Presumably before this has happened the propagule is not ready to establish itself as a seedling. By 40 days, virtually all propagules show root growth (Banus and Kolehmainen 1975). Most will strand in a horizontal position and erect themselves after rooting in the mud…Propagules which do not successfully root after 30 days or so may regain buoyancy and float off again in a horizontal position. They may remain viable for a year or more (Rabinowitz 1978b). Occasionally, propagules are still viable after being transported tens of kilometers inland by hurricanes…The timing of these events is affected by circumstances. In sunny conditions, virtually all floating Rhizophora propagules pivot to the vertical by 30 days and root within a further 10 days or so; about half of shaded propagules are still floating horizontally after several months. This behaviour will facilitate settling in forest clearings rather than directly under adult trees (Banus and Kolehmainen 1975).” (Hogarth 1999:24, 27-30)

Last Updated August 18, 2016

References

Book

The Biology of Mangroves and Seagrasses (Biology of Habitats Series)

Oxford University Press | 31/05/2007 | Peter Hogarth

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