Wednesday, 21 May 2008

Hibernation of Canna seed

Cannas are unique in many ways, one of them being that they are the only member of the the Liliopsida Class (monocot family) in which hibernation of seed is known to occur, due to its hard, impenetrable seed covering.

Seed hibernation, is different from seed dormancy. Seed dormancy can be defined as a seed not germinating when conditions, e.g. light, water/nutrient availability or the presence of activating substances like smoke, are favourable. While, hibernation is the ability of a seed to remain in hibernation when there is a lack of things essential to their development (water, sunlight, nutrients, etc.) or in harsh conditions (extreme cold, extreme heat, hard ground, etc.). There is not a specific time limit in which a seed may hibernate; some seeds found in the arctic grew after an estimated 10,000 years. When conditions are right for the particular type of seed, they can come out of hibernation and grow.

We have a famous instance of Canna compacta seed that was discovered by archeologists in an Inca rubbish tip, and subsequently germinated! See left. I have never been able to trace anybody with any personal knowledge of that well-documented experience, and the fact that the plant grown from it has been lost to us is a cause of frustration.

Seeds display hibernation for very short periods of time every year. In autumn, seeds produced in summer may have a chance to grow while the soil is still warm, but instead do not grow until spring. The seeds have chemicals encoded into them that will not allow them to germinate until the soil warms up again, a sign of spring. This is called after ripening.

Seeds may also go into hibernation for other causes. Many pines have adapted to forest fires that destroy trees by creating pine cones that open in extreme heat (fire) and release seeds coated with a material that will break down only in extreme heat. The seeds hibernate until this happens.

Canna seeds have a hard coating that must be broken down by contact with rough rock, soil, etc. before they begin to grow. Until the shell is broken and water reaches the seed, the seed remains dormant.

When released, some seeds (especially those in a desert environment) must wait for rain before the seed is triggered out of hibernation and begins to grow. There are many other causes of dormancy. It is an unclear phenomenon; the limits of seed hibernation are still unknown, and many triggers have yet to be discovered.

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