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A time for every purpose

All life moves. Or, more precisely, all life moves purposefully.  This is true even for trees and plants.  Movement is essential for maintaining life.  Animals migrate; plants disperse.  Some form of migration is an ingredient of all life.  For many organisms, it is a key function of reproduction.  We don't reproduce merely to create a new organism, but also to disperse the population - finding new fertile ground, or resources. Reproduction is a form of migration. Reproduction isn't merely to replicate. Reproduction produces change and diversity.  While we may have strong resemblences in families, we also have differences.  Creating a difference is how evolution works.  In this sense, nature is a continuous exploratory process, finding what works best.  Nature senses change and responds.  Some of this is immediate and physiological or behavioural; some of it is over generations. 

If we look at a forest over long periods of time, we would see that it shifts. There is a movement over generations.  But we see movement in plants on a daily basis.  Flowers open and stems bend toward the sun - they are phototropic.  As all gardeners know, light is vital to the plant. 

Charles Darwin and his son carried out an elegant experiment on grass stems.  They put a cap on the very tip of the young stems.  What they found was that the stems no longer bent towards the light.  The bending is produced by elongation of the plant cells on one side of the stem.  Clearly, some kind of signal was being sent to these cells from the light-sensitive tip.  The Danish physiologist, Peter Boysen-Jensen, later showed that this signal was a chemical that travelled down from the tip only on the shaded side of the stem. The tips contain light-sensitive proteins - phototropins - that cause a hormone - auxin - to be transported down the stem. 

Day length matters to a plant. Plants are good time-keepers. The Earth spins as it orbits the sun, and it is the measure of day length that really matters.  Some plants - short-day plants - such as rice, will only flower when the day length drops below a certain threshold.  Others, such as spinach and sugar beet are long-day plants - flowering only when the day length rises above a certain level.  In this way, the plants monitor the seasons. Some are day-length neutral. 

Hello, darkness my old friend.  We refer to short-day plants, but it is the night that matters - the period of darkness.  A short-day plant will only flower if it gets a continuous period of darkness for a given length of time.  So, how do plants do this? 

One idea is that it involves a synchrony - a lining up - of an internal physiological clock with the light/dark periodicity.  Plants flower when these are in synchrony. But how would this work?

The plant produces a bloom inducing protein in a rhythmic cycle - the protein production ebbs and flows, but it is usually broken down as soon as it is produced, and this prevents the concentration rising.  As the evenings get lighter, this breakdown of the protein is blocked, and the concentration increases and triggers flowering.  That is one idea, but plants may have found different ways to solve the problem. 

Production of seed is only half of the solution.  Dispersal is a major part of the trick, for which nature has produced a variety of means.  And this is where plants use animals - animals move at greater speeds and distance. They may collect and bury nuts; their fur may pick up seed.  For plants and trees, animals make ideal dispersal kits.  Evolution is an interactive process. 

Ray Noble is a chartered biologist and Fellow of the Royal Society of Biology


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