Wednesday, August 26, 2009

Tropisms 101

Is there anything more undeniably cheerful than the open face of a large sunflower?

You don't even have to like them to get the effect. Your brain will lift you up anyway because of the color: yellow stimulates the nervous system, activates the memory, stirs up feelings of optimism and happiness, and encourages communication (yellow legal pads come to mind). 85% of all pencils are yellow, but that's because in the 1880s the best graphite came from China where the color yellow is associated with royalty (Ming yellow) and respect; pencils were painted yellow to borrow a little of the glory. That was just the beginning of its use in advertising. You will see plenty of evidence in any fast food operation that yellow is powerful in its effect on consumers' moods: be jolly, enjoy your food, eat fast, and then get out of here so someone else can use the table.

Sunflowers (Helianthus anuus) are all native to North America. There is enormous variety among members of the helianthus family, but the specimen familiar to most people is the tall annual that bears one large flower head atop a very sturdy stem.

Sunflowers demonstrate the remarkable property of phototropism, which is to say they all face the rising sun. In a mechanism on the plant's stem, turgidity is controlled by varying the water pressure within the cells, and the side that swells bends toward the side that does not (in effect it is much like the way hair curls by uneven growth one side to the other).

Why would a plant do this? The more surface area a flower can expose to the morning sun, the warmer it becomes (I have read that this difference can be up to 14 degrees Fahrenheit higher than the ambient air). Given a choice from among all the flowers in the garden, where are cold-blooded insects likely to go to jump start their days? The sunflower sauna. And, in this way, the sunflower is sure to be pollinated.

There is a slightly different phenomenon called heliotropism: this mechanism works all day to bring either the flower's face (such as the common buttercup Ranunculus repens) or the leaves (nasturtium Tropaeolum majus) and garden heliotrope (Heliotropium peruvianum, hence its name) into perpendicular alignment to the sun for maximum exposure. The flower turns all day long to face the sun, maximizing opportunities for photosynthesis. Leonardo da Vinci was first to describe heliotropism in a work on the nature of plants. But hadn't anyone noticed this before? Look in any meadow and you can practically watch the buttercups move like little satellite dishes primed to pick up ancient noise in the heavens.

There is another light-related characteristic of plants that is equally intriguing: skototropism (Strong and Ray, 1975), or negative phototropism, which means exactly what you think it means (skotos is Greek for dark). Skototropic plants grow not only away from the sun, but emphatically toward darkness. (Have you ever had an errant English ivy Hedera helix grow through the concrete window casement of your basement? You've seen skototropism.)

One thing the ivy gains by this process is a way for a plant to attach itself to something else for support as it grows. Pretty as it may be, don't let ivy fool you: if you let it grow up against your beautiful brick facade, the mortar will deteriorate and soon you will have a pile of bricks where your wall once stood. What you see in the photograph above is an explosion of the growths which can develop in one of two ways: they can attach themselves to a structure and bind the plant to it with great strength; or, they can touch down on the soil and extend downward (gravitropism, of course, which was first explored by Charles Darwin). Darwin also worked on understanding heliotropism by covering the growing tips of seedlings with foil to block the light; they stopped turning toward the sun.

Grativropism describes the movement of roots as affected by gravity. It used to be called geotropism, but that is no longer seen to be accurate since epiphytes like orchids, for example, have gravitropic roots and they grow downward but they extract their nutrients from the air, not the soil/earth (geo-).

So many questions swirl in my mind related to the implications of the various tropisms. This list covers a mere handful, though I find these to be the ones that merit particular attention for considering psychology and spirituality in the garden.

I'll pick up the threads tomorrow. I'm still thinking all this through, and want to sleep on it.

Photography: Bumble Bee, Darren Hachter, 19 May 2006; orchid Holcoglossum kimballianum (Rchb.f) Garay, 1972; buttercup Ranunculus repens, Sannse, Great Holland Pits, Essex, 6 June 2004; nasturtium Tropaeolum majus, Armon, 22:19, 17 February 2006; Hedera helix root system, Bialowieza, 2005; Sunflowers in Fargo, South Dakota, Bruce Fitz; Sunflower in Victoria, Australia, Fir2002, November 2008; heliotrope Heliotropium peruvianum, Algirdas, 2005

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