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Dear List,

As many of you know, from time to time I come across a botanical thought that perplexes me. Today's thought concerns phototropism, that stalwart of Botany 101 students worldwide. The aroids I grow (Cryptocoryne) often have very elongated, rather weak petioles. When growing in crowds (their typical habit) the crisscrossing petioles and leaves make for a veritable swarm of vegetation floating on water surfaces. Among this leafy mess is considerable movement on the time order of days. A time-lapse movie of a patch of Crypt leaves would be a seething turmoil. It is only because of our own time sense that plants seem to be passive, quiescent beings.

I assume that much or most of this movement pertains to the struggle for light. A new leaf emerges and starts to shade out some existing ones, which then bend and turn to avoid the dreaded shade. I figure this is a manifestation of positive phototropism, the leaves seeking to maximize their uptake of light energy. Fine.

Every once in a while I have to manage the plants and I can never seem to get all the leaves back in a situation where the upper surfaces are happily pointed to the sky. Some leaves always get turned over because the petioles are soft and can twist, exposing the underside of the leaf to the sky and the incoming light. While you can see that the plants attempt, in their way, to get the leaf turned back the right way, often this is not possible. Even though the leaf is bathed in light, this light is shining onto what is anatomically the underside and the plants do not like it this way. If the leaf does not get turned back upright, it dies. Why is that?

My first theory is that it has something to do with stomata, those little openings with which plants maintain their air/moisture balance. Everybody from Botany 101 knows that the bottom side of the leaf is where the lion's share of these gizmos lie. If the underside is exposed to the sun and weather the leaf gets off-kilter and the plant shuts it down. That's a pretty good theory even if we are dealing here with aquatic plants where the relative humidity (at least in my setup) is 100%.

My second theory is that the photosynthetic apparatus in plants is directional, meaning that reception of photons for photosynthesis is a one-way mechanism. By this theory photons coming in through the backside, as it were, are not efficiently captured, basically rendering it the same as if that leaf were in the dark. A leaf in the dark is a wasted leaf, so the plant gives orders to withdraw the sap-soluble goodies and abandon that leaf in favor of producing a new one that can orient itself properly.

Any ideas from the botanists?

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