A drop of water on a brown leaf does not cause any forest fire: damage can still cause water on leaves.
Read out It is an iron rule for hobby gardeners: Never in the midday sunshine pour the plants, otherwise threaten bad burns on the leaves. That's right, have Hungarian researchers shown now? but only if the leaves of said plants are not quite smooth and even. Such leaves are not affected by drops of water and the sun, while leaves with fine hairs or other bumps burn even relatively quickly. In the hairs, the water droplets can catch, so that they hover slightly above the leaf level, so to speak, and the leaf itself lies directly in the focal length of the water droplet. One can explain well with the results, why so many beach vacationers are burned from top to bottom cancer red, the researchers say? After all, human skin is also hairy and therefore very vulnerable if it is still wet from the bath in the sea.

A drop of water has a shape similar to that of a convex lens, such as that used in a magnifying glass. Loupes are known to act as burning glass: they focus the rays of the sun so that they converge in one point. At the same time, the energy of light is also concentrated at this point, which in turn can ignite flammable objects that are located there. The Hungarians now wanted to know if a drop of water that clings to a plant leaf is also capable of doing so. So they irradiated small glass beads and drops of water of different shapes and sizes with sunlight and also simulated the conditions in the computer.

Interestingly, on sunny days, the glass beads caused severe burns on smooth leaves, such as a maple tree, while spherical or shallow water droplets were unable to do so? possibly because the leaf was sufficiently cooled at the point of contact with the drop, the researchers speculate. The situation was different when the leaves were covered by small waxy hairs, as in the case of the swimming fern: Here, the drops caught in the hair and remained just above the leaf level. Thus, on the one hand, the surface of the leaves came within the range of the focal length, where the energy density of the collimated light is highest. On the other hand lacked by the non-existent contact of the cooling effect. The result was a severe sunburn, the researchers write.

"When the focal plane of a drop hits a dry plant surface, intensely focused sunlight can actually cause a fire, " says Horváth. The probability for it is however quite small? the water droplets would probably have evaporated before the leaves could ignite.

ddp / science.de? Ilka Lehnen-Beyel Gábor Horváth (Eötvös University, Budapest) et al .: New Phytologist, online pre-publication, doi: 10.1111 / j.1469-8137.2009.03150.x Display

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