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 Light Intensity

 At the top of this photograph there is a branch from the sunny crown of a poison-oak bush. At the bottom of the picture is a branch from the shaded base of the same plant. The leaflets vary in their biggest dimension from 4/16 to 15/16 inches:

 1

At noon on a cloudless on June 21st the sun delivers about 100,000 lumens of visible light. Simultaneously, the sun delivers an equal amount of energy as heat. The bush tries to use the light while staying cool.

Leaflets in the sun lose heat by air cooling and evaporation. According to Lambers, the speed of air cooling increases 4xs when the biggest part of a leaflet halves. In this picture, the smallest leaflet air cools 15/4 x 15/4 = 14 times faster than the largest leaflet.

In order to document the effects of light intensity further, the most typical one of ten representative end leaflets were chosen in each of five light conditions. From left to right those conditions were: deep shade, medium shade, light shade, partial sun, and full sun.

 2

Nothing grows in deep shade, except a little poison-oak. The poison-oak leaflets are held horizontally, and do not overlap. The leaflets are paler, and the veins less pronounced than in other situations. Deep shade can be seen under the clusters of big trees in the center of Los Osos Oaks Reserve. Deep shade illumination is about 1,000 lumens.

Poison-oak grows most vigorously in light shade, at 20,000 lumens. The stems grow 5 feet a year, toward the light; and the leaves are more apt to have a waxy coat. However, the plants fruit best in partial sunlight.

Sun and shade leaves are common in nature. Here is coast live oak:

3

 

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