The Effects of Exposure Upon the Structure of Certain Heath-Plant

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Abstract
Specimens of Pteridium aquilinum, Vaccinium myrtillus, and Molinia coerulea growing in full sun were collected from the valleys and from different heights on the slopes of Hindhead Common, Surrey (the ridges being 750 to 850 ft. above sea-level) . The exposure factors considered were wind velocity, evaporation, relative humidity, temp. of the air and soil, and transpiration, although it was impossible to separate the exposure effects from those of soil differences at the different levels. The morphological and anatomical differences noted were probably in part a result of soil differences, which may in some cases have been of greater importance than the differences of exposure. Increased exposure was accompanied by a very marked and progressive decrease in the dimensions of the plants and reduction of the transpiring surface. In Pteridium the laminae were much smaller and possessed fewer pinnae, which were shorter, tougher, thicker, and less divided. In Vaccinium the leaves of exposed specimens were smaller than those of sheltered ones. In Molinia the exposed leaves were shorter than the others and usually more strongly inrolled. No variation in hairiness was noticed. The thickness of the outer epidermal walls, including the cuticle, increased with increased exposure, particularly in the stem of Vaccinium. No appreciable differences were found in the size or position of the stomata with reference to the other epidermal cells. Fewer stomata, however, were usually found in the exposed than in the sheltered plants, but this was not the case in V. myrtillus. The vascular tissue showed a tendency to decrease in amount with increased exposure. The general result of the studies showed a marked and progressive variation in external and internal features in response to increasing exposure. The modifications brought about by increased exposure are such as will tend to decrease transpiration, conserve the water supply of the plant, provide additional mechanical tissue, and increase photosynthesis.

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