The movements of the leaves of tropical Leguminosae had previously been studied by others—we may mention in particular the admirable investigations of Charles and Francis Darwin—but a definite explanation of their biological significance has only been arrived at through the most recent researches, carried out in the tropics and under the natural conditions of life of the plants. A visit to the botanical station at Buitenzorg, in Java, enabled Stahl and Haberlandt to shed some new light on the subject, and to show that the closing and erecting of the feather-like leaves of these plants is essentially connected with a proportionate diminution of the force of the tropical showers of rain, as well as with the regulation of the amount of light that falls upon them; for the more they are exposed to the sun the more do they become directed upwards, until eventually they close completely. This is at any rate the case in numerous other tropical Leguminosae, in which it is not the impulse of touch which starts the movement, but a certain intensity of the light:—the closing of the leaves here serves as a protection not only from the rain, but also from the excessive effects of the sun's rays. The leaves of these plants close when the light is strong, as well as when it is not so, and in the latter case the adaptation is looked upon by Haberlandt as an arrangement which causes the closing of the leaves even before the rain begins, when the sky is usually considerably clouded over. Compare Stahl, 'Regenfall u. Blattgestalt.; ein
Page:The Effect of External Influences upon Development.djvu/59
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