. Notes on the life history of British flowering plants. Botany; Plant ecology. b II GERANIACE^ 135 acid flavour. They are very delicate, and adapt them- selves to diflferences of light in four ways—by movements of the whole leaf, by change of angle, by movements of the chlorophyll within the leaf, and by changes in the form of the chlorophyll grain. In strong sunshine the leaflets close slightly and move downwards (Fig. 78), so that the rays fall on them less directly ; and also at night, serving as a protec- tion against cold. Dar- win's experiments clearly showed that leaves which were comp
. Notes on the life history of British flowering plants. Botany; Plant ecology. b II GERANIACE^ 135 acid flavour. They are very delicate, and adapt them- selves to diflferences of light in four ways—by movements of the whole leaf, by change of angle, by movements of the chlorophyll within the leaf, and by changes in the form of the chlorophyll grain. In strong sunshine the leaflets close slightly and move downwards (Fig. 78), so that the rays fall on them less directly ; and also at night, serving as a protec- tion against cold. Dar- win's experiments clearly showed that leaves which were compelled to remain horizontal at night suifered more from frost than those which were allowed to Fig. IS.—OxaHs Acetusella. a, Leaf seen assume a vertical position.^ abo™trnight ^''^'' *' ^^^^''''' *^'™' The motile organs are the very short stalks which connect the leaflets with the common leaf-stalk. Fig. 79 represents a trans- verse section in Oxalis carnea. Beneath the skin or epidermis is a mass of parenchyma surrounding an axial strand, containing the vascular tissue enclosed in a sheath. Fig. 80 gives a longitudinal section. The par- enchyma presents no important difl"erences. If the lower part swells and the upper contracts, as in Fig. 78, the leaflet rises; if the reverse, it sinks. The change is rendered possible by the extrusion or absorption of water. It is remarkable that if the leaves are covered up at night so that they receive no light they will still open in the. morning.^ The movements are mainly due to the chemical rays. If the yellow, orange, and red rays are excluded, the plant reacts as if in white light; on the contrary, if the violet and blue rays are shut out, and the red end of the spectrum only allowed to "pass through, the plant behaves as if in darkness. Under normal conditions the cell represents a turgid bladder; the watery cell-sap being imprisoned by the I Movements of Plants. ^ Saclis, Lectures on Physiology of Please not
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