. Experimental morphology. Protoplasm; Growth. 438 EFFECT OF LIGHT [Ch. xvn tropism), the following turn from it (negative phototropisra): the hypocotyl of the seedling mistletoe; the roots of many- plants, Sinapis (Fig. 124), Helianthus, Vicia faba, Zea mais, etc.; stems of some recumbent dicotyledons, the moneywort; the root hairs of the prothalli of ferns and hepat- ics; the tendrils of the vines Vitis and Ampelopsis. As we shall see later, however, the sense of turning is, within limits, dependent upon the intensity of the light. Finally, we observe that plants differ greatly in
. Experimental morphology. Protoplasm; Growth. 438 EFFECT OF LIGHT [Ch. xvn tropism), the following turn from it (negative phototropisra): the hypocotyl of the seedling mistletoe; the roots of many- plants, Sinapis (Fig. 124), Helianthus, Vicia faba, Zea mais, etc.; stems of some recumbent dicotyledons, the moneywort; the root hairs of the prothalli of ferns and hepat- ics; the tendrils of the vines Vitis and Ampelopsis. As we shall see later, however, the sense of turning is, within limits, dependent upon the intensity of the light. Finally, we observe that plants differ greatly in the degree of their phototropism. Thus aquatic plants and non-chloro- phyllaceous phanerogams are only very slightly phototropic (compare Hochkeutinbe, '96). The general phenomena of positive phototropism are seen when a seedling which has been growing in the dark is illu- minated upon one side by a horizontal ray. The tip of the seedling, which is normally constantly "nutating" about the vertical line passing through its axis, now begins to move towards the light side of the vertical. The quickness with which it does so seems to vary with the species and with the intensity of illumination of the plant and other conditions of the environment; the turning may be evident in 15 minutes * or it may be delayed for several hours. There is apparently a certain, not precisely determined, latent period elapsing be- tween illumination and response. The curvature first appears just behind the tip of the seedling, but later almost the whole stem above the ground becomes in- volved, so that after several hours it points straight towards the source of light (Fig. 125). The intensity of light necessary to provoke the maximum response „ ,„, ^ . ^ ^ ^ . varies with the species. Wiesnee Fig. 125. — Course of phototropic ^ curving of the cotyledon of ('93) especially has made accurate Avena sativa. a, before iiiu- determinations on this subject. mination; 6, after It hours; c, aft
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