. Plant physiology. Plant physiology. INFLUENCE OF EXTERISTAL CONDITIONS ON GROWTH 245 of individual organs. The most common phenomenon to be noted in this con- nection is the daily periodicity of growth. Plants grow more slowly by day than by night so that it appears that light exerts a retarding influence upon growth.^ The growth maximum occurs in the early morning hours and the minimum occurs in the evening. The curve 32 of Fig. 125 shows the diurnal march of the rate of plant growth, which is seen to increase gradually from about 6 to about 6 , after which it gradually decreases,
. Plant physiology. Plant physiology. INFLUENCE OF EXTERISTAL CONDITIONS ON GROWTH 245 of individual organs. The most common phenomenon to be noted in this con- nection is the daily periodicity of growth. Plants grow more slowly by day than by night so that it appears that light exerts a retarding influence upon growth.^ The growth maximum occurs in the early morning hours and the minimum occurs in the evening. The curve 32 of Fig. 125 shows the diurnal march of the rate of plant growth, which is seen to increase gradually from about 6 to about 6 , after which it gradually decreases, from morning until evening. The accelerated growth of the night hours occurs in spite of the lower night temperature, as may be seen from the figure just mentioned, where the curve f represents the diurnal march of temperature corresponding to that of growth. This periodicity is mainly dependent upon light, although it continues to be manifest—but with less regularity—when the plant is kept continuously. 6a, JZjv 6m, iZt 6a, i2n 6m fZt Fig. -Graph showing daily periodicity of growth, the broken line 32;. The full (<°) is corresponding graph of temperature. {After Sachs.) "in darkness. The latter fact has been explained as an induced rhythm; the ancestors of the present plants have been exposed for countless generations to the diurnal alteration of light and darkness and the periodicity of growth ap- pears to have become a habit (due to internal conditions), which is more or less markedly inherited. One-sided illumination brings about a bending of plant organs, this response being termed phototropism or heliotropism.^^ When an organ bends toward the more brightly lighted side it is said to be positively phototropic, when it bends away from the more intense light it is negatively phototropic. Positive photo- tropism is very common among plants and is usually observed when growing stems are subjected to one-sided illumination. iBaranetzky, J., Die tagliche Perio
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