. Principles of modern biology. Biology. Fig. 14-3. Sleep movements in Oxa/is stricta. Left, doy position of leaflets; right, night position of leaflets. Another example of turgor movement. Holland and the United States, the nature growth substances began to become clearer. The early experiments showed that replacing the decapitated tip of meristem tissue re- stores the growth of the cells of the stump. Moreover, Went found that this effect was still obtained even when a fairly thick (2 mm) layer of agar was interposed between the cut surfaces, which proved that some diffusible substance—rathe
. Principles of modern biology. Biology. Fig. 14-3. Sleep movements in Oxa/is stricta. Left, doy position of leaflets; right, night position of leaflets. Another example of turgor movement. Holland and the United States, the nature growth substances began to become clearer. The early experiments showed that replacing the decapitated tip of meristem tissue re- stores the growth of the cells of the stump. Moreover, Went found that this effect was still obtained even when a fairly thick (2 mm) layer of agar was interposed between the cut surfaces, which proved that some diffusible substance—rather than a wave of excitation —is responsible for the growth effect. Phytohormones, Especially the Auxins. Since these early experiments, three natural growth substances, or auxins, have been identified; and a variety of synthetic prod-. Fig. 14-4. Positive phototropism of the stem of a bean seedling. Successive photographic exposures taken at 40-minute intervals. ucts have been found that exert analogous effects. The three natural auxins—auxin a (C1SH3206), auxin b (C1SH3„04), and hetero- auxin (C10H15NO2)—are all relatively simple, readily diffusible organic compounds, al- though heteroauxin has been obtained mainly from animal sources. All the natural auxins have been prepared in pure form, and all have been synthesized artificially. Virtu- ally no specificity is found in the action of the different compounds; that is, all have similar effects when applied to corresponding parts of different plants. The auxins are pro- duced primarily by meristem tissue in the growing points of the plant, whence they are transported to the nearby regions, where cell differentiation is occurring. The auxins exert various effects on the metabolism and rate of multiplication of plant cells (see below). But one outstanding function of the auxins is to regulate—by accelerating or retarding—the lengthwise growth of individual cells in the growing parts of the plant. It is important to
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