The mechanism of phototropic bending and branching inVaucheria geminatawas analyzed. Using the half-side-illumination method it was shown that phototropic bending is initiated by the formation of a new growth center on the illuminated side of the apical dome of the tube, not by the difference in growth rate between the lighted and shaded sides of the tip. Branching was also initiated when the part proximal to the apical dome was illuminated. Blue light was effective for branching as it was for phototropic bending. The refractive indices measured at the growing tip and an area other than the tip were 1.36 and 1.34 respectively. A hyaline patch resembling the hyaline cap, which appeared prior to the onset of tip-growth in the apical dome, was invariably formed shortly before the initiation of a new branch at the illuminated locus.
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