The geotropic behaviour of rhizomes of Aegopodium podagraria has been investigated by time-lapse photographic recording using infra-red radiation in complete absence of light. They have been shown to be very sensitive to light. Even a single exposure for 30 seconds to red light evokes a characteristic response in which the rhizome first turns downwards and then turns up again to the horizontal position. The exposure to light during the process of digging up the rhizomes and setting up in the apparatus has similar effects which die away after about 24 hours. After recovery from this initial light effect they continue to grow roughly horizontal if maintained in darkness or subjected to infra-red radiation only. If inverted (turned through 180°) the side which was lowermost during the inversion grows for about 2 hours relatively the more quickly, thus becoming convex; then a rapid straightening and bending in the opposite direction occurs. This is followed by further up-and-down movements which gradually die out and the rhizome then continues growing straight. Inversion for 10 minutes also evokes an essentially similar response, which begins about 20 minutes after the inverted rhizome has been returned to its original position. The important theoretical significance of this is discussed in the text. Displacement of rhizomes through angles other than 180° results in to-and-fro bendings in which the side of the rhizome which was uppermost before the stimulus of displacement first becomes convex. The rhizomes then turn gradually towards the horizontal plane. Rhizomes which have recovered from the initial light effect and are thus growing in darkness horizontally are caused to turn upwards when the air surrounding them is replaced by air+5 per cent, carbon dioxide. The theoretical significance of these findings is discussed.