THE MOVEMENTS OF THE LEAVES OF THE COMPASS–PLANT LACTUCA SCARIOLA

Abstract

Among the various leaf movements which have been studied, very little attention has been paid to the phototropic movements of the leaves of the so-called compass-plants. In the older American literature the phenomenon of the north-south direction of the leaves of Silphiutm laciniatum is often mentioned (Bay, 2), but little experimental work has been done with this plant. Many investigators (Erman, 3; Stocker, I2; Karsten, 9; Schanderl, 8) have considered these movements merely from an ecological point of view, the physiological basis of the movement being ignored. According to Stahl (i i), who was the first to study the leaf movements of Lactuca scariola, the leaves are diaphototropic, so that they tend to bring their laminae perpendicular to the sunlight. Stahl assumed that the growth conditions were favorable only during the early morning and late evening, that is, only the east and west sun would have an effect on the movements, with the result that all the laminae should point north and south. According to Mayer (5), however, the leaves twist in such a way as to place their laminae in the position in which they are equally illuminated from both sides. In nature, therefore, the leaves should only be in equilibrium if the laminae were pointing north and south. The occurrence of torsions is explained by Mayer by assuming that, if a leaf is unequally illuminated from both sides, the result will be two curvatures at different points of the leaf-base, which would combine to produce a torsion. The same explanation is given by Erman for the movements of the leaves of Iris pseudacoruts. Seybold (9), who studied the torsions in the leaves of Lactutca scariola, was able to prove that the leaves of plants rotating on a clinostat were still twisted. He was, however, unable to detect the northsouth orientation of the leaves, probably on account of the unfavorable weather conditions. As the conditions in Southern California are very favorable for these movements, I was able to study them further. The species used was Lactuca sctriola, which grows abundantly in waste places. The plants were dug out and grown in pots; some experiments were also carried out with plants growing in the field. In the case of a plant grown in the open field, all the leaves point distinctly north and south (text fig. 5). Closer inspection of the leaves shows that these positions are obtained either by torsions of the leaf base or by simple curvatures. The leaves inserted at the north and south sides of the