Clockwise and Counterclockwise Spirality in the Phyllotaxy of Tobacco >

Only two species of tobacco, Nicotiana tabacum L. and N. rustica L., have found an important place in cultivation for chewing or smoking purposes. N, rustica is now of little importance in agriculture, having been supplanted by AT', tabacum except in parts of Poland, Union of Soviet Socialist Republics, Asia Minor, and India. In both of these species the fundamental arrangement of the leaves always follows a clockwise or a counterclockwise spiral course from the cotyledons to the inflorescence around the stem. A previous study (ly involving comparatively few tobacco plants indicated that the spirality of about half the plants was clockwise and that of the other half was counterclockwise. It was suggested, however, that the proportion of the plants of either type might be changed by breeding and rigid selection. To test this hypothesis the direction of the spirals of several thousand tobacco plants, including the progeny from selected clockwise and counterclockwise parents, was determined in 1944 and 1945. The spiral course always follows from older to younger leaves up the stem. To avoid confusion it is important to define precisely the clockwise and counterclockwise directions. In this paper, as in a previous one by the writer (1), the same procedure has been consistently followed in defining the clockwise and counterclockwise arrangements. If a string is wound around the stem to connect the consecutively younger leaves, the visible sections in the clockwise spirality will incline upward from right to left. In the counterclockwise spirality the visible sections will run up the stem from left to right. In tobacco one course or the other is normally followed throughout the entire leaf arrangement of the main stem of the plant. Occasionally a node may show a whorled arrangement of the leaves, but this is exceptional and in some instances appears to be due to the failure of the internode to elongate. The writer has observed on the main stem no instance of the reversal of the spiral from that which began above the cotyledons. The leaf arrangement of the branches and suckers, however, may show an entirely independent behavior with respect not only to the spiral course but also to the leaf arrangement itself as expressed in the Fibonacci system. The alternate arrangement of the leaves in tobacco, unlike that of many other plants, is