Theories concerning the causes of phyllotaxis may be divided conveniently into two groups. According to those of the first group, the arrangement of the leaves depends upon some unknown properties of the stem or the stem-apex. According to those of the second group, the positions in which the leaf-primordia arise are determined by the positions of the older primordia with which they make contact, or of any other members below them with which they may be in contact, such as cotyledons. Among the theories of the first group is that of SCHIMPER and BRAUN, who suggested that spiral sequences were due to a spiral growth impulse which travelled up the stem, the leaves arising at regular intervals along the course of this spiral

summary When in shoot apices of potatoes the presumptive area of I1, the next leaf due to arise, or of 12, was isolated from the growing‐point by a vertical cut, the isolated pieces regularly formed dorsiventral leaves, even when slips of mica were left in the cuts. These results differ from those of Sussex, who reported that he obtained radial leaves from nearly similar operations on potatoes, and interpreted them as due to interruption of an induction of dorsiventrality by the apex. The difference in the results cannot be explained by supposing that our isolated pieces were larger than his, since the evidence indicates that they were smaller. It may have been due to differences in the conditions of culture. In apices of Epilobium hirsutum on the other hand, I1 areas when isolated by vertical cuts developed often into radial leaves, though sometimes into dorsiventral leaves. So also did P, primordia if isolated on small pieces at early plastochron. These and other experiments on E. hirsutum make it probable that in that species the dorsiventrality of the leaves is induced by the apex. But this cannot be concluded with certainty, since the experiments show also that reduction of size contributes towards making an isolated I1 or P1 develop as a radial leaf, and it remains possible that this factor may have been effective alone. In earlier experiments on E. hirsutum P1, I, or I2 was narrowly confined between two vertical radial cuts, without being isolated from the apex, and several of these confined primordia or presumptive areas formed radial leaves, which are illustrated. But in more numerous later experiments this result was not obtained again. The determination of the dorsiventrality of leaves in these and other species is briefly discussed.

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Part 1.— Introduction. (1) Nature and Purpose of the operations, and Methods. Experiments on Lupinusalhus were reported previously in which the arrangement of the subsequent leaves was changed as a result of the isolation from the stem apex of the region from which the next leaf or the next but one was due to arise (Snow and Sn o w , 1931). The results led to the conclusion that each new leaf-primordium arises in the first space that attains both a certain minimum width and a certain minimum distance below the apex (p. 36), a conclusion which strongly supports v a n It e r so n ’s theory of phyllotaxis (1907). The purpose of the present experiments was to test this conclusion further by means of a different operation performed on the same plant. The conclusion of the previous paper will therefore be taken again as a working hypothesis, and an attempt will be made to explain the present results on the basis of it. In the present experiments a slight vertical cut was made in a radial plane through the area from which the next primordium was due to arise, or in other words through the presumptive area of Ix (for terminology see section 4). The cut sometimes extended downwards a little way below this presumptive area, but probably never reached more than a very little above it. This operation was considered to be a suitable method for testing the conclusions reached previously for the following reasons. Firstly, if the centres of primordia arise only in positions that allow room for their stipules, as the previous results indicated (p. 23), any primordia arising in contact with the sides of the wound should arise with their centres at some distance from it, and consequently at some distance from the normal position of the centre of Ix. Secondly, as a result of these displacements, the positions of the subsequent primordia should also be changed, if they arise in accordance with the working hypothesis