Experiments on bijugate apices

Abstract
The bijugate phyllotaxis and the shoot apices of seedlings of Dipsacus laciniatus are briefly described and discussed. The normal mean angle between successive pairs of leaves was found to be 72°.7. Experiments are reported which were designed to test a suggestion that in bijugates the deviation of each new pair from a position at right angles to that of the previous pair is due to an inhibition from the second older pair below. The leaves of the apical buds were removed as far as the youngest pair (the P 1 's) inclusive, or as far as the P 2 's or P 3 's only. After the operations the angles between successive pairs increased considerably, and the increases continued for five or even six plastochrons. Also some angles increased beyond 90° and reversed the spirals. It is pointed out that these results are difficult to reconcile with the inhibition hypothesis. Comparison is made with the bijugy of apices of Salvia horminoides , which can be explained junctionally as due to the contacts 1, 1 and 2 of the leaf bases. For the bijugy of Dipsacus an explanation is suggested in terms of the combined working of two factors. The first factor is junctional and depends on the shapes of the leaf bases with their united rims. The second factor is a pressure exerted by the upstanding parts of the P 2 's on the sides of the stem apex, which they were seen to touch. The results of the experiments can also be understood on this basis.
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