IX. Experiments on Phyllotaxis. II. — The effect of displacing a primordium

Abstract

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

Fibonacci or quasi-symmetric phyllotaxis. Part I: why?

“…Although Hofmeister's statement is far from a mechanistic explanation of phyllotaxis, it conveys the essential idea that the position of new primordia is determined by the position of pre-existing primordia as though their presence at the periphery of the meristem inhibited the formation of primordium in their close vicinity. In the decades that followed, various types of ablation experiments have confirmed the inhibitory effect that older primordia have on the initiation of new primordia [16,19].…”

INCOMPOSITA: a MADS-box gene controlling prophyll development and floral meristem identity inAntirrhinum

“…According to the theory of the 'first available space', based on microsurgical manipulations, the timing and positioning of organ initiation is regulated by the availability of the minimal free area on the meristem surface, at a minimal distance from the summit and from pre-existing primordia (Snow and Snow, 1931;Snow and Snow, 1933). In other interpretations, space itself is not decisive; a new primordium emerges at the position of weakest inhibition where the most recently formed primordium is the strongest source of inhibition (Tooke and Battey, 2003;Wardlaw, 1949).…”

Organ positions and pattern formation in the shoot apex

“…The cells that give rise to the leaf primordia apparently have some mechanism by which they interpret their position. A wide variety of evidence suggests that the positions where the leaf primordia form are influenced by the center of the SAM as well as previously formed leaf primordia (Marc and Hackett, 1991;Mitchison, 1977;Richards, 1951;Schoute, 1913;Snow and Snow, 1931, 1933, 1952Wardlaw, 1949;Young, 1978). Hence, the study of phyllotaxy provides a system by which pattern formation can be studied in plants.…”