I. Experiments on Phyllotaxis. I. - The effect of isolating a Primordium

Abstract

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

Organ positions and pattern formation in the shoot apex

“…For example, our preliminary analysis of the Forever young mutant shows that leaf primordia contain abnormal numbers of cells and thus vary in size (Callos and Medford, unpublished). In such cases the order of primordia formation is determined by the degree of development of the vascular tissues (Esau, 1965;Snow and Snow, 1931). However, numerous samples should be examined to be certain that there is no effect on vascular development.…”

“…The center of the SAM is indicated by a centrally located black circle and leaf primordia are numbered in the order of formation. Drawn after Snow and Snow (1931).…”

“…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.…”

The forever young gene encodes an oxidoreductase required for proper development of the Arabidopsis vegetative shoot apex

“…However, in fey apices leaf pdmordia often have an abnormal number of cells, so primordia age can not be determined by size alone. As such, primordia age was determined by the degree of differentiation of the vascular tissues as described by Esau (1977) and in previous phyllotactic studies (Snow and Snow, 1931).…”

The shoot apical meristem: the dynamics of a stable structure

“…To explain the highly ordered arrangements of the primordia, different models have been proposed. On the basis of microsurgery experiments, Snow & Snow (1931, 1933, for instance, proposed that a new organ is initiated as soon as there is enough space for it. Wardlaw, in 1949, on the basis of similar experiments, proposed that existing primordia themselves produce signals that influence the positioning of new ones (for reviews, see Steeves & Sussex 1989;Lyndon 1998).…”