Phyllotaxis and Organogenesis in Ferns

Organ positions and pattern formation in the shoot apex

“…Although the models of Wardlaw (1949) and Snow (1931, 1933) propose different mechanisms for positioning leaf primordia, they can not be distinguished experimentally. In addition, models involving inhibitory gradients have existed for over 80 years (Schoute, 1913), yet a substance(s) controlling leaf positioning in plants has not been identified.…”

“…For example, leaf primordium 14 (black) will arise in a position between leaf primordia 9 and 11. center of the meristem and wide enough to contain a leaf primordium. Wardlaw (1949) applied a similar approach in his study of phyllotaxy in a fern, Dryopteris. Although the results of these experiments were essentially the same as those of Snow and Snow, Wardlaw concluded that physical space could not be the mechanism controlling leaf positioning in Dryopteris because the leaf primordia are so widely spaced that they are not in physical contact with each other.…”

“…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.…”

Development, Differentiation, and Yield

“…What evidence there is relates to the "determination" of lateral appendages during vegetative growth (Cutter, 1965). Operative experiments (Wardlaw, 1949;Sussex, 1954Sussex, , 1955 and organ culture (Steeves, 1962) suggest that the fate of a primordium initiated on a flank of the apical meristem is affected by influencespresumably chemical in nature-reaching it from the apical dome itself, and by interaction with other, neighboring primordia. Thus the youngest prospective leaf primordia produce centric structures when isolated from the growth cone; but later, after some decisive event or events occurring over a definable period of time, changes occur in the primordia which constrain them to develop in a leaf-like manner, expressing dorsiventral symmetry.…”

A Conserved microRNA Signal Specifies Leaf Polarity

“…Leaves of higher plants exhibit a varying degree of asymmetry along their adaxial/abaxial (upper/lower) axis. This asymmetry is thought to reflect inherent positional differences in the developing organ relative to the SAM (Wardlaw 1949). The adaxial/dorsal side of the leaf develops in close proximity to the stem cells in the SAM, whereas the abaxial/ventral side develops at a greater distance from the apex.…”