Polymerous Leaf Whorls in Vascular Plants: Developmental Morphology and Fuzziness of Organ Identities

Abstract

In vascular plants there are at least eight ways to develop polymerous whorls, i.e., whorls with four or more leaves. Six ways are presented and compared with literature to estimate organ identity (morphological significance) of the leaflike whorl members. New shoots (also seedlings) may start with dimerous or trimerous whorls. Then leaf number per whorl rises as follows: (1) Many taxa add more leaves per whorl continuously with increasing size of the apical meristem (e.g., Equisetum, Hippuris). (2) Taxa provided with interpetiolar stipules replace their stipules by leaves (e.g., Galium and allies). (3) Taxa with the capacity to form compound leaves shift basal leaflets around the whole node (e.g., Limnophila, probably also Ceratophyllum). Various whorled plants start shoot development with leaf inception along a helix, which is continued into the whorled region. Then polymerous whorls develop as follows: (4) Acacia longipedunculata forms helically arranged fascicles instead of single leaves before the production of complete whorls. (5) Acacia baueri and Acacia verticillata add supernumerary leaves between a first series of helically arranged leaves. (6) Hydrothrix produces an annular bulge around the node of each first-formed leaf. All additional leaves of a whorl arise on this annular bulge. Leaf identity of whorl members cannot be defined unequivocally in whorls with asynchronous (i.e., nonsimultaneous) development, dorsoventral distribution of lateral buds, and/or fewer vascular traces than leaves per node. It is heuristically stimulating to accept structural categories (e.g., shoot, leaf, leaflet, stipule) as fuzzy concepts, as developmental pathways that may overlap to some degree, leading to developmental mosaics (intermediates). For example, the whorled leaves of Utricularia purpurea resemble whole shoots, corroborating Arber's partial-shoot theory.

Acacia (wattle) and Cananga (ylang-ylang): from spiral to whorled and irregular (chaotic) phyllotactic patterns – a pictorial report

“…8). The author has published on various aspects of unusual phyllotaxes in acacias already before [11,13,16]. Stipules, as typical for Leguminosae, are present in the genus Acacia.…”

“…This species belongs to section Lycopodiifoliae, a species group which is characterized by more or less regular phyllode whorls [13,25]. It appears as monophyletic in molecular trees [18].…”

“…In Acacia adoxa, there are 6-to 10-merous phyllode whorls whereas in other members of the Lycopodiifoliae group the whorls may consist of up to 27 phyllodes and as many interpetiolar stipules in between, e.g., 10-19 phyllodes per whorl in A. hippuroides, 15-27 ones in A. longipedunculata [11][12][13]. There are never less than five to nine phyllodes per whorl in the Lycopodiifoliae group, as observable in A. galioides [25].…”

“…The following species are studied (alphabetic order, for details of the used Acacia specimens see also [13,16]). Voucher specimens for each species are deposited at Z/ ZT.…”

“…If readers are interested in fuzziness of structural categories, continuum approaches, and complementary perspectives to describe plant architecture they are referred to earlier publications of the Sattler school [11][12][13][14][15].…”