Usher Posluszny

Comparative Shoot Development and Evolution in the Lemnaceae

Shoot development in three duckweed (Lemnaceae) species (Spirodela polyrhiza, Lemna minor, Wolffia borealis) was studied and compared. Duckweed shoots are extremely reduced, and no evidence of a shoot apical meristem was seen during development. Duckweed shoots generally consist of a single unit (a frond) that is interpreted as a developmental hybrid (of leaf and stem origin) and may best be described and conceptualized as a metameric unit (an internode and associated node with its appendages). In the pocket(s) of older fronds, successive buds arise from meristematic tissue at the base of the previous bud. This bud development appears homologous to supernumerary bud development (multiple buds at a node) that occurs in Pistia stratiotes. First‐formed buds and pockets develop from tissue on the dorsal surface near the base of frond primordia. We suggest that the morphology of Lemnaceae plants can be understood as a result of the progressive simplification of shoots from Spirodela to Lemna to Wolffia, all of which have evolved from a Pistia‐like shoot system. Understanding the shoot architecture of duckweeds has helped to clarify the growth potential of these prolific plants.

Primary Vascular Patterns in the Vitaceae

Vitaceous shoots can be classified into five distinct architectural patterns based on a three‐node sequence of tendril and axillary bud presence. The relationship between two of the more commonly occurring patterns and their primary vasculature was examined. Cissus alata was chosen to represent pattern 5 (distichous phyllotaxy and continuous leaf‐opposed tendril/inflorescences) and Vitis riparia to represent pattern 4 (distichous phyllotaxy and a three‐node modular pattern of interrupted leaf‐opposed tendril/inflorescences). Both species show architectural dorsiventrality in that the prophyll of the first‐order axillary bud is ventral and vascular dorsiventrality in that all midvein leaf traces arise from ventral vascular sympodia. Both taxa have an even number of vascular sympodia, with four in C. alata and six in V. riparia. Leaf traces are multilacunar, with seven traces in C. alata and five in V. riparia. The leaf‐opposed tendril/inflorescences have the same vascular architecture as the axillary buds and are derived from the same vascular sympodia, although there is no evidence from this study that the tendrils represent a vertically displaced serial axillary bud. Vascular architecture reflects the underlying three‐node modularity of these shoot patterns in two ways: first, leaf traces in both species most commonly arise three nodes below their point of departure from the stem, and second, the number of internodes the axillary bud traces traverse is dependent on the position of the tendril within the shoot module in V. riparia (pattern 4). Vegetative characters such as shoot architecture and primary vascular pattern should prove useful in phylogenetic analyses of this architecturally unique family.

Evolution of the helobial flower

Abstract We propose a hypothesis for the derivation of flowers and reproductive structures in the Alismatidae (Helobiae) which is based on evolution of features of ‘flower’ and ‘inflorescence’ in different ways from an ancestral pre-floral condition with a multiaxial reproductive structure. We base this proposition on the existence of many cases of structures with attributes of both ‘flower’ and ‘inflorescence’. In one suite of cases the lateral axes appear to have become flower-like while the main axis has become an inflorescence axis; in another the axes has become inflorescence-like proximally and flower-like distally. Like other evolutionary plans, it can also be read in the reverse direction, and would then imply that features of ‘flower’ and ‘inflorescence’ had become re-partitioned in different ways in the group. We interpret as reduced the simpler forms of flower among the group.

Developmental Morphology of Reproductive Structures of Zostera and a Reconsideration of Heterozostera (Zosteraceae)

The Zosteraceae, a family of marine sea grasses that are aquatic monocotyledons, is presently composed of three genera: one dioecious genus, Phyllospadix, and two monoecious genera, Zostera and Heterozostera. The reproductive structures of all genera in this family are unique, consisting of a flattened spadix with flowers on one side, all enclosed within a spathe. Using techniques of epi-illumination light microscopy, SEM, and paraffin embedding, we examined the developmental morphology of the complex and unusual reproductive structures in Zostera and Heterozostera Floral development studies showed that retinacules (speculated to be modified perianth parts) arise first, followed by anthers consisting of two bisporangiate thecae joined by a connective, and finally the gynoecium, consisting of a unilocular ovary. The best interpretation of these reproductive structures is that they are male and female parts of a complex inflorescence. Based on reproductive and vegetative comparative studies, the similarities between the genera Zostera and Heterozostera do not warrant placing Heterozostera tasmanica into a distinct genus but do warrant reinstating Zostera tasmanica into the genus Zostera in the subgenus Heterozostera.

Developmental and morphological analyses of homeotic cytoplasmic male sterile and fertile carrot flowers

Floral development and morphology were observed for two homeotic cytoplasmic male sterile carrot lines and their isonuclear fertile maintainers. For one sterile line, W33A stamens are replaced by petal-like organs; for the other, W259A, both stamens and petals are replaced by green bract-like structures. Both isonuclear maintainers, W33B and W259B respectively, have stamens and white petals. The different sterile phenotypes result from the interactions of distinct nuclear genotypes with one sterility-inducing cytoplasm. Early stages of floral development were similar among all four lines; the third whorl primordia were radial while those in the second whorl were dorsiventral. However, the third whorl primordia were splayed outward in the sterile lines and inward in the fertile lines. Subsequently, radial anthers on filaments differentiated in fertile lines and dorsiventral hastate and cordate shaped structures appeared in W33A and W259A, respectively. In the mature flower, the third whorl organs were cordate in W33A and ovate in W259A. Based on epidermal cell morphology, the second whorl organs of the two sterile lines had characteristics of both petals and bracts, but of opposite degrees; cells of W33A and W259A were most similar to those of petals and bracts, respectively. The third whorl organs of the sterile lines had characteristics of their respective second whorl organs; however, structures of W33A also had filament-like cells and those of W259A were more bract-like than their respective second whorl organs. The cytoplasm affected when homeosis was manifested during development. Nuclear factors interacting with cytoplasm were most important for determining differentiation. The significance of cytoplasm to current models of nuclear-gene-controlled homeosis is discussed.

The Developmental Morphology of Leea guineensis. II. Floral Development

The floral development of Leea guineensis G. Don is described, using three-dimensional and histological observations. Inflorescences may be terminal or axillary. Axillary inflorescences, however, arise only in the axil of the uppermost leaf on the shoot in conjunction with a terminal inflorescence. Inflorescence branches are initiated spirally and are preceded by the formation of subtending bracts. Subsequent orders of branches arise as pairs of primordia, each at 90⚬ from the previous pair. The ultimate inflorescence pattern is a compound dichasium. Flowers are pentamerous. Sepals arise spirally. Petals arise simultaneously, alternate with the sepals, and are cucullate, valvate, and reflexed at anthesis. Stamens are petal-opposed. The anthers are connivent and hook over the floral disc prior to anthesis. At anthesis the anthers are reflexed and may appear extrorse. Pollen is tricolporate. The gynoecium arises as a ring primordium, from which three units are formed as the result of inward growth of three primary septa from the gynoecial wall. Two ovules form at the base of the septum of each unit, and subsequently three secondary septa form, effectively forming six locules at maturity. Ovules are bitegmic, anatropous, and crassinucellate. The ovary is half-inferior at maturity. Flowers are markedly protandrous. Fruits were not formed in the material observed.

Development of the axillary bud complex in Echinocystis lobata (Cucurbitaceae): interpreting the cucurbitaceous tendril

In the Cucurbitaceae, the tendrils, coiling organs used for climbing and mechanical support, are part of an axillary bud complex (ABC). Although the morphological nature of tendrils and the branching pattern of the ABC in the Cucurbitaceae have been much studied, their homology remains unresolved, with hypothesized candidates being the leaf, flower, stem, or stem-leaf combination. We used Echinocystis lobata as a model to study the early ontogeny of the ABC with epi-illumination microscopy and serial resin sections. The ABC produces four structures (proximal to distal, relative to the subtending leaf) as the result of two successive subdivisions: an inflorescence of staminate flowers, a solitary pistillate flower, a lateral bud, and a tendril. The first separates the tendril primordium from the continuation of the ABC, and the second separates the staminate inflorescence and the ABC. The pistillate flower apparently forms between the staminate inflorescence and the lateral bud. Because there is no subtending leaf during these subdivisions and the first lateral appendages in the resulting primordia arise in the same plane, we conclude that the tendril and other organs formed by the ABC are lateral branches of equal morphological value. This study is the basis for continuing comparative and functional morphological studies.

Influence of Age and Growth Rate on Radial Anatomy of Annual Rings of Thuja occidentalis L. (Eastern White Cedar)

Increment cores and stem disks were collected from Thuja occidentalis L. (eastern white cedar) and divided into young (<30 yr) and old (>300 yr) age classes. A further subdivision of the material based on growth rate (annual rings <0.5 mm versus annual rings ≥0.5 mm) was made. This allowed for analysis of the effect of tree aging independent of growth rate. Number of tracheids per annual ring, tracheid radial diameter, and ratio of latewood to earlywood did not vary with tree age. In contrast, the annual rings of both young and old slow-growing trees had fewer tracheids, decreased mean tracheid diameter (both earlywood and, to a lesser extent, latewood) and an increased proportion of smaller latewood tracheids. In fast-growing trees regardless of age, tracheid radial diameter was larger in earlywood and remained relatively constant (plateau phase) for most of the width of the annual ring. Radial diameter then declined sharply, before the initiation of latewood. In slow-growing trees, initial earlywood tracheid radial diameter was smaller and the plateau phase short or absent, and the decline in tracheid radial diameter for both fast- and slow-growing trees was not coupled with latewood production. It is proposed that plasticity of tracheid radial diameter is a safety feature that reduces the risk of embolisms for trees growing on xeric sites.

Inflorescence morphology and development in the basal rosid lineage Vitales

This review summarizes inflorescence developmental morphology in the grape order Vitales within a phylogenetic context. Inflorescences in the shrubby Leeaceae are terminal thyrses that appear leaf‐opposed once renewal growth begins. Plants of the Vitaceae are mainly tendrilled lianas and form five well‐defined clades. Inflorescences develop from the unique, non‐leafy, uncommitted primordium which arises opposite a leaf on the flank of the shoot apical meristem, and may mature into an inflorescence, tendril, or a combination of the two. The Ampelopsis‐Rhoicissus clade, Cissus antarctica group and Yua have a tendril/inflorescence, with cymose branching on the inflorescence axis. Inflorescences in the core Cissus clade and Cyphostemma‐Tetrastigma clade arise on a compressed axillary shoot, and leaf‐opposed tendrils arise on the main shoot, resulting in different branch orders. In Parthenocissus, tendrils are leaf‐opposed on long shoots, and inflorescences are leaf‐opposed on axillary branches on short shoots. In the Ampelocissus‐Vitis clade, the leaf‐opposed tendrils and thyrse inflorescences are of the same branching order, and are often combined. Terminal inflorescences such as in Leeaceae are common in angiosperms, in contrast to the unique leaf‐opposed tendril/inflorescence of Vitaceae. The further separation of the tendrils and inflorescences onto different orders of branching (core Cissus, Cyphostemma‐Tetrastigma), or separate short and long shoots (Parthenocissus), have allowed each component to optimize its own function. The Ampelocissus‐Vitis clade, however, has returned to a thyrse inflorescence, which may allow for larger or unusual lamellate inflorescences (Pterisanthes), and has reunited tendrils and inflorescences. Thus, each clade has developed its own set of inflorescence morphology.

Developmental Morphology of Reproductive Structures of Phyllospadix (Zosteraceae)

The floral structures of members of the Zosteraceae are unusual and difficult to interpret. Developmental sequences were reconstructed for both male and female inflorescences of two species of the dioecious genus Phyllospadix (P. scouleri and P. torreyi). Reproductive structures initiate in two rows along a flat tonguelike spadix. The retinacules (perianthlike structures) initiate first followed by anthers consisting of two bisporangiate thecae, which initiate joined by a connective as a single primordium in P. scouleri, but as two separate primordia in P. torreyi The gynoecia consisting of a unilocular ovary initiate last. Although the anthers initiate, they do not develop to maturity but remain on the spadix as small residual structures while the gynoecia mature. The developmental sequence is similar to that of the genus Zostera. and it is best to interpret the individual floral units as part of a complex inflorescence. Based on the specialized habitat and the presence of large well-developed retinacules and residual stamens on female inflorescences, Phyllospadix appears to be more derived than Zostera.