Phyllotaxis in two species ofRubia,R. akane andR. sikkimensis

Phyllotaxis and vascular course in the vegetative shoots ofRubia akane andR. sikkimensis were studied. Each node of both species has a whorl of four leafy members among which two are true leaves. Arrangement of the true leaves is not decussate but bijugate, i.e., opposite leaves are arranged spirally. Bijugy was ascertained not only by gross morphology but also by arrangement of primordia around the shoot apex and vascular course through several internodes. Divergence angle differed widely with internodes even within a single shoot and with shoots even in the internodes which are separated by a same number of nodes from the apex. Mean divergence angles obtained for five youngest internodes of some shoots were between 49.4° and 61.8° inR. akane and between 53.6° and 59.4° inR. sikkimensis. Young seedlings ofR. akane showed decussate phyllotaxis in the lowermost several internodes. In the internodes near the lower end of the bijugate part, the divergence angle was wider than in the upper internodes. The directions of the phyllotactic spirals in the main axis and the lateral branches were either homodromous or antidromous, and those in the oppositely paired branches also were either homo- or antidromous.     

MORPHOLOGICAL STUDIES OF THE NYMPHAEACEAE (SENSU LATO). XIII. CONTRIBUTIONS TO THE VEGETATIVE AND FLORAL STRUCTURE OF CABOMBA

Six species of Cabomba have been examined although the anatomy of the vegetative axes is based on the study of only C. caroliniana and C. palaeformis. A plant consists of an erect short shoot with decussate leaves which bears axillary flowering shoots and rhizomes. A rhizome bears decussate leaves and may also form axillary flowering shoots or turn upward and become a new short shoot. The phyllotaxies of the flowering shoots are proximately decussate or ternate (C. piauhyensis). The flowering shoots with decussate phyllotaxy change to 1/3 phyllotaxy distally; they bear axillary flowers proximally, and extra-axillary flowers distally. Flowering shoots with ternate phyllotaxy do not change distally but each produces first axillary and then extra-axillary flowers. Decussate vegetative axes and flowering shoots have four vascular bundles; ternate vegetative axes and flowering shoots have six vascular bundles, distantly paired into two or three vascular bundle-pairs, respectively. An elliptical vascular plexus occurs at each node. Each leaf receives one bundle-pair from one trace and each flower three bundle-pairs. A two-level receptacular vascular plexus occurs in flowers; the proximal, larger portion provides traces to perianth and stamens and the distal, smaller portion becomes carpellary traces. Each of the three sepals typically receives five branch traces from a basal principal trace, and each of the three petals receives, typically, three branch traces from a basal principal trace. Sepals and petals generally occur in a single, basally connate whorl. Each stamen receives one trace. Each stamen of three-stamen flowers is opposite a petal; each stamen of six-stamen flowers is aligned with an interval between a petal and adjacent sepal. Each staminal trace, which is just above the principal petal trace, in a three-petal flower, is frequently adnate to the latter trace. Each carpel receives one principal trace from the distal, small extension of the receptacular plexus, and each principal trace becomes three conventional veins of a carpel. Ovules may be borne directly over one of the veins or in any position between veins and are supplied by branches of the nearest vein or nearest two veins. All traces, ovular supply veins and the proximal portions of all veins are amphicribral. The several anatomical and morphological differences in vegetative axes and flowers between Cabomba and Brasenia suggest a greater taxonomic distance between the two genera than commonly supposed. It is suggested that extra-axillary flowers in 1/3 helical and ternate flowering shoots of Cabomba might be advantageous in preventing anthesis of flowers beneath peltate leaves. The aberrant position might be the initial evolutionary step toward what, in other nymphaeaceous genera, has shifted each flower to an adjacent helix. It is proposed that the zigzag stem accompanying the trigonal and sympodial flowering shoots may offer greater stability and floatability in water than the monopodial form. Several suggestions are offered for the variability of ovular positions: 1) the variability is a vestige of former laminar placentation in conduplicate carpels; 2) it is a vestige of a primitive condition antedating the current close association of ovules with ventral carpellary veins; 3) it is an early stage of evolution which might have terminated in laminar placentation and cantharophily, but which was replaced by a trend toward myophily.     

Phyllotaxis, phenology and architecture in Cephalotaxus, Torreya and c(Coniferales)

In Amentotaxus, Cephalotaxus and Torreya there is a regular seasonal alternation of foliage leaves and bud-scales, with foliage leaves largely preformed, i.e. initiated in the season before they expand. On most plagiotropic shoots phyllotaxis in the production of foliage leaves may be either bijugate ( Cephulotaxus, Torreya ) or decussate ( Amentotaxus ). In bijugate phyllotaxis successive leaf pairs originate at an angle of about 68° to each other, i.e. approximately one-half of the 'ideal' or Fibonacci angle of 137.5°. Secondary leaf orientation in Cephulotaxus and Torreya , by twisting of the leaf base, produces the dorsiventrality of plagiotropic shoots, whereas in Amentotaxus secondary orientation involves a twisting of the stemc as well as the leaf base. In Cephalotaxus cc condition is constant in the production of the numerous but imprecise number of bud-scales and in the production of foliage leaves. However, in Torreya the phyllotaxis changes from bijugate in the production of foliage leaves to decussate in the production of bud-scales, which are constant in number (about eight pairs). This allows a precise analysis of the biphasic production of leaf primordia in the seasonal cycle. The phyllotactic change in Torreya may not be the result of reported changes in shoot apex dimensions since Cephalotaxus , with its constant phyllotaxis, has a comparable seasonal change in apex dimensions. Information on architecture, chirality and cone morphology is also included.     

DIFFERENTIATION OF LATERAL SHOOTS AS THORNS IN ULEX EUROPAEUS

Ulex europaeus is a much-branched shrub with small, narrow, spine-tipped leaves and axillary thorn shoots. The origin and development of axillary shoots was studied as a basis for understanding the changes that occur in the axillary shoot apex as it differentiates into a thorn. Axillary bud primordia are derived from detached portions of the apical meristem of the primary shoot. Bud primordia in the axils of juvenile leaves on seedlings develop as leafy shoots while those in the axils of adult leaves become thorns. A variable degree of vegetative development prior to thorn differentiation is exhibited among these secondary thorn shoots even on the same axis. Commonly the meristems of secondary axillary shoots initiate 3–9 bracteal leaves with tertiary axillary buds before differentiating as thorns. In other cases the meristems develop a greater number of leaves and tertiary buds as thorn differentiation is delayed. The initial stages in the differentiation of secondary shoot meristems as thorns are detected between plastochrons 10–20, depending on vigor of the parent shoot. A study of successive lateral buds on a shoot shows an abrupt conversion from vegetative development to thorn differentiation. The conversion involves the termination of meristematic activity of the apex and cessation of leaf initiation. Within the apex a vertical elongation of cells of the rib meristem initials and their immediate derivatives commences the attenuation of the apex which results in the pointed thorn. All cells of the apex elongate parallel to the axis and proceed to sclerify basipetally. Back of the apex some cortical cells in which cell division has persisted longer differentiate as chlorenchyma. Although no new leaves are initiated during the extension of the apex, provascular strands are present in the thorn tip. Fibrovascular bundles and bundles of cortical fibers not associated with vascular tissue differentiate in the thorn tip and are correlated in position with successive incipient leaves in the expected phyllotactic sequence, the more developed bundles being related to the first incipient leaves. Some secondary shoots displayed variable atypical patterns of meristem differentiation such as abrupt conversion of the apex resulting in sclerification with limited cell elongation and small, inhibited leaves. These observations raise questions concerning the nature of thorn induction and the commitment of meristems to thorns.     

LEAF DEVELOPMENT AND PRIMARY VASCULAR ORGANIZATION IN SHOOTS OF ANISOPHYLLEA DISTICH A

Anisophyllea disticha is characterized by strong shoot dimorphism. Orthotropic shoots with helically arranged scale leaves produce tiers of plagiotropic shoots, while plagiotropic shoots are anisophyllous and bear dorsal scale and ventral foliage leaves arranged in a unique tetrastichous system. In this study we compare the patterns of leaf development and primary vascular organization in the two types of shoots. Orthotropic shoots have an open vascular system with five sympodia. Expansion of orthotropic shoot scale leaves occurs from P1 to P10–12, and leaf tissues mature precociously. Plagiotropic shoots have a closed vascular system with six sympodia. Leaves in ventral and dorsal orthostichies do not differ significantly in size until ca. P15, but ventral leaves are distinct histologically from the second node in an orthostichy, P4–6. Ventral foliage leaves have a diffuse plate meristem, and leaf expansion continues until ca. P30. Differentiation of ventral and dorsal leaf trace procambium parallels the divergent patterns of leaf expansion. These observations demonstrate the strong correlation among shoot symmetry, leaf development, and vascular differentiation within dimorphic shoots of one species.     

ESTABLISHMENT OF THE VASCULAR SYSTEM IN SEEDLINGS OF POPULUS DELTOIDES BARTR.

Seven seedlings ranging from 1 to 25 days old were embedded in Spurr's resin and serially sectioned at 1–2 μm. Sectioning extended from well above the apex downward to the hypocotyl base in the 1–day seedlings and to varying levels in the hypocotyl in the older seedlings. Procambial development was analyzed in its entirety for each seedling, and a composite two-dimensional diagram representing the procambial system of a 25-day-old seedling was prepared. Each cotyledon was served by a double-trace, one-half of which was derived from each of two embryonic bundles. The central traces serving the four primary leaves were in turn derived from the four cotyledonary bundles comprising the double traces. The procambial system serving the cotyledons and the four primary leaves approximated a decussate phyllotaxy. The central traces serving the secondary leaves were arranged in a helix that conformed at first to a 1/3 and then to a 2/5 phyllotaxy. Transitions to higher phyllotactic orders were systematic and reproducible, and they occurred in an orderly sequence in both the central and lateral leaf traces. The manner in which leaf traces diverged from parent traces to serve new leaf primordia provided for vascular redundancy. Thus, the entire vascular system was integrated into a highly functional whole.     

SHOOT VASCULAR SYSTEM AND PHYLLOTAXIS OF CASUARINA (CASUARINACEAE)

In species of Casuarina with multileaved whorls, each stem vascular bundle divides radially into two at the site of a leaf trace separation, and the same two bundles rejoin acropetally to where the trace supplies a leaf. Such divisions are divisions of a single vascular bundle, and the rejoining of bundles forms a single bundle. Proposals that the extant primary vascular systems of dicotyledons may have been derived as in conifers are incorrect in so far as Casuarina is concerned, or the system has evolved beyond that so far proposed for dicotyledons. Reasons are offered, however, for considering that fernlike leaf gaps are not present. Leaf traces supply leaves at the first nodes distal to their origins. The ways by which an increase or decrease of stem bundles occur are described. Phyllotactic patterns range from helical (rare) to whorled. In the embryo, where leaves occur decussately, of certain species with multileaved whorls, and in the shoot apices of species with tetramerous whorls, slight differences in the levels of leaf attachments and the bending of leaf traces indicate the probable evolution of extant whorled phyllotaxies from one or more helical arrangements. Stages in the evolution are suggested. The leaves in most species with multileaved whorls are in true whorls. The original periderm of branchlets lies internally to the internodal traces and chlorenchyma, but is otherwise external to the vascular system. It is concluded that each leaf originates at its level of separation from the axis despite several structural features suggesting that the leaf bases have become congenitally adnate to the stem.     

COMPARATIVE MORPHOLOGY OF THE PRIMARY VASCULAR SYSTEMS IN SOME SPECIES OF ROSACEAE AND LEGUMINOSAE

The structural patterns of the primary vascular systems in some species of Leguminosae and Rosaceae have been determined by tracing the longitudinal course of the vascular bundles in terminal stem segments. These systems are interpreted as consisting of sympodia. Each sympodium is composed of an axial bundle which is continuous through the length of the segment and from which arise trace bundles that supply leaves and axillary buds. A compact arrangement of vascular bundles seems to correlate with the woody habit. Regardless of the degree of compactness of the primary vascular system, the structural identity of the individual sympodia is maintained. The total number of vascular bundles at a particular level is related to the number of axial bundles in the system, the number of traces per leaf and per axillary bud, and the number of internodes traversed by the traces prior to entering a lateral appendage. Shrubs and trees have more vascular bundles than herbs. Data from this study and the literature indicate that the vascular system is predominantly of the open type in dicotyledonous plants which have helically arranged leaves and, further, that in such plants with a 3-trace, trilacunar nodal structure, the number of sympodia coincides with the number of orthostichies (which is also the denominator of the phyllotactic fraction). In open systems leaf gaps cannot be morphologically delimited. Because of the resemblance of the open type of angiosperm vascular system to that of certain gymnosperms, previously interpreted to have evolved from a protostele, we suggest that the eustele of angiosperms is homologous with the stele of gymnosperms. We believe, also, that angiosperms, like gymnosperms, are probably not characterized by leaf gaps of filicinean type. We provide, furthermore, a rationale for the view that the axial bundle of a sympodium is a cauline structure.     

The Pathway of Assimilate Flow from Source to Sink in Urtica dioica L., Studied with14C under Ambient Atmospheric Conditions

The contribution of individual vascular bundles of the stemto the flow of assimilates from a selected source leaf to thesink regions was investigated inUrtica dioica L., a plant witha decussate leaf arrangement. Two homologous sets of eight vascularstrands were recognized, arranged in mirror symmetry in thestem internodes. In each set, three of the bundles were identifiedas traces of one leaf merging into the vascular system of thestem one node below the origin of the leaf. The main bundleof a stem-half bifurcates at each end of the internode intotwo subdominant bundles, which combine in the next but one nodeto form the dominant bundle again. Each set of vascular strandsalso contains two minor bundles which pass more or less withoutinterruption through the whole stem. The uppermost mature source leaf (leaf number 5 as counted fromthe tip) was exposed to¹⁴CO₂in a closed gas circuit. The concentrationof the carbon-labelled CO₂was maintained at the ambient CO₂levelto maintain the natural source strength of the leaf. By theend of the usual nocturnal dark phase, carbon from the sourceleaf had been imported predominantly by sink leaves of the sameorthostichy. Lesser, but significant amounts of radiocarbonwere also incorporated into the sink leaves of the adjacenttwo orthostichies via the marginal leaf traces. In spite ofthe junction of the vascular strands in the nodes and an interfascicularconnection of the stem bundles, randomization of the photosynthatesfrom individual leaves was minimal in the vascular system ofthe stem in the upward direction, and also low in the flux tothe roots. Substantial amounts of radioactivity were also foundin the lately-formed xylem elements of the vascular strandsand their interfascicular connections, indicating active secondarygrowth. Assimilate distribution; source–sink connections; Urtica dioica ; vascular architecture     

ANOMALOUS SECONDARY GROWTH IN LIANAS OF THE BIGNONIACEAE IS CORRELATED WITH THE VASCULAR PATTERN

Vascular pattern and anomalous secondary growth were studied in shoots of Clytostoma callistegoides, a liana having two types of phyllotaxy, one decussate and the other whorled. In shoots with decussate phyllotaxy, typical of bignoniaceous lianas, the vascular pattern has four major vascular strands that extend continuously from internode to internode, whereas in shoots having a whorled phyllotaxy the pattern has six major vascular strands. The first unidirectional cambium segments which result in the anomalous secondary growth were initiated precisely opposite each of the major vascular strands in both types of shoots. It is concluded that positioning of unidirectional cambium segments responsible for anomalous growth is correlated morphogenetically with the vascular pattern.     

Morphological and anatomical studies inTribulus terrestris

Seedling morphology and vascular course inTribulus terrestris were studied. This species has no erect stem, but four buds appear immediately above the cotyledonary node and grow into prostrate shoots. They were determined to be the main axis of the seedling and the axillary branches of the earliest three foliage leaves, which arise very close to each other. All the leaves, including cotyledons, are vascularized with four bundles among which two are related to a single median gap. When two leaves are attached to one node, lateral traces to the opposed leaves are derived by bifurcation of a single bundle at either side of the stem. In the shoot with a series of alternate leaves, the median pair of traces to every other leaf are found on the same orthostichy. In the branch of which the first node bears no flower but an anisophyllous pair of leaves, the smaller leaf at the node was proven to be the first prophyll because its median traces are superposed by those to the leaf at the next node.     

MORPHOLOGY AND VASCULATURE OF THE LEAF OF POTATO (SOLANUM TUBEROSUM)

The leaf and stem of the potato plant (Solanum tuberosum L. cv. Russet Burbank) were studied by light microscopy to determine their morphology and vasculature; scanning electron microscopy provided supplemental information on the leaf's morphology. The morphology of the basal leaves of the potato shoot is quite variable, ranging from simple to pinnately compound. The upper leaves of the shoot are more uniform, being odd pinnate with three major pairs of lateral leaflets and a number of folioles. The primary vascular system of the stem is comprised of six bundles, three large and three small ones. The three large bundles form a highly interconnected system through a repeated series of branchings and arch-producing mergers. Two of the three large bundles give rise to short, lateral leaf traces at each node. Each of the small bundles in the stem is actually a median leaf trace which extends three internodes before diverging into a leaf. The three leaf traces enter the petiole through a single gap; thus the nodel anatomy is three-trace unilacunar. Upon entering the petiole, each of the laterals splits into an upper and a lower lateral. Whereas the upper laterals diverge entirely into the first pair of leaflets, the lower laterals feed all of the lateral leaflets through a series of bifurcations. Prior to their entering the terminal leaflet, the lower laterals converge on the median bundle to form a single vascular crescent which progresses acropetally into the terminal leaflet as the midvein, or primary vein. In the midrib, portions of the midvein diverge outward and continue as secondaries to the margin on either side of the lamina. Near the tip of the terminal leaflet, the midvein consists of a single vascular bundle which is a continuation of the median bundle. Six to seven orders of veins occur in the terminal leaflet.     

MORPHOLOGY AND ANATOMY OF THE LEAF OF COLEUS BLUMEI (LAMIACEAE)

Leaves of the Princeton and a variegated clone of Coleus blumei Benth. were examined with the light microscope to determine the course of their vasculature and the spatial relationship between the mesophyll, bundle sheath, and vascular tissues. In Princeton clone leaves two leaf traces enter the petiole at the node and quickly branch to form an arc of bundles which undergo further divisions as well as fusions in the distal half of the petiole. The anastomosing arc of bundles reaches its greatest complexity in the base of the midvein, where its lateral-most bundles unite and diverge outward to form secondary veins. As the midvein bundles continue acropetally, they gradually fuse more and divide less until only a single bundle remains, from which secondaries and smaller veins branch. Major (ribbed) veins include not only the midvein and secondaries but also tertiary and quaternary veins. Decreasing vein size is accompanied by increasing direct contact between vascular and photosynthetic tissues. Minor veins, which make up 86% of the total vein length, are completely surrounded by photosynthetic bundle sheaths and mesophyll consisting of palisade and spongy parenchyma. Statoliths occur in a layer of cells just outside the phloem of the petiole-midrib axis and secondary veins. Functional hydathodes are present at the apices of the marginal teeth. The overall organization of tissues in variegated leaves differs little in either the green or albuminous areas from corresponding (but always green) regions of Princeton leaves. Chloroplasts are lacking in mesophyll, bundle-sheath, and most guard cells of the albuminous region but are present in guard cells which are within 1 mm of green areas.     

Floral development of Urospatha: merosity and phylogeny in the Lasioideae (Araceae)

In this paper we study merosity in the genus Urospatha within the framework of a resolved phylogeny of the Araceae. We analyse how a transition from dimerous or tetramerous merosity to pentamerous or hexamerous merosity can occur developmentally in the Lasioideae. In Urospatha, initiation of floral primordia along the inflorescence is acropetal, while development of flowers is basipetal. This indicates the presence of two distinct phases in the development of the Urospatha inflorescence. The first phase corresponds to initiation of flowers and establishment of the phyllotactic pattern, and the second phase to differentiation of floral organs. Urospatha is characterized by the presence of trimerous, tetramerous, pentamerous and rarely hexamerous flowers. In all types of flowers, the stamens are closely associated and opposite to the tepals. Pentamerous flowers are formed by addition of a sector comprising a stamen and tepal. Likewise, in the case of hexamerous flowers, two sectors are added. In the Lasioideae, the increase in the number of tepals and stamens is linked with two developmental processes that have appeared independently in the subfamily: (1) addition of one or two stamen?Cpetal sectors (Anaphyllopsis and Urospatha), and (2) independent increase in the number of tepals and stamens on whorls, more or less organized and inserted in alternate position (Dracontium). Tetramerous whorls as they occur in basal Lasioideae would be homologous to two dimerous whorls from an evolutionary point of view.     

THE TENDRIL OF PARTHENOCISSUS INSERTA: DETERMINATION AND DEVELOPMENT

Tendrils on long shoots of Parthenocissus inserta occur in a regular pattern opposite the alternate distichous leaves at two successive nodes of each three nodes. Ontogenetic study shows that the tendril is initiated at the flank of the shoot apex during the second plastochron in an essentially axillary position. It is carried upward with growth of the internode above the axillant leaf and ultimately is situated opposite the next younger leaf. In a rhythmic pattern a different group of appendages is produced by the shoot apex at each node in the sequence of three. In acropetal order these are: at the tendrilless node the leaf subtends an axillary bud complex which in turn subtends a tendril; the leaf at the lower tendril-bearing node directly subtends a tendril, and the leaf at the upper tendril-bearing node subtends an axillary bud. Tendril primordia were not induced to develop as foliaceous shoots when cultured in vitro or in decapitation experiments, indicating that the meristem which becomes a tendril is determined early in its inception. Although built on a shoot pattern, the tendril is regarded as an organ sui generis with a possible relationship to the inflorescence. The morphological nature of the tendril is discussed in the light of theories postulated in the literature.     

Organization of the vascular system in the stems of Diplazium and Blechnum (Filicales)

Detailed analysis of the three-dimensional vascular organization in species of Diplazium and Blechnum indicates that these ferns possess reticulate (dictyostelic) vascular systems that closely reflect the helical phyllotaxis of the shoot. In each species, the vascular pattern shows a specific relationship to the phyllotaxis, so that the phyllotactic fraction can be determined by examination of the number of cauline vascular bundles (meristeles) in cross section of the stem. The number of meristeles in a cross section equals the denominator of the phyllotactic fraction, i.e., the number of foliar orthostichies on the stem. The same numerical relationship also exists in the eusteles of seed plants between the number of axial (sympodial) stem bundles and the phyllotaxis. There is a further parallel between the three-dimensional reticulate pattern of fern dictyosteles and the reticulate patterns that characterize some herbaceous dicotyledons. However, the hypothesized separate origins of seed plant eusteles and fern dictyosteles from protostelic precursors preclude any direct homologies between these similar patterns. The parallel evolution of presumably more physiologically efficient reticulate systems in herbaceous seed plants and in ferns that have only a primary plant body is noteworthy. The similar relationships between the primary stem vascular patterns and phyllotaxy in both ferns and seed plants further emphasize the likely similarity of the morphogenetic events that occur at the shoot apex in these taxonomically disparate groups.     

THE ABPHYL SYNDROME IN ZEA MAYS. I. ARRANGEMENT. NUMBER AND SIZE OF LEAVES

A range of phyllotactic variation in Zea mays has been obtained from progeny derived from a single ‘opposite-leaved’ plant. Some segregants exhibit a form of alternate phyllotaxy with poor separation between nodes, while others duplicate the original decussate condition. Numerous intermediate examples have also been observed. Some have both spiral and opposite arrangements on the same plant; others, which begin their ontogeny with the normal distichous arrangement, switch at different stages of maturity to spiral or decussate arrangement. Leaves from ABPHYL plants are up to one-half the width of comparable normal leaves although their length is similar. Since ABPHYL plants may have twice as many leaves as do normal siblings, total area and dry weight of leaf blades per ABPHYL plant are greater than in normal siblings. Leaf width of both ABPHYL and normal plants correlates well with the number rather than the width of epidermal cells.     

The architecture of Zeylanidium olivaceum (Podostemaceae) inferred from the structure of the primary shoots

Zeylanidium olivaceum (Podostemaceae-Podostemoideae) is the only crustose-rooted species of the genus that still develops prominent primary shoots from the seedling in addition to the secondary (root-borne) shoots forming the clonal plant body. The primary shoots are articulated into an up to 8.5 cm long and 4 mm thick stalk (hypocotyl) and a copiously foliated paint-brush-like shoot which is sympodially branched in the form of a helicoid cyme. The helicoid branching pattern indicates a transversal prophyll position, typical of the dicotyledons, but replaced in most other Podostemoideae by a median prophyll position. The short stems within the leafy head do not separate, but are fused to a dense aggregate (coenosome). Branches are mainly vegetative with a rosette of about 20 elongate subulate leaves. The primary shoots branch in the vegetative stage and thus differ from other Podostemoideae where ramification is confined to the floriferous shoots. The leaves adhere together at the base, forming an apical furrow-like hollow surrounding the shoot tip. The tiny shoot apex is one-layered, radially symmetrical, and develops leaf primordia in a decussate pattern. The erect primary shoots thus differ from the distichously foliated plagiotropic secondary shoots by the decussate phyllotaxis, and by the presence of more than 20 leaves on a shoot as compared to the about six leaves on the vegetative and floriferous secondary shoots. The features observed in the primary shoots are interpreted as primitive as compared to those of the secondary shoots. Z. olivaceum is thus characterised by heterobathmy, i.e., the occurrence of plesiomorphic (primary shoots) and apomorphic features (secondary shoots). The primary shoots exhibit primitive features that apparently have been lost in secondary and primary shoots of most other members of subfamily Podostemoideae.     

Dancing on the platform: Lability of floral organs of Beilschmiedia appendiculata (Lauraceae)

Floral characters are important for the systematics of the Lauraceae. However, structure and development of the flowers remain poorly known in the family. In this study, we observed the variation and early development of flowers of Beilschmiedia appendiculata, which belongs to the Cryptocarya clade of the family. The results indicate that the shoot apical meristems (SAMs) of the floral buds are enlarged and become a platform for the programmed initiation of the floral organs; floral organs develop basically in an acropetal pattern; phyllotaxis is whorled, initiation of floral primordia within a whorl is asynchronous; floral merosity is extremely variable, for example, dimerous, trimerous, tetramerous, dimerous plus trimerous, and trimerous plus tetramerous. In addition, this species has lost the innermost staminal whorl and glands are not closely associated with stamens of the third staminal whorl, which is unusual in the family Lauraceae. Our new observations broaden our knowledge of the variation of floral structure in Beilschmiedia and pose a fundamental question regarding the ecology underlying the lability of floral organs in B. appendiculata.     

Interrelations between Phyllotaxis, Leaf Development and the Primary-secondary Vascular Transition in Populus deltoides

The procambial system of Populus deltoides Bartr. ex Marsh.plants progressed from phyllotaxy in the cotyledon stage throughthe phyllotactic orders 3/2;5/13. The nodal position at whicheach of these phyllotactic transitions occured was determinedby anatomical analyses; they were found to be remarkably consistentin a large population of young plants. The data were used todiagrammatically reconstruct the procambial system of a typical16 leaf plant. Because all plant parts grew continuously anduninterruptedly, it was not possible to verify the positionsof the phyllotactic transitions by morphological criteria. However,several measured parameters (the number and lengths of primordiawithin the terminal bud, the plastochron interval, and the numberof leaf traces with birefringent xylem elcments) attained constantvalues following establishment of the 5/13 phyllotaxy, suggestingthis to be the stable phyllotactic order for the species. Althoughbud size continued to increase in plants exhibiting 5/13 phyllotaxy,it could be accounted for by the increased number and size ofbasipetal subsidiary bundles in the procambial leaf traces.It was suggested that these phyllotactic transitions in theprocambial system are programmed in the plant to occur at ratherspecific stages of ontogeny. The process is mediated by theolder leaves and it is therefore modified by plant vigour. Locationof the primary-secondary vascular transition zone was also relatedto the order of phyllotaxy. It advanced acropetally in the stemin close association with leaf maturation, but this associationwas further influenced by plant vigour. Populus deltoides Bartr. ex Marsh., cottonwood, vascular anatomy, phyllotaxis, leaf growth, xylem