DOES SHADE PROLONG JUVENILE DEVELOPMENT? A MORPHOLOGICAL ANALYSIS OF LEAF SHAPE CHANGES IN CUCURBIT A ARGYROSPERMA SUBSP. SORORIA (CUCURBITACEAE)

Several studies have concluded that shade extends the juvenile phase of plant development based on the prolonged production of juvenile-looking leaves along the shoot. Until now, the alternative hypothesis that leaves produced in shade converge in shape with more juvenile leaves through plastic responses of individual leaves has not been investigated. The literature has shown that differences in shape among leaves in a heteroblastic series are manifest very early in development, often at or near inception, whereas divergence in development between sun and shade leaves does not become apparent until considerably later. This study is the first to distinguish between these alternatives by comparing the developmental morphology of young leaves of the heteroblastic plant Cucurbita argyrosperma subsp. sororia. Differences in shapes of mature leaves along the shoot in sun and shade were quantified in terms of leaf area, perimeter, and shape using truss analysis. Developmental morphology from initiation through expansion was examined for representative transition and for later (adult) leaves using scanning electron microscopy and allometry. Determinants of shape established very early in development were the same for leaves at the same position grown in sun and shade. Differences in morphology between sun and shade leaves at the same position did not arise until these leaves reached lamina lengths greater than 1,000 μm. Thus, the less-lobed, more juvenile looking leaf produced at later positions in the shade arose through later developmental responses of individual leaves to shade, rather than through a prolonged phase of juvenile development.     

MORPHOLOGICAL CHANGES ACCOMPANYING THE TRANSITION FROM JUVENILE (ATMOSPHERIC) TO ADULT (TANK) FORMS IN THE MEXICAN EPIPHYTE TILLANDSIA DEPPEANA (BROMELIACEAE)

Two distinct morphological forms characterize the ontogenetic development of the epiphytic bromeliad Tillandsia deppeana. Juveniles are characterized by a non-impounding rosette of small, linear leaves covered with elaborate trichomes possessing a 4 + 8 + 16 + 64 shield cell pattern. The broader transitional leaves, which form an impounding rosette prior to the initiation of true adult leaves, also possess trichomes with the 4 + 8 + 16 + 64 cell pattern. Adult individuals have large, broad leaves with overlapping, sheathing bases which impound water and debris. These leaves have trichomes with shields exhibiting a 4 + 8 + 32 cell pattern. Trichome density is fairly uniform in the juvenile leaves with trichomes covering 100% of leaf surfaces, whereas in the adult leaves density is high at the base and diminishes significantly toward the apex. Stomatal density of both juvenile and adult leaves increases from the base to the apex, although this is most pronounced in the adults. Stomata in the adults are also arranged in longitudinal series parallel and abaxial to parallel rows of mesophyll tissue. The results of this study indicate that juveniles of T. deppeana are more similar morphologically to adult atmospheric-type tillandsioid species than to the tank-forming adults into which they eventually develop.     

EVOLUTIONARY ENDOCRINOLOGY OF JUVENILE HORMONE ESTERASE: FUNCTIONAL RELATIONSHIP WITH WING POLYMORPHISM IN THE CRICKET,GRYLLUS FIRMUS

The existence, nature, and physiological consequences of genetic variation for juvenile hormone esterase (JHE) activity was studied in the wing-polymorphic cricket, Gryllus firmus. Hemolymph (blood) JHE activity was sixfold lower in nascent short-winged (SW) females, relative to nascent long-winged (LW) females during the last juvenile stadium (stage). Morph-associated genetic variation for JHE activity had two causes, variation in loci: (1) regulating whole-organism enzyme activity; and (2) controlling the degree to which JHE is secreted into the blood Reduced JHE activity in nascent SW-selected individuals was associated with reduced in vivo juvenile hormone catabolism. This suggests that variation in JHE activity during juvenile development may have important physiological consequences with respect to the regulation of blood levels of juvenile hormone and consequent specification of wing morph. This is the first definitive demonstration of genetic variation for hormonal metabolism in any insect and a genetic association between hormone metabolism and the subsequent expression of morphological variation (wing morph). However, we have not yet firmly established whether these associations represent causal relationships In contrast to the clear association between JHE activity and wing morph development, we observed no evidence indicating that variation in JHE activity plays any direct or indirect role in causing the dramatic differences in ovarian growth between adult wing morphs. Variation in JHE activity also does not appear to be important in coordinating the development of wing morph with the subsequent expression of reproductive differences between adult morphs. Finally genetic variation for the developmental profiles of JHE activity during juvenile and adult stages are remarkably similar in three Gryllus species. This suggests that genetic correlations between JHE activities during different periods of development, which underlie these activity profiles, have been conserved since the divergence of the three Gryllus species.     

Ontogeny of sexual dimorphism and phenotypic integration in heritable morphs

In this study we investigated the developmental basis of adult phenotypes in a non-model organism, a polymorphic damselfly (Ischnura elegans) with three female colour morphs. This polymorphic species presents an ideal opportunity to study intraspecific variation in growth trajectories, morphological variation in size and shape during the course of ontogeny, and to relate these juvenile differences to the phenotypic differences of the discrete adult phenotypes; the two sexes and the three female morphs. We raised larvae of different families in individual enclosures in the laboratory, and traced morphological changes during the course of ontogeny. We used principal components analysis to examine the effects of Sex, Maternal morph, and Own morph on body size and body shape. We also investigated the larval fitness consequences of variation in size and shape by relating these factors to emergence success. Females grew faster than males and were larger as adults, and there was sexual dimorphism in body shape in both larval and adult stages. There were also significant effects of both maternal morph and own morph on growth rate and body shape in the larval stage. There were significant differences in body shape, but not body size, between the adult female morphs, indicating phenotypic integration between colour, melanin patterning, and body shape. Individuals that emerged successfully grew faster and had different body shape in the larval stage, indicating internal (non-ecological) selection on larval morphology. Overall, morphological differences between individuals at the larval stage carried over to the adult stage. Thus, selection in the larval stage can potentially result in correlated responses in adult phenotypes and vice versa.     

Leaf shape variation and herbivore consumption and performance: a case study with Ipomoea hederacea and three generalists

The effect of leaf shape variation on plant-herbivore interactions has primarily been studied from the perspective of host seeking behavior. Yet for leaf shape to affect plant-herbivore coevolution, there must be reciprocal effects of leaf shape variation on herbivore consumption and performance. We investigated whether alternative leaf morphs affected the performance of three generalist insect herbivores by taking advantage of a genetic polymorphism and developmental plasticity in leaf shape in the Ivyleaf morning glory, Ipomoea hederacea. Across four experiments, we found variable support for an effect of leaf shape genotype on insects. For cabbage loopers (Trichoplusia ni) and corn earworms (Helicoverpa zea) we found opposing, non-significant trends: T. ni gained more biomass on lobed genotypes, while H. zea gained more biomass on heart-shaped genotypes. For army beetworms (Spodoptera exigua), the effects of leaf shape genotype differed depending on the age of the plants and photoperiod of growing conditions. Caterpillars feeding on tissue from older plants (95 days) grown under long day photoperiods had significantly greater consumption, dry biomass, and digestive efficiency on lobed genotypes. In contrast, there were no significant differences between heart-shaped and lobed genotypes for caterpillars feeding on tissue from younger plants (50 days) grown under short day photoperiods. For plants grown under short days, we found that S. exigua consumed significantly less leaf area when feeding on mature leaves than juvenile leaves, regardless of leaf shape genotype. Taken together, our results suggest that the effects of leaf shape variation on insect performance are likely to vary between insect species, growth conditions of the plant, and the developmental stage and age of leaves sampled. Handling editor: May Berenbaum.     

MORPHOLOGY OF MARSILEA VESTITA. I. ONTOGENY AND MORPHOLOGY OF THE SUBMERGED AND LAND FORMS OF THE JUVENILE LEAVES

During their ontogeny, the primordia of the juvenile leaves of Marsilea plants in sterile culture develop 1, 2 or 4 marginal meristems, and these, in turn, contribute cells to the young leaf by anti- and periclinal cell divisions. The final leaves are unifid, bifid, or quadrifid, depending on how many marginal meristems develop, and this is determined early in the ontogeny of the leaf. The mechanism which determines whether or not a marginal meristem develops may fluctuate, as shown by the existence of trifid leaves. Two forms of juvenile leaves are produced, those in a liquid medium, which in many respects resemble the adult quadrifid submerged leaves, and those on a solid medium, which in many respects resemble the adult land leaves.     

THE EFFECT OF SUGARS AND APPLIED CHEMICALS ON HETEROBLASTIC DEVELOPMENT IN IPOMOEA PURPUREA GROWN IN ASEPTIC CULTURE

Seedlings of Ipomoea purpurea were grown in aseptic culture in order to investigate more directly than had been possible in pot culture the effect of carbohydrate and protein nutrition on heteroblastic development which in this species involves a change from entire juvenile to lobed adult leaves. The effect of sugars was to accelerate the development of the adult leaf form while casein hydrolysate retarded it. The growth retardants, AMO-1618 and CCC, also promoted the formation of the adult leaf. These results are discussed in the light of current views on the factors controlling heteroblastic development.     

THE EVOLUTION OF THRESHOLD TRAITS: A QUANTITATIVE GENETIC ANALYSIS OF THE PHYSIOLOGICAL AND LIFE-HISTORY CORRELATES OF WING DIMORPHISM IN THE SAND CRICKET

Many traits are phenotypically discrete but polygenically determined. Such traits can be understood using the threshold model of quantitative genetics that posits a continuously distributed underlying trait, called the liability, and a threshold of response, individuals above the threshold displaying one morph and individuals below the threshold displaying the alternate morph. For many threshold traits the liability probably consists of a hormone or a suite of hormones. Previous experiments have implicated juvenile hormone esterase (JHE), a degratory enzyme of juvenile hormone, as a physiological determinant of wing dimorphism in the crickets Gryllus rubens and G. firmus. The present study uses a half-sib experiment to measure the heritability of JHE in the last nymphal stadium of G. firmus and its genetic correlation with fecundity, a trait that is itself genetically correlated with wing morph. The phenotypic and genetic parameters are consistent with the hypothesis that JHE is a significant component of the liability. Comparison of sire and dam estimates suggest that nonadditive effects may be important. Two models have been proposed to account for the fitness differences between morphs: the dichotomy model, which assumes that each morph can be characterized by a particular suite of traits, and the continuous model, which assumes that the associated fitness traits are correlated with the liability rather than the morphs themselves. The latter model predicts that the fitness differences will not be constant but change with the morph frequencies. Variation in fecundity and flight muscle histolysis are shown to be more consistent with the continuous model. Data from the present experiment on JHE are inconclusive, but results from a previous selection experiment also suggest that variation in JHE is consistent only with the continuous model.     

Leaf-shape variation of Paederia foetida in Japan: reexamination of the small, narrow leaf form from Miyajima Island

Variations in Paederia foetida L. leaf shape were examined to evaluate the taxonomic validity of the small, narrow leaf form of P. foetida f. microphylla Honda from Miyajima Island, Honshu, Japan. There is considerable variation in P. foetida individuals in terms of leaf size and leaf index (leaf length:leaf width ratio). On Miyajima Island, some individuals have narrow leaves with a high leaf index value, a phenotype represented by the type specimen of P. foetida f. microphylla, and some do not. Given that the leaf size of individuals from Miyajima Island is smaller than that of individuals from other localities in Japan, and that the small leaf phenotype is stable even under cultivation, P. foetida f. microphylla is classified as the form having the smallest leaf size. Anatomical examination of leaf blades revealed that the large variation in leaf size was attributable to variation in the number of leaf cells but not to differences in cell size or cell shape. Based on these results, we discuss the endemism of P. foetida f. microphylla.     

THE MORPHOLOGICAL EFFECTS OF PROTEIN SYNTHESIS INHIBITION IN MARSILEA

Marsilea vestita and M. drummondii were grown in sterile cultures to which concentrations of the protein synthesis inhibitors, 2-thiouracil (10 mg/liter) and 5-fluorouracil (1 mg/liter) had been added. When young sporelings are grown in a solution of thiouracil at optimum concentration, there is an inhibition of the rate of leaf formation, a retardation of the leaf heteroblastic series, and all leaves develop as land forms. When thiouracil is added to plants which are already producing typical adult, quadrifid leaves, the effects depend on whether the treated plants are water or land forms. Plants which are typically water forms convert to land forms. After treatment successive leaves develop typical sunken stomata on both leaf surfaces. The tissues of the rhizome, root and petiole are more compact and, in general, the cells of the plant have thicker walls. Vascular patterns are not changed, though the size of the rhizome, root and petiole may be reduced. Plants which are typically land forms are less affected than the water forms, but they show a small reduction in apex volume and an apparent reversion of the leaflet number from the typical quadrifid leaf to a trifid, bifid, or single lamina condition. In both land and water forms apical dominance may be broken by treatment with 10 mg/liter thiouracil or 1 mg/liter fluorouracil and numerous lateral branches develop. Higher concentrations (15–25 mg/liter of thiouracil) may result in abnormal development of lateral axillary buds, petiole bases and leaflets. The meristems of the plant are differentially sensitive to thiouracil; leaflet meristems are most sensitive, the root meristems are the least sensitive. It appears that a true reversion to juvenile leaf development need not occur even though protein synthesis and the volume of the apex are reduced. The development of the land or water form in Marsilea appears to depend on rate of growth. Hence inhibition of the growth of typical water forms, through inhibition of protein synthesis, causes a shift in development toward the morphology typical of land forms.     

STRUCTURE AND DEVELOPMENT OF LEAVES IN CARLUDOVICA PALMATA (CYCLANTHACEAE) WITH REFERENCE TO OTHER CYCLANTHACEAE AND PALMAE

The adult leaf of Carludovica palmata consists of a plicate lamina, adaxial hastula, petiole, and sheath. The leaf is unusual in the angiosperms because about two-thirds of the apical meristem is utilized in its initiation. The adult leaf requires about 4–5 plastochrons to mature. Shortly after its initiation the adult leaf and apical meristem collectively appear pyramid-shaped and various parts of the mature adult leaf may be traced back to particular portions of the pyramid. Plications develop by differential growth within the lamina, not by splitting of leaf tissue. Quantitative studies indicate that certain regions of the developing adult leaf elongate more rapidly or slowly than other regions depending upon the stage of leaf development. The adult leaf of C. palmata develops differently from those of previously studied palms in various ways. It therefore appears less justifiable to consider the superficial similarity between the adult leaves of various Cyclanthaceae (particularly those of Carludovica sensu strictu) and those of fan palms as evidence of especial affinity between the Cyclanthaceae and Palmae. Juvenile leaves of C. palmata differ from adult leaves both in their mode of origin and appearance at maturity. The juvenile leaf appears homologous to the entire adult leaf.     

GEOGRAPHIC VARIATION AND HETEROCHRONY IN TWO SPECIES OF COWRIES (GENUS CYPRAEA)

Geographic variation in the marine, Indo-Pacific cowry, Cypraea caputserpentis, involves clinal variations that parallel the ontogenetic development of adult shell characteristics. Cypraea caputdraconis, a closely related species endemic to Easter Island and Sala y Gómez, is morphologically similar to juvenile C. caputserpentis. Using multivariate measures of size and shape, I examine these patterns as a possible outcome of heterochrony, or changes in the timing of developmental events in ontogeny. Whorl-expansion rates of juvenile shells are significantly higher in C. caputdraconis when compared to C. caputserpentis and are negatively correlated with surface seawater temperatures among populations of C caputserpentis. High expansion rates, often associated with slow growth, result in a delay in the onset of lateral callus development and subsequent paedomorphosis. Ontogenetic trajectories calculated from growth series of adult and preadult shells indicate that paedomorphosis results from the combined effects of neoteny and post-displacement. Paedomorphosis among cowries may result from the advantages of larger body size relative to shell size under reduced predation intensities and associated increases in fecundity.     

Shoot morphology of Aucuba japonica incurred by anisophylly: ecological implications

Anisophylly, having leaves different in size and/or shape, was quantified in adult Aucuba japonica and simulations were carried out to evaluate the effects of anisophylly on the extent of self-shading at the single-shoot level as well as at the whole-canopy level. Clear anisophylly was observed in the individual after switching from the single-stemmed juvenile stage to the multi-stemmed adult stage. In such plants, leaf area in the canopy abruptly increased. The effective display of adult foliage involved a variety of morphological changes in addition to anisophylly, most prominently reduction in leaf size compared to juveniles. The simulation results indicate that diversity of leaf size and shape is an effective means of minimizing self-shading as well as allowing the efficient exploitation of a larger canopy volume in adult plants. Anisophylly also increased the biomass use efficiency of individual plants at maturity. Taken together, having diverse leaf forms is superior to having a single leaf form for maximizing area acquisition and for efficiently filling the acquired area. We therefore conclude that the anisophylly expressed in A. japonica is adaptive.     

WING SHAPE VARIATION ASSOCIATED WITH MIMICRY IN BUTTERFLIES

Mimetic resemblance in unpalatable butterflies has been studied by evolutionary biologists for over a century, but has largely focused on the convergence in wing color patterns. In Heliconius numata, discrete color‐pattern morphs closely resemble comimics in the distantly related genus Melinaea. We examine the possibility that the shape of the butterfly wing also shows adaptive convergence. First, simple measures of forewing dimensions were taken of individuals in a cross between H. numata morphs, and showed quantitative differences between two of the segregating morphs, f. elegans and f. silvana. Second, landmark‐based geometric morphometric and elliptical Fourier outline analyses were used to more fully characterize these shape differences. Extension of these techniques to specimens from natural populations suggested that, although many of the coexisting morphs could not be discriminated by shape, the differences we identified between f. elegans and f. silvana hold in the wild. Interestingly, despite extensive overlap, the shape variation between these two morphs is paralleled in their respective Melinaea comimics. Our study therefore suggests that wing‐shape variation is associated with mimetic resemblance, and raises the intriguing possibility that the supergene responsible for controlling the major switch in color pattern between morphs also contributes to wing shape differences in H. numata.     

DOSAGE EFFECTS OF THE LANCEOLATE GENE IN TOMATO

In contrast to the odd-pinnately compound leaf of the normal (+/+) tomato plant (Lycopersicon esculentum), the single-gene mutant lanceolate (La/+) generally has a simple leaf. Lanceolate plants, also, have small fruits and flowers, weak apical dominance, and exhibit variation in the position and fusion of cotyledons. Homozygous mutants (La/La) appear in 3 different phenotypes, 1 of which, narrow, has narrow simple leaves, sterile inflorescences, and extremely weak apical dominance. The other 2, modified, and reduced, lack an organized shoot. After selfing La/ + plants for 9 generations, autotetraploids were produced with the aid of colchicine. In addition, several triploid plants arose spontaneously. The study of diploid, triploid, and tetraploid material with various proportions of the La allele revealed in many characters a graded series as a function of the La dosage. With increasing La dose, there was a gradual reduction in: (1) total leaf length; (2) the number and size of primary and secondary lateral leaflets; (3) the number and size of marginal lobes of the terminal leaflet, associated with an increase in the proportional length of the terminal leaflet. Many leaves were found with the basal lobes of the terminal leaflet resembling incompletely separated lateral leaflets. The differences in leaf shape between different genotypes came about before the leaf primordium was 3 mm long. There was a progressive delay in the initiation of lateral primordia with increasing La dosage. It is proposed that the gradual changes from compound to simple leaves with increasing La dosage are produced by successively greater restrictions of meristematic activity after the terminal leaflet is formed. With increasing proportion of La alleles, the reproductive structures showed: (1) a decrease in the number of flowers per inflorescence; (2) a decrease in the length of the sepals; (3) an increase in the proportion of flowers with dialytic anthers. Dialytic anthers had narrow adaxial lobes and were frequently twisted along their main axes. The common denominator for most trends affected by the La allele seems to be a general reduction of growth, but more so in lateral than in longitudinal growth. Histological data suggest that the reduction in lateral growth is mainly brought about by a reduction of cell division in lateral meristems.     

NATURAL SELECTION ON BILL CHARACTERS IN THE TWO BILL MORPHS OF THE AFRICAN FINCH PYRENESTES OSTRINUS

Evidence for natural selection on seven bill and body characters is examined in the two bill morphs of the African estrildid finch Pyrenestes ostrinus. Two regression methods are used in examining natural selection in association with survivorship: a parametric (Lande and Arnold, 1983) and a non-parametric (Schluter, 1988) method. Selection was estimated in adult males, females and juveniles over a four-year period in a population in south-central Cameroon. Selection was common among groups but patterns differed and depended on the method used in detecting selection. The non-parametric method revealed evidence for disruptive selection occurring on bill width and is explained within the context of known feeding efficiencies and the hardness of important seeds in finch diets. Directional selection was common on bill characters in all groups, but infrequent on other characters. There was no evidence of selection on generalized size or shape characters. Selection on bill characters was common across groups despite low annual variation in rainfall. This contrasts with studies of Galápagos finches in which selection is frequently associated with dramatic changes in food supply caused by high variance in annual rainfall. Patterns of selection on bill traits in P. ostrinus also differ from those in song sparrows and Galápagos finches by exhibiting evidence for natural selection on all bill dimensions.     

A comparative analysis of leaf shape of wheat, barley and maize using an empirical shape model

Background and Aims

The phenotypes of grasses show differences depending on growth conditions and ontogenetic stage. Understanding these responses and finding suitable mathematical formalizations are an essential part of the development of plant and crop models. Usually, a marked change in architecture between juvenile and adult plants is observed, where dimension and shape of leaves are likely to change. In this paper, the plasticity of leaf shape is analysed according to growth conditions and ontogeny.

Methods

Leaf shape of Triticum aestivum, Hordeum vulgare and Zea mays cultivars grown under varying conditions was measured using digital image processing. An empirical leaf shape model was fitted to measured shape data of single leaves. Obtained values of model parameters were used to analyse the patterns in leaf shape.

Key Results

The model was able to delineate leaf shape of all studied species. The model error was small. Differences in leaf shape between juvenile and adult leaves in T. aestivum and H. vulgare were observed. Varying growth conditions impacted leaf dimensions but did not impact leaf shape of the respective species.

Conclusions

Leaf shape of the studied T. aestivum and H. vulgare cultivars was remarkably stable for a comparable ontogenetic stage (leaf rank), but differed between stages. Along with other aspects of grass architecture, leaf shape changed during the transition from juvenile to adult growth phase. Model-based analysis of leaf shape is a method to investigate these differences. Presented results can be integrated into architectural models of plant development to delineate leaf shape for different species, cultivars and environmental conditions.     

Fungal species diversity in juvenile and adult leaves of Eucalyptus globulus from plantations affected by Mycosphaerella leaf disease

In recent years, Mycosphaerella leaf disease (MLD) has become very common in Eucalyptus globulus plantations in Galicia, northwest Spain. The aetiology of MLD is complex and is associated with several species of Mycosphaerella and Teratosphaeria. A survey of the fungal mycobiota associated with juvenile and adult leaves and with leaf litter of the same trees in MLD‐affected plantations was made. The goal was to identify pathogens and endophytes, to determine whether the mycobiota of each leaf type differed and whether leaf litter might be a reservoir of MLD inoculum. Fungi belonging to 113 different species were isolated from the leaves of juvenile and adult trees sampled at 10 locations; 81 species occurred in juvenile and 65 in adult leaves. The average number of species obtained from juvenile leaves was significantly greater (P > 0.01) compared to adult leaves. This difference suggested that juvenile leaves are not only more susceptible to a group of pathogens, but to a wide range of fungi. Therefore, a general resistance mechanism might be lacking or be less effective in juvenile than in adult leaves. Several pathogenic species were identified in both leaf types. Leaf litter and living leaf mycobiotas were very different. However, some of the species they shared were MLD pathogens, suggesting that leaf litter could contribute to the inoculum of MLD.     

THE ORGANIZATION OF THE VASCULAR SYSTEM IN THE STEMS OF THE NYMPHAEACEAE. I. NYMPHAEA SUBGENERA CASTALIA AND HYDROCALLIS

The vascular system in the stems of Nymphaea odorata and N. mexicana subgenus Castalia, and N. blanda subgenus Hydrocallis consists of continuing axial stem bundles with eight being the usual number. The stem bundles are concentric and xylem maturation is mesarch. Xylem elements consist of tracheids with spirally or weakly reticulated secondary wall thickenings. The phloem is made up of companion cells and short sieve tube members with simple sieve plates that are nearly transverse. At the node each leaf is supplied with two lateral leaf traces and a median leaf trace. A root trace is also present and supplies a series of adventitious roots borne on the leaf base. Flowers and vegetative buds develop directly from the apical meristem and occupy leaf sites in a single genetic spiral. Each flower or vegetative bud is related to a leaf through specific spatial and vascular association. The related leaf is separated from the related flower by three members of the genetic spiral and occupies an adjacent orthostichy. Vascular tissue for the related flower arises from the inner surfaces of the four stem bundles supplying leaf traces to the related leaf and extends through the pith to the flower or vegetative bud via a peduncle fusion bundle. The vascular system organization in the investigated species of Castalia and Hydrocallis is not typically monocotyledonous or dicotyledonous, nor can it be considered transitional between them. The ontogeny of the vascular system is similar to typical dicotyledons and the investigated species of Nymphaea can, therefore, be considered to represent highly specialized and modified dicotyledons.