VASCULARIZATION OF A MULTILACUNAR SPECIES: POLYSCIAS QUILFOYLEI (ARALIACEAE) I. THE STEM

The odd-pinnate leaves of Polyscias quilfoylei have a sheathing leaf base that completely encircles the stem. At each node, many traces depart the vascular cylinder and traverse an obliquely upward course through the leaf base before aggregating in the rachis. Lateral traces diverge from parent traces in the stem vasculature at variable times relative to the leaf they serve, from variable positions in the vascular cylinder and from parent traces of variable ages. The stem vasculature is formed by the coalescing of leaf traces from as many as five leaves. All bundles departing the vascular cylinder at a node to serve a leaf are true leaf traces originating independently in the stem. Leaf traces develop acropetally from their positions of origin on parent traces. Primordial leaves are first served by the median trace and later by lateral traces. Many traces were recognized in the internodes subtending embryonic leaves, but they could not be related either to a specific leaf or to a specific position within a leaf. Because these traces had not yet achieved contact with a primordial leaf site, they were assumed to be in the process of developing acropetally at the time of sampling. Observations suggest that the multiple traces in this species might perform a similar function of integrating the vascular cylinder that subsidiary bundles perform in certain uni- and trilacunar species.     

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.     

THE ORGANIZATION OF THE VASCULAR SYSTEM IN THE STEMS OF THE NYMPHAEACEAE. II. NYMPHAEA SUBGENERA ANECPHYA,LOTOS, AND BRACHYCERAS

The anatomy and organization of the stem vascular system was analyzed in representative taxa of Nymphaea (subgenera Anecphya, Lotos, and Brachyceras). The stem vascular system consists of a series of concentric axial stem bundles from which traces to lateral organs depart. At the node each leaf is supplied with a median and two lateral leaf traces. At the same level a root trace supplies vascular tissue to adventitious roots borne on the leaf base. Flowers and vegetative buds occupy leaf sites in the genetic spiral and in the parastichies seen on the stem exterior. Certain leaves have flowers related to them spatially and by vascular association. Flowers (and similarly vegetative buds) are vascularized by a peduncle trace that arises from a peduncle fusion bundle located in the pith. The peduncle fusion bundle is formed by the fusion of vascular tissue derived from axial stem bundles that supply traces to certain leaves. The organization of the vascular system in the investigated taxa of Nymphaea is unique to angiosperms but similar to other subgenera of Nymphaea.     

VASCULAR ORGANIZATION IN SHOOTS OF CACTACEAE. I. DEVELOPMENT AND MORPHOLOGY OF PRIMARY VASCULATURE IN PERESKIOIDEAE AND OPUNTIOIDEAE

Primary shoot vasculature has been studied for 31 species of Pereskioideae and Opuntioideae from serial transections and stained, decorticated shoot tips. The eustele of all species is interpreted as consisting of sympodia, one for each orthostichy. A sympodium is composed of a vertically continuous axial bundle from which arise leaf- and areole-trace bundles and, in many species, accessory bundles and bridges between axial bundles. Provascular strands for leaf traces and axial bundles are initiated acropetally and continuously within the residual meristem, but differentiation of procambium for areole traces and bridges is delayed until primordia form on axillary buds. The differentiation patterns of primary phloem and xylem are those typically found in other dicotyledons. In all species vascular supply for a leaf is principally derived from only one procambial bundle that arises from axial bundles, whereas traces from two axial bundles supply the axillary bud. Two structural patterns of primary vasculature are found in the species examined. In four species of Pereskia that possess the least specialized wood in the stem, primary vascular systems are open, and leaf traces are mostly multipartite, arising from one axial bundle. In other Pereskioideae and Opuntioideae the vascular systems are closed through a bridge at each node that arises near the base of each leaf, and leaf traces are generally bipartite or single. Vascular systems in Pereskiopsis are relatively simple as compared to the complex vasculature of Opuntia, in which a vascular network is formed at each node by fusion of two sympodia and a leaf trace with areole traces and numerous accessory bundles. Variations in nodal structure correlate well with differences in external shoot morphology. Previous reports that cacti have typical 2-trace, unilacunar nodal structure are probably incorrect. Pereskioideae and Opuntioideae have no additional medullary or cortical systems.     

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.     

THE ORGANIZATION OF THE VASCULAR SYSTEM IN THE STEMS OF THE NYMPHAEACEAE. III. VICTORIA AND EURYALE

Organization of the stem vascular system was analyzed in Victoria species and Euryale ferox. The stem vascular system consists of a number of concentrically-organized continuing axial stem bundles. At the node each leaf is supplied with a root trace, two lateral leaf traces, and a median leaf trace. A peduncle fusion bundle is also present at each node. The peduncle fusion bundle supplies vascular tissue to the median leaf trace and to the peduncle trace. Flowers are nonmedian axillary but have specific vascular, spatial, and developmental relationships to leaves in a manner that resembles the genus Nymphaea. On the basis of the analysis of the stem vascular system, Victoria and Euryale are more similar to each other than to Nymphaea. However, the vascular system in Victoria and Euryale is similar enough to Nymphaea to suggest that Nymphaea, Victoria, and Euryale form a natural taxon of unique angiosperms. The organization of the stem vascular system in Victoria and Euryale is dicotyledonous.     

Variation in Arundo donax stem and leaf strength: Implications for herbivory

We determined leaf and stem strength for Arundo donax from plants grown in different geographic areas and at different times within their growing cycle. Mean leaf strength for plants collected within California was 1.72 Newtons (N) and ranged from 0.36 to 6.32 N, based on 1170 individual determinations. For leaves collected from 30 plants within four states between July 11 and 20, 2007, mean leaf strength was 1.58 N based on 936 determinations. Values ranged from 0.24 to 4.90 N. Overall, leaf strength showed statistically significant changes depending on the sampling date, sampling location, type of leaf sampled, and position within the leaf where the measurement was taken. In general leaf strength was greater near the base of the leaf and decreased with distance away from the base; leaf strength changed as the growing season progressed; and first year leaves had leaf strength values about 25% greater than leaves produced on stems >1-year old. This represents two of the three age categories of leaves which may be present on giant reed at any one time. Stem strength and stem wall thickness were strongly correlated (Kendall's Tau b = 0.92, P < 0.0001, N = 26). Linear regression indicated that mean stem strength decreased by approximately 6.8% (95% confidence limits 5.8-7.7%) from one node to the successive node progressing from the stem base to the shoot tip. These results imply that the ability of biological control agents to damage A. donax leaves may not be the same across the locations this plant occurs or at all times during the growing season within a given location.     

Anatomy of the gas canal system of Nelumbo nucifera

Nelumbo nucifera (Gaertn.) grows by extending a creeping rhizome through anaerobic sediments. Nodes form at intervals along the rhizome, each producing a single leaf, and gas canals channel air from the leaves throughout the petioles and rhizomes. The gas flow pathway was mapped by casting the canals in growing shoots with silicone and by blowing air through complexes of rhizomes and petioles. Air from a leaf flows to a rhizome through one of two petiolar canal pairs, joining with the lowermost of three canal pairs in the rhizome through a chamber in the node. The lowermost canal pair links these nodal chambers along the length of a rhizome, allowing air from a node to flow both forward, toward a growing shoot, and backward, toward preceding leaves. These linked chambers also connect with the middle pair of canals on their proximal side, enabling flow to proceed backward along the rhizome to an adjacent node. A chamber in the next node then diverts the flow into the upper canal pair. This pair leads to a third node and chamber from which the air vents to the atmosphere through the second petiolar canal pair. Thus, pressurised air from one leaf must flow backward through two nodes before it returns to the atmosphere. Forward flow also ventilates a shoot's growing tip, with air from the lowermost canal pair entering a chamber in the developing node which, as described above, connects with the middle canal. This allows the air to reverse direction at the tip and enter the vent flow pathway.     

Morphological studies on the genusClematis linn

In four-sepaled flowers ofClematis the sepal is supplied by three main traces. The basic pattern of the vascular supply to sepals is found inC. alpina var.ochotensis which invariably has six-bundled pedicels. It is as follows: the median traces to the first pair of opposite sepals, as well as all the lateral traces, arise directly from pedicel bundles, while those to the second pair are formed secondarily, after fusion and subsequent division of two adjacent pedicel bundles. As to the manner of origin of the median traces, the pattern is similar to that of the vascular supply to foliage leaves. This gives further evidence for the generally accepted view that the sepals ofClematis, like foliage leaves, are decussately arranged. In most other species such asC. apiifolia, C. stans, etc. the number of pedicel bundles tends to be reduced from six to four so as to coincide with that of the sepals, so patterns are much simplified and specialized: all the traces arise directly from pedicel bundles. InC. japonica an iconsistent pattern is observed, since the number of pedicel bundles from which sepal traces arise is much higher and varied.     

Some aspects of the morphology and distribution of Microsoriurn linguaeforrne (Polypodiaceae)

Microsorium linguaeforme is reported for the first time from India. Its rhizome is slender, much elongated with a highly dissected stele and leaf gaps in a single median dorsal row. The characteristic branching pattern is a modification of the common pattern in Polypodiaceae, resulting from the displacement of leaf-associated branches and the precocious development of the most basal secondary branch of each primary branch. The first two or three leaves of juvenile plants have no associated branches; thereafter, abaxially-originating traces t o solitary branches are progressively displaced so that the branch trace is close to the preceding leaf trace. Stomata of adult leaves are Copolomesoperigenous and of juvenile leaves Eupolomesoperigenous. The spores are monolete and with a smooth exine. Spore germination is of the Vitiariatype and prothallial development of the Kauliniatype. It is concluded that M. linguaeforme is closely allied to Leptochilus and is probably parental to it.     

AXILLARY BUD DEVELOPMENT IN POPULUS DELTOIDES. II. LATE ONTOGENY AND VASCULARIZATION

A nearly mature axillary bud of Populus deltoides was embedded in epoxy and serially sectioned at 6 μm. Sectioning extended from the cataphyll tips to a level in the subtending internode about 6 mm below the bud base. Vascular development was followed through the serial microsections and the vascular system was mapped in its entirety from initiation of the original bud traces to termination of the last recognizable leaf trace beneath the bud apex. Each vascular trace was identified as to its origin, its termination within a foliar organ, and its relation to other traces comprising the bud vascular cylinder. Analysis of these data confirmed the procambial patterns found in Part I of this study. Two original bud traces that diverged from the central trace of the axillant leaf gave rise to two pairs of scale traces in quick succession, and these scale traces become the progenitors of all subsequent vascular traces that were perpetuated within the bud. Just before the bud vascular system separated from that of the stem, a third pair of scale traces diverged from the original bud traces; the latter then receded toward the stem to eventually merge with its vasculature. The third pair of scale traces produced a horizontal vascular connection between stem and bud before terminating in the adaxial cataphyll. The vascular system at first conformed with a ½ vascular phyllotaxy when the original bud traces were initiated, progressed through a ⅓ vascular phyllotaxy in the scale trace system, and terminated at the time of sampling with a ⅖ vascular phyllotaxy in the foliage leaf primordia.     

Transport of 14C-Assimilates in Seedlings of Phaseolus vulgaris L. in Relation to Vascular Anatomy

Patterns of ¹⁴C-photosynthate translocation in bean (Phaseolusvulgaris L.) seedlings have been investigated in relation tovascular-bundle continuity between exporting and importing organsby use of radioassay and tissue-clearing techniques. Assimilatefrom the primary leaves reaches the first trifoliate leaf byan indirect route. There is no direct vascular connection betweenthe primary leaves and distal tissues. Bundles of the primaryleaf petiole connect with an anastomosis at the node, and allbundles which originate from this structure descend the stem.Assimilate from primary leaves moves down the stem, and is thentransferred to an ascending pathway, the bundles of which traversethe anastomosis at the second node. The lateral leaflets ofthe first trifoliate leaf are served differentially by primaryleaves with respect to assimilate supply. Differences are relatedto position, and may be accounted for by differences in vascularcontinuity.     

Morphological and anatomical studies inTribulus terrestris I. Phyllotaxis and branching

Phyllotaxis and branching system inTribulus terrestris were studied. Besides three nodal types already known in the species, a new type was found. The four nodal types were designated as A (with one leaf and no flower), B (with one leaf and one flower), C (newly found type, with two leaves of different sizes and no flower) and D (with two leaves of different sizes and one flower). A-type nodes are found only in basal parts of shoots, while distal parts have exclusively D-type nodes. Between the nodes of the two types there is always one node of B-type. Occurrence of C-type is rare and restricted to the first node of the lateral branch. The first leaf of the lateral branch superposes to the leaves of the mother axis at maturity, but it is situated at first more or less laterally. On the basis of anatomy, as well as gross morphology, the branching system at B- and D-type nodes was interpreted as sympodial, the main axis ending in a flower. With regard to the manner of overlapping of sepal margins, right- and left-handed calyces were recognized. The handedness always corresponds to the direction to which the peduncle is inclined.     

Sink to Source Transition of Populus Leaves

Development of the Populus leaf is presented as a model system to illustrate the sequence of events that occur during the sink to source transition. A Populus leaf is served by three leaf traces, each of which consists of an original procambial trace bundle that differentiates acropetally and continuously from more mature procambium in the stem and a complement of subsidiary bundles that differentiates bidirectionally from a leaf basal meristem. During development these subsidiary bundles maintain continuity through the meristematic region of the node. The basipetally developing subsidiary bunles form phloem bridges that serve to integrate adjacent leaf traces of the stem vasculature. Distal to the node the acropetally developing bundles from all three leaf traces are reoriented in a precise and orderly sequence to form tiers of petiolar bundles. These tiers of bundles extend into the midrib where bundles diverge at intervals as the major lateral veins. The dorsal-most tier of bundles extends to the lamina tip and each successive tier of bundles contributes to lateral veins situated more proximally in the lamina. Although the midrib and the major vein system differentiate acropetally in the lamina, they mature basipetally. Maturation of the mesophyll and other lamina tissues also mature basipetally. As a consequence of the basi-petal maturation process, the lamina tip matures very early and begins exporting photosynthates while the lamina base is still importing from other leaves. The transition of a leaf from sink to source status must therefore be considered as a progression of structural and functional events that occur in synchrony.     

THE RELATIONSHIPS BETWEEN THE PRIMARY VASCULAR SYSTEM AND PHYLLOTACTIC PATTERNS OF ANAGALLIS ARVENSIS (PRIMULACEAE)

The various primary vascular systems of shoots of Anagallis arvensis L. (Primulaceae) can be distinguished in relation to the number of leaves (two, three, or four) at each node. In this study, shoot segments (single intemodes and the nodes above them) were examined. The arrangement of segments within shoots was also recorded. The vasculature forms a closed system with the number of sets of bundles usually equal to twice the number of leaves. Irregularities are found in the following features of the system: the number of bundles composing leaf half-traces; occurrence of anastomosing bundles; the number of intemodes through which bundles extend; levels of leaf attachment to the stem at the node; and distribution of parenchyma within the vascular cylinder, which determines the number of bundles in sets and the number of bundle sets. The irregularities occur with different frequencies for segments exhibiting different phyllotactic patterns. Comparison of these frequencies leads to the following conclusions: anastomosing bundles occur only in decussate or trimerous shoot segments, whereas sets of bundles united within intemodes and displaced leaves occur only in tetramerous or trimerous ones; decrease of the number of bundles per leaf and displacement of leaves at the nodal level are correlated; variation between segments exhibiting the same phyllotactic pattern is greatest for trimerous, less for tetramerous, and least for decussate segments; the vascular system of decussate shoot segments is more stable than that of the other systems; and trimerous segments seem to be intermediate between the other two segment types.     

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     

ONTOGENY OF THE COTYLEDONARY REGION OF CHAMAESYCE MACULATA (EUPHORBIACEAE)

Development of the cotyledonary region in Chamaesyce maculata is described from germination of the seed through formation of the dense mat of branches which characterize this common weed. The cotyledonary node is trilacunar with split-lateral traces. Epicotyl development is limited to a pair of leaves (“V-leaves”) inserted directly above and decussate to the cotyledons. The two V-leaves are also vascularized by three traces and insertion of these traces relative to the vasculature at the immediately subjacent cotyledonary node is asymmetrical; four of the six V-leaf traces arise on one side of the intercotyledonary plane and two arise on the opposite side. Further shoot development is limited to lateral branches that develop sequentially from cotyledonary axillary buds, and then from de novo buds which arise at bases of previously developed lateral branches. The first axillary bud to develop is located on that half of the seedling which supplies the V-leaves with four traces. Development or insertion of the third and fourth branches (first and second de novo branches) relative to the first and second (cotyledonary) branches occurs in two patterns, termed cis and trans. Subsequent de novo branches generally develop between the two most recently developed branches on that half of the seedling, gradually forming a large branch plexus at the cotyledonary region. In mature robust specimens, however, the sequence of lateral branch development may become irregular. Structure of the cotyledonary region of C. maculata does not readily support widely held concepts of homology between the pleiochasium of Euphorbia subgenus Agaloma and the lateral branch system of Chamaesyce.     

A QUANTITATIVE STUDY OF SIEVE-TUBE DIFFERENTIATION IN VEGETATIVE SHOOT APICES OF COLEUS

Quantitative methods, with round-the-clock collecting of large samples in successive years, have uncovered several new phenomena of sieve-tube differentiation in young leaves of Coleus vegetative shoots. In small leaves (1–350 μ), there are no sieve tubes in the leaf itself, but they differentiate acropetally in the two traces to each leaf. Regression lines fitted to the data for leaf length vs. most distal position of sieve tubes in the traces support the view that differentiation is steady and acropetal, but they also reveal that differentiation in the traces falls steadily farther behind elongation of the leaf. Leaves more than 500 μ long have sieve tubes close to their tips. An intensive search of leaves of intermediate lengths revealed an isolated locus of sieve-tube differentiation. These relationships were reproducible year after year. Every plant with discontinuous strands of sieve tubes in the second leaf pair had discontinuous xylem in the third. This isolated locus was not seen before, probably because of small samples and daytime collections; most of our cases were from night collections.     

Observations on phyllotaxis, stelar morphology, the shoot apex and gemmae of Lycopodium lucidulum Michaux (Lycopodiaceae)

Phyllotaxis in Lycopodium lucidulum consists of low alternating spirals, with the adult shoots corresponding to a system of 5 + 5 contact parastichies in which there are ten orthostichies. Each major stelar lobe is a sympodium of the leaf traces of two orthostichies and each lobe has two mesarch xylem poles, Differentiation of both the procambium and xylem of the leaf traces is bidirectional, that is differentiation first commences in the leaf base and then is acropetal into the leaf and basipetal into the stem. Furthermore, the procambium of the axis does not extend above that of the youngest leaf primordium and the axial procambium is in part a composite of that of the leaf traces. Thus, it is concluded that the stele in this taxon is not a strictly cauline structure. The shoot apex consists of four zones—a zone of surface initials, a zone of subsurface initials, a peripheral zone and a rib meristem. This zonation pattern is essentially the same as that of the seed plants. From their inception, gemma primordia also exhibit shoot apical zonation and are entirely different from leaves in their subsequent growth pattern and vascularization. Although the gemmae occupy leaf sites in the phyllotactic sequence, they are interpreted as arrested stem dichotomies on the basis of their development and vascular system.     

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.