Dynamic changes in hydraulic conductivity in petioles of two savanna tree species: factors and mechanisms contributing to the refilling of embolized vessels

Diel variation in specific hydraulic conductivity (k_s) was recorded in petioles of two savanna tree species, Schefflera macrocarpa and Caryocar brasiliense, from central Brazil. These two species have compound leaves with long petioles (10–30 cm). In both species, petiole k_s decreased sharply with increasing transpiration rates and declining leaf water potentials (ψ_L) during the morning. Petiole k_s increased during the afternoon while the plants were still transpiring and the water in the non‐embolized vessels was still under tension. Dye experiments confirmed that in both species diel variation in k_s was associated with embolism formation and repair. When transpiration was prevented in individual leaves, their petiole k_s and water potential remained close to their maximum values during the day. When minimum daily ψ_L on selected branches was experimentally lowered by 0.2–0.6 MPa, the rate of k_s recovery during the afternoon was slower in comparison with control branches. Several field manipulations were performed to identify potential mechanisms involved in the refilling of embolized petiole vessels. Removal of the cortex or longitudinal incisions in the cortex prevented afternoon recovery of k_s and refilling of embolized vessels. When distilled water was added to petiole surfaces that had been abraded to partially remove the cuticle, k_s increased sharply during the morning and early afternoon. Evidence of starch to sugar conversion in the starch sheath cells surrounding the vascular bundles of the petioles was observed during periods of rapid transpiration when the abundance of starch granules in the starch sheath cells surrounding the vascular bundles decreased. Consistent with this, petiole sugar content was highest in the early afternoon. The most parsimonious explanation of the field observations and the experimental results was that an increase in osmotically active solutes in cells outside the vascular bundles at around midday leads to water uptake by these cells. However, the concurrent increase in tissue volume is partially constrained by the cortex, resulting in a transient pressure imbalance that may drive radial water movement in the direction of the embolized vessels, thereby refilling them and restoring water flow. This study thus presents evidence that embolism formation and repair are two distinct phenomena controlled by different variables. The degree of embolism is a function of tension, and the rate of refilling a function of internal pressure imbalances.     

Pressure and flow relations in vascular bundles of the tomato plant

In the tomato plant water flows through primary xylem in accordance with Poiseuille's law. This relation and the analogy between Poiseuille's and Ohm's law were employed to calculate rates of flow and differences in pressure within vascular bundles when transpiration rates from individual leaves were known. The resistance of vascular bundles to flow was calculated from a modification of Poiseuille's law and from measurements of vessels in all bundles. The rates of flow in all bundles were derived from a set of simultaneous linear equations of flow, written to correspond with the nature of the vascular network. Values of the difference in pressure associated with flow in bundles were derived from resistances and flow rates in individual bundles. These agreed substantially with values observed in a comparable plant.

In large bundles, vessels occur in a frequency distribution that is approximately normal with respect either to the logarithms of their radii or to the fourth power of their radii. The largest vessels in a bundle transport most of the water when they are functioning.

The tomato plant contains 2 types of vascular bundle. The large bundles of the stem form a network by joining above each node in combinations of 2 at a time. The small bundles of the stem and petiolar bundles are independent of other bundles from their origins at junctions to their termini. The small bundles offer high resistance to flow, whereas the resistance of large bundles is low. The average conductance of large bundles decreases from the base to the apex of the stem. That of small vascular bundles remains low and more or less constant throughout the plant.

Only a small difference in pressure is required to maintain flow in large bundles. For lower leaves, the driving pressure required to move water to the base of a petiole is considerably less than that which moves water through petioles. The difference in pressure that maintains flow increases steadily for successively higher nodes. However, the pressure that drives flow to leaves is not always greater for higher leaves than for intermediate ones. For the plant examined, the highest leaves required a smaller amount of energy to move water from the ground than intermediate leaves did. This was also true of the power expended in moving water to individual leaves.

In the large network bundles, significant cross transfer of flow occurs at junction points from one bundic to another. Because of the interconnections between large bundles. pressure and flow relations are apparently not greatly altered when localized dysfunction occurs in the vessels of large bundles. In small, independent bundles, a localized dysfunction in vessels produces a significant effect on pressure and flow relations.

     

The Vascular System of Maize Stems Revisited: Implications for Water Transport and Xylem Safety

The plexus of vascular bundles in the nodes of grasses is notoriouslycomplex, where long axial bundles pass through a network oftransverse bundles. The xylem pathways for water in maize stemshave been investigated anatomically and with dye and particulatetracers, revealing some of the details of this complexity. Onlyapprox. 3% of axial vessels pass through nodes without beinginterrupted by end walls. Axial bundles at nodes differ fromthose in internodes in having the metaxylem and protoxylem vesselsconnected by small tracheary elements. So it is only at nodesthat exchange of sap occurs between the large vessels withina bundle. End walls, acting as filters for particles and gasbubbles, always separate axial vessels from vessels in transversebundles. The high redundancy of bundle connections in the nodalplexus is interpreted as providing alternative water pathwaysto bypass embolisms and damaged or diseased sections of thexylem. The pores in the filters at the base of nodes and betweenaxial and transverse vessels within nodes are <20 nm in diameter.Where axial vessels connect to transverse vessels, a varietyof unusual shapes of vessel elements mediate two- and three-wayconnections within the plexus.Copyright 2000 Annals of BotanyCompany Zea mays, cryoSEM, maize, node, pits, pit membranes, vessel ends, vessels, xylem embolism, xylem pathogens     

Can cell walls bending round xylem vessels control water flow?

Vascular bundles of petioles below wilted leaves of Nymphoides peltata (S.G. Gmel. O. Kuntze) were frozen intact and freeze-fractured for electron microscopy. Cell walls in them appeared drawn in against the helical thickenings of xylem vessels. By contrast, walls round vessels which had been frozen in vascular bundles below turgid leaves, and walls round vessels which had been fixed, embedded and sectioned, were straight or bulged outwards slightly. Walls bulged outwards slightly also from cut vessels filled with sucrose solution before freezing. Movement of vessel walls could produce the clicks audible when water cavitates in vessels, and might explain a variable resistance to the flow of water through plants.     

COMPARATIVE STUDIES OF THE NODAL AND VASCULAR ANATOMY IN THE NEOTROPICAL CYATHEACEAE. III. NODAL AND PETIOLE PATTERNS; SUMMARY AND CONCLUSIONS

Comparative studies of the nodal and vascular anatomy in the Cyatheaceae are discussed as they relate to the taxonomy and phylogeny of the family. There is in the Cyatheaceae (excluding Metaxya and Lophosoria) a basic nodal pattern consisting of four major phases of leaf trace separations. Abaxial traces arise from the leaf gap margins, and the last abaxial traces from each side of the gap are larger and undergo numerous divisions. Distally adaxial traces separate from the gap margins, and the last adaxial traces are usually larger and undergo multiple divisions. In addition, medullary bundles frequently become petiole strands of the adaxial arc in the petiole. Rarely, cortical bundles form petiole strands in the abaxial arc in the petiole. Leaf gaps of the squamate genera of the Cyatheaceae are fusiform and possess prominent lateral constrictions which result from medullary bundle fusions and the separation of leaf traces. A characteristic petiole pattern is found in all members of the Cyatheaceae. There is an increase in the complexity of the petiole vascular tissue which results in a gradation from the undivided strand in Metaxya, to the three-parted petiole pattern in Lophosoria, and finally to the much-dissected petiole vascular tissue in the advanced genera. Nodal and vascular anatomy data basically support Tryon's phyletic scheme for the family. The Sphaeropteris-Alsophila-Nephelea line shows certain tendencies toward increased complexity of nodal and vascular anatomy, whereas the Trichipteris-Cyathea-Cnemidaria line shows the same anatomical and morphological characters in a direction of increased simplification or reduction.     

The kinetics of sucrose concentration in the phloem of individual vascular bundles of the Ricinus communis seedling measured by nuclear magnetic resonance microimaging

The sucrose concentration was measured at 70-min intervals in the phloem of individual bundles of the hypocotyl of Ricinus seedlings by ¹H nuclear magnetic resonance (NMR) spectroscopic imaging. The sucrose concentration stayed fairly constant in all bundles for more than 7 h if the cotyledons were embedded in the endosperm or excised and incubated in 100 mM sucrose. If, however, the sucrose solution was replaced by sucrose-free buffer solution, the sucrose levels in the phloem decreased with a kinetic depending on the seedling: in some cases there was a smooth decline, in some a decline followed by a slight recovery and in some cases a clear-cut oscillation. The sucrose concentration was often not identical in the phloem of the individual bundles. The oscillations were larger in the phloem at the apex of the hypocotyl than in the phloem at the base of the hypocotyl. Cutting the petiole of one cotyledon led to a decrease in sucrose not only in the four bundles directly connected to the severed petiole but in all eight bundles of the hypocotyl. Cutting the petiole and dividing the vascular ring at the cotyledonary node and at the root crown did not prevent the decline of sucrose in all eight bundles. Therefore, a functional equilibration of translocated solutes between the eight bundles may occur within the 1-h measuring interval by radial diffusion through the parenchyma of the hypocotyl. Received 4 July 1997 / Accepted: 4 October 1997     

ORGANIZATION AND ONTOGENY OF THE VASCULAR SYSTEM IN THE PETIOLE OF EASTERN COTTONWOOD

The topologic arrangement of petiolar bundles varies within the length of the cottonwood petiole. Each petiolar bundle is formed by the subdivision and aggregation of acropetally differentiating subsidiary bundles in a predictable pattern. The subsidiary bundles provide vascular continuity between the stem and specific portions of the leaf lamina. Spot-labeling of individual veins with ¹⁴CO₂, freeze substitution, and microautoradiography were used to establish the relation between the secondary veins of the lamina and the vasculature of the petiole. Within the petiole vasculature each subsidiary bundle was continuous with a specific portion of the lamina and seemed to have a separate function. Subsidiary bundles continuous with the central leaf trace were closely related functionally to the tip region of the lamina, while the subsidiary bundles continuous with the lateral leaf traces were functionally related to the middle and basal portions of the lamina.     

单芽狗脊营养器官的解剖学研究

采用离析法和石蜡切片法对单芽狗脊营养器官进行形态解剖研究。结果表明：单芽狗脊叶为异面叶,上、下表皮细胞均为不规则型,仅下表皮有气孔器分布;叶柄维管束有2~6个,自叶柄基部向上至叶轴仅有2个较大的维管束;根状茎薄壁细胞之间有多个维管束散生分布,且富含丰富的淀粉粒;皮层在根的横切结构中占比较大,木质部的发育方式为外始式;单芽狗脊珠芽的发育过程分为三个阶段,珠芽原基的形成期、珠芽原基的分化期、成熟期。     

14C Translocation pathways in honeylocust and green ash: Woody plants with complex leaf forms

Long-distance transport in plants requires precise knowledge of vascular pathways, and these pathways differ among species. This study examines the ¹⁴C translocation pathways in honeylocust (Gleditsia triacanthos L.) and green ash (Fraxinus pennsylvanica Marsh.), species with compound leaves, and compares them with those of cottonwood (Populus deltoides Bartr. ex Marsh.), a species with simple leaves. The stem vasculature of honeylocust conforms to a 2/5 helical phyllotaxy and that of green ash to a decussate phyllotaxy. The plastochron is relatively long in both species – 2.5+ days in honeylocust and 4.5+ days in green ash. Consequently, the transition from upward to downward translocation from mature source leaves is abrupt and occurs close to the apex. Export of ¹⁴C from localized treatment positions within a leaf was found to vary both quantitatively and spatially. To determine export patterns, ¹⁴CO₂ was administered to either individual leaflets of once-pinnate or pinnae of bipinnate leaves of honeylocust, and to either individual veins of simple or leaflets of compound leaves of green ash. Transections of either the petiole or rachis base were then examined for ¹⁴C by micro-autoradiography. In all cases, as treatment positions advanced acropetally in the leaves, the bundles translocating ¹⁴C were situated more dorsally in the basal petiole and rachis vasculatures. ¹⁴C was confined to the right side of the vasculature when structures on the right side of a leaf were treated. Compound leaves of both species mature acropetally. Thus, mature basal pinnae of honeylocust and basal leaflets of green ash translocate acropetally to younger leaf parts that are still rapidly expanding. All translocation pathways, both in the stem and leaf, conformed with vascular organization previously determined by anatomical analyses.     

Tissue-specific localization and dynamic changes of endogenous IAA during poplar leaf rhizogenesis revealed by in situ immunohistochemistry

Poplar 741 [Populus alba × (P. davidiana + P. simonii) × P. tomentosa] leaves were rooted within 8 days when cultured on 1/2 MS medium. The spatial distribution of endogenous indole-3-acetic acid (IAA) and its dynamic changes in the rhizogenesis were investigated, using an immunohistochemical approach. Anatomical analyses showed that the root primordia arose from vascular cambium cells in the basal regions of the petioles of the leaves. Before root induction, immunostaining patterns showed a basipetally decreasing gradient of IAA along the leaves. Three days after induction, the IAA immunostaining pattern observed along the leaves was high at both ends and low in the middle. And IAA in the basal regions of the petiole was distributed mainly in the vascular bundles. Localized application of 2,3,5-triiodobenzoic acid (TIBA) on laminas of the leaves delayed the accumulation of IAA in the vascular bundles of the basal regions of the petioles, but not in the mesophyll of the laminas. These data indicate that an accumulation of IAA in the vascular bundles of the basal regions of the petioles induces the occurrence of rhizogenesis of poplar leaves. And IAA accumulated in the vascular bundle of the basal region of the petiole results from its polar transportation from mesophyll of the laminas, rather than by in situ IAA generation.     

Histological events leading to somatic embryo formation in cultured petioles of alfalfa

Summary An optimum 10-day exposure of petioles of alfalfa [Medicago sativa ssp.falcata (L.) Arcangeli] to 2,4-dichlorophenoxyacetic acid or 2,4,5-trichlorophenoxyacetic acid results in the semisynchronous production of somatic embryos starting about 4 days after transfer to a non-auxin-containing medium. The timing of cell division induction in the petiole explants was found to vary depending on the petiole tissue type. Cells adjacent to the vascular bundles divide first at about 48 h after exposure to auxin, closely followed by those of the inner parenchyma, whereas most of the cells of the subepidermal and epidermal layers start to divide later, between 72 and 120 h. Two different sources of callus were also evident. Cells adjacent to the vascular bundles and the inner parenchyma cells were the primary source of callus when a short, 2-day (non-embryo-producing) exposure to auxin was used. In contrast, the subepidermal and epidermal cells were the primary source of callus tissue when a longer, 10-day (embryo producing) exposure was used. It is concluded that the source of somatic embryos is primarily the daughter cells of the subepidermal or epidermal tissue or both.     

COMPARATIVE ANATOMY OF THE PETIOLE AND INFRAGENERIC RELATIONSHIPS IN JATROPHA (EUPHORBIACEAE)

Anatomical features of the petiole in several species of Jatropha L. (Euphorbiaceae) are presented as evidence in support of infrageneric relationships. A trilacunar 3-trace nodal pattern is typical for the genus. The vascular supply to the stipules is derived from the branching of the two peripheral leaf traces. The number of vascular bundles range from 11 through 9, 7, 5 and 3, and occur in a ring, as free traces, a medullated cylinder, or as U-shaped free traces. The reduction from nine to three bundles is correlated with the gross morphological features while 11, which occurs only in the section Peltatae (Pax) Dehgan & Webster, presents an increase. Reduction in the number of petiolar traces follows the evolutionary advancement of various taxa. This reduction in traces corresponds with south-north distribution of the species and consequential adaptation to colder and more arid climates in Central America and Africa. Smaller leaves, fewer primary veins and fewer vascular traces have resulted as a response to reduced need for water. Presence of dorsal (super-numerary) bundles which supply the petiolar glands in subgenus Jatropha (= Adenoropium Pax) is considered significant, since African taxa of the section (subsection Pubescentes Pax) have retained these bundles despite the loss of petiolar glands. The latter glands are prominent in the South American and Indian species. Sectional lines in the genus can, therefore, be drawn generally on the basis of numerical constancy and relative uniformity in the arrangement of petiolar traces. The continuity of vascular bundles from the stem into the petiole and variations of bundle arrangements are depicted in three-dimensional drawings.     

Indole-3-acetic acid accumulation during poplar rhizogenesis revealed by immunohistochemistry

Poplar hybrid 741 [Populus alba × (P. davidiana + P. simonii) × P. tomentosa] leaves were rooted within 8 d when cultured in vitro on 1/2 Murashige and Skoog (MS) medium. The spatial distribution of endogenous indole-3-acetic acid (IAA) in the rhizogenesis was investigated, using an immunohistochemical approach. In addition, the effect of 2,3,5-triiodobenzoic acid (TIBA) on IAA distribution was also analyzed. The results showed that a strong IAA signal was detected in the vascular bundles of the basal regions of the petioles 3 d after root induction. Furthermore, the signal in vascular bundles of the basal regions of the petioles was stronger than that of the middle regions of the petioles. Application of TIBA on lamina delayed both the accumulation of IAA in the vascular bundles and rhizogenesis. These data indicate that an endogenous IAA rise in vascular bundles is among the first signals leading to the rhizogenesis, and that it results from transportation of the hormone from the lamina of the leaf to the base of the petiole, rather than by in situ IAA generation.     

云南箭竹不同年龄段假鞭的解剖学结构特征及木质化观察

该研究以云南箭竹不同年龄段的假鞭为实验材料,采用滑动切片法并利用光学显微镜观察,分析云南箭竹假鞭的解剖结构特征及其随年龄的动态变化,为假鞭结构研究提供新的解剖学数据信息。结果显示:(1)云南箭竹假鞭节间的表皮层只有1层细胞,皮下层由3～4层细胞壁加厚的纤维细胞组成,皮层一般有20～25层不规则的薄壁细胞,成熟的皮层细胞会形成皮层气道,髓实心不具髓腔。(2)云南箭竹假鞭纤维壁厚随鞭龄增加而增加,且同一年龄假鞭的内侧韧皮部面积大于外侧;纤维腔径随鞭龄增加而逐渐减小,但同一年龄假鞭内侧纤维腔径大于外侧;韧皮部的面积、维管束和导管的直径均随着鞭龄的增加而增大。(3)假鞭维管束一般不具有原生导管,外部维管通常有2个较大的后生导管,在假鞭中部及内部通常只有1个后生导管,另1个后生导管不发育或发育不全。(4)在0.5年生到2年生的云南箭竹假鞭中,被染成紫红色的木质素在纤维细胞壁、薄壁细胞壁、导管细胞壁中都有分布,且随着假鞭年龄的增加染色逐渐加深,表明云南箭竹假鞭木质素含量随着鞭龄的增长而不断增加,木质化程度随鞭龄的增长逐渐提高。     

PERIASTRON RETICULATUM UNGER AND AEROCORTEX KENTUCKIENSIS,N. GEN. ET SP., FROM THE NEW ALBANY SHALE OF KENTUCKY

Segments of anatomically preserved axes of the Lower Mississippian genus, Periastron, are analyzed in detail, and new features of histology and the pattern of vascular bundles are described. The name P. perforatum is shown to be a synonym of P. reticulatum. Division and fusion of vascular bundles in the axis result in variation in both their number (5–10) and form (in transverse section, from circular to elongate). In none of the 13 specimens studied is there any evidence of traces to lateral appendages, providing, with other evidence, support for the position that Periastron represents the petiole of a large leaf. Aerocortex kentuckiensis, a new name, is established for two specimens that resemble Periastron but which differ from it in being characterized by 2–4 vascular bundles in contrast to the 5–10 (or 11?) of Periastron, and by having centrally, rather than peripherally, located secretory ducts. Aerocortex and Periastron might represent, respectively, proximal and distal regions of a petiole.     

The development of phloem anastomoses between vascular bundles and their role in xylem regeneration after wounding in Cucurbita and Dahlia

The differentiation of phloem anastomoses linking the longitudinal vascular bundles has been studied in stem internodes of Cucurbita maxima Duchesne, C. pepo L. and Dahlia pinnata Cav. These anastomoses comprise naturally occurring regenerative sieve tubes which redifferentiate from interfascicular parenchyma cells in the young internodes. In all three species, severing a vascular bundle in a young internode resulted in regeneration of xylem to form a curved by-pass immediately around the wound. The numerous phloem anastomoses in these young internodes were not involved in this process, the regenerated vessels originating from interfascicular parenchyma alone. Conversely, in mature internodes of Dahlia, the regenerated vessels originated from initials of the interfascicular cambia, and their phloem anastomoses did not influence the pattern of xylogenesis. On the other hand, in old internodes of Cucurbita, in which an interfascicular cambium was not yet developed, the parenchyma cells between the bundles had lost the ability to redifferentiate into vessel elements, and instead, regenerated vessels were produced in the phloem anastomoses. Thus, the wounded region of the vascular bundle was not bypassed via the shortest, curved pathway, but by more circuitous routes further away from the wound. Some of the regenerated vessels produced in the phloem anastomoses were extremely wide, and presumably efficient conductors of water. It is proposed that the dense network of phloem anastomoses developed during evolution as a mechanism of adaptation to possible damage in mature internodes by providing flexible alternative pathways for efficient xylem regeneration in plants with limited or no interfascicular cambium.This paper is dedicated to the memory of the late Isaac Blachmann (deceased 19 November 1995), father-in-law of the senior author, for encouragement and advice throughout the yearsThis research was supported by an International Scientific Exchange Award to R.A. from the Israel Academy of Sciences and The Royal Society.     

STUDIES ON THE LEAF OF POPULUS DELTOIDES (SALICACEAE): MORPHOLOGY AND ANATOMY

Mature field- and growth-chamber-grown leaves of Populus deltoides Bartr. ex Marsh. were examined with light and scanning electron microscopes to determine their vasculature and the spatial relationships of the various orders of vascular bundles to the mesophyll. Three leaf traces, one median and two lateral, enter the petiole at the node. Progressing acropetally in the petiole these bundles are rearranged and gradually form as many as 13 tiers of vascular tissue in the petiole at the base of the lamina. (Most leaves contained seven vertically stacked tiers.) During their course through the midrib the tiers “unstack” and portions diverge outward and continue as secondary veins toward the margin on either side of the lamina. As the midvein approaches the leaf tip it is represented by a single vascular bundle which is a continuation of the original median bundle. Tertiary veins arise from the secondary veins or the midvein, and minor veins commonly arise from all orders of veins. All major veins–primaries, secondaries, intersecondaries, and tertiaries–are associated with rib tissue, while minor veins are completely surrounded by a parenchymatous bundle sheath. The bundle sheaths of tertiary, quaternary, and portions of quinternary veins are associated with bundle-sheath extensions. Minor veins are closely associated spatially with both ad- and abaxial palisade parenchyma of the isolateral leaf and also with one or two layers of paraveinal mesophyll that extend horizontally between the veins. The leaves of growth-chamber-grown plants had thinner blades, a higher proportion of air space, and greater interveinal distances than those of field-grown plants.     

Hydraulic characteristics of water-refilling process in excised roots of Arabidopsis

Plants have efficient water-transporting vascular networks with a self-recovery function from embolism, which causes fatal discontinuity in sap flow. However, the embolism-refilling process in xylem vessel is still unclear. The water-refilling processes in the individual xylem vessels of excised Arabidopsis roots were visualized in this study using synchrotron X-ray micro-imaging technique with high spatial resolution up to 1 μm per pixel and temporal resolution up to 24 fps. In normal continuous water-refilling process, we could observe various flow patterns affected by the morphological structures of the xylem vessels, especially when water passed through perforation plates. A simple criterion based on the variation in dynamic pressure was suggested to evaluate the contribution of individual perforation plates to the water-refilling process. Meanwhile, the water-refilling embolized sections of xylem vessels through radial pathways were also observed. Separated water columns were formed from this discontinuous water-refilling process and the water influx rates through the radial pathways were estimated to be 478 and 928 μm³ s^?1. The dynamic behavior of the separated water columns were quantitatively analyzed from the stoppage of volume growth to the translational phase. These water-refilling processes in excised roots of Arabidopsis may shed light on understanding the water refilling in the embolism vessels of intact plants and the interconnectivity of xylem vessel networks in vascular plants.     

Complexity untwined: The structure and function of cucumber (Cucumis sativus L.) shoot phloem

Cucurbit phloem is complex, with large sieve tubes on both sides of the xylem (bicollateral phloem), and extrafascicular elements that form an intricate web linking the rest of the vasculature. Little is known of the physical interconnections between these networks or their functional specialization, largely because the extrafascicular phloem strands branch and turn at irregular angles. Here, export in the phloem from specific regions of the lamina of cucumber (Cucumis sativus L.) was mapped using carboxyfluorescein and ¹⁴C as mobile tracers. We also mapped vascular architecture by conventional microscopy and X-ray computed tomography using optimized whole-tissue staining procedures. Differential gene expression in the internal (IP) and external phloem (EP) was analyzed by laser-capture microdissection followed by RNA-sequencing. The vascular bundles of the lamina form a nexus at the petiole junction, emerging in a predictable pattern, each bundle conducting photoassimilate from a specific region of the blade. The vascular bundles of the stem interconnect at the node, facilitating lateral transport around the stem. Elements of the extrafascicular phloem traverse the stem and petiole obliquely, joining the IP and EP of adjacent bundles. Using pairwise comparisons and weighted gene coexpression network analysis, we found differences in gene expression patterns between the petiole and stem and between IP and EP, and we identified hub genes of tissue-specific modules. Genes related to transport were expressed primarily in the EP while those involved in cell differentiation and development as well as amino acid transport and metabolism were expressed mainly in the IP.