Anatomical Studies on Secondary Phloem of Euonymus bungeanus

The anatomical structures, especially the type, distribution and arrangement of the constituent elements in the secondary phloem of Euonymus bungeanus Maxim. have been studied. The results showed that the secondary phloem was thicker, consisted of sieve-tube elements, companion ceils ,phloem parenchyma cells ,secretory ceils and rays. Sieve-tube elements, phloem parenchyma cells and secretory cells were alternately arranged in tangential bands, forming a conspicuous zone-like constitution. There was no obvious boundary between the functional phloem and the non-functional phloem. Sieve-tube elements were long, slender cells with very oblique end walls and compound sieve plates. Sieve areas on lateral wall were highly differentiated. Companion cells were triangular in transection and slender in radial section. Mostly,two or three companion cells stayed along with one sieve-tube element. In the functional phloem, phloem parenchyma cells were also slender, containing a few starch grains;but in the nonfunctional phloem they enlarged and contained abundant starch grains. Secretory cells were longer than sieve-tube elements, consisting of rubber-like material. Rays were uniseriate. Finally, the authors also discussed the phylogenetic position of E. bungeanus, which may provide some references for further study of the classification of different genera of Celastraceae.     

ONTOGENY AND STRUCTURE OF THE SECONDARY PHLOEM IN PYRUS MALUS

Evert , Ray F. (U. Wisconsin, Madison.) Ontogeny and structure of the secondary phloem in Pyrus malus. Amer. Jour. Bot. 50(1): 8–37. Illus. 1963.—The secondary phloem of apple consists of sieve-tube elements, companion cells, phloem parenchyma cells, fiber-sclereids, and ray parenchyma cells. The sieve-tube elements are generally long, slender cells with very oblique end walls and much-compounded sieve plates. All sieve-tube elements initially possess nacreous thickenings. Similar wall thickenings were observed in the differentiating fiber-sclereids and xylem elements. Of the 245 sieve-tube elements critically examined, 242 were associated with companion cells. All of the companion cells were shorter than their associated sieve-tube elements. Young companion cells possess slime bodies which later become dispersed. Callose is often found on the sieve-tube element side of the common wall between sieve-tube element and companion cell. In several collections, callose was found on both sides of that wall. The parenchyma cells are of 3 types: crystal-containing cells; tannin-and/or starch-containing cells; and those with little or no tannins or starch. Any type parenchyma cell may be on to genetically related to a sieve-tube element, that is, may be derived from the same phloem initial as the sieve-tube element. Morphologically, the phloem parenchyma cells intergrade with the companion cells, the tannin- and starch-free parenchyma cells often being difficult to distinguish from companion cells. Most of the tannin- and starch-free parenchyma cells collapse when the contiguous sieve-tube elements become nonfunctional. The fiber-sclereids arise from parenchyma cells which overwinter on the margin of the cambial zone and differentiate in nonfunctional phloem.     

STUDIES ON THE ROOT OF HORDEUM VULGARE L.– ULTRASTRUCTURE OF THE SEMINAL ROOT WITH SPECIAL REFERENCE TO THE PHLOEM

Seminal root tissue of Hordeum vulgare L. var. Barsoy was fixed in glutaraldehyde and osmium tetroxide and studied with the light and electron microscopes. The roots consist of an epidermis, 6–7 layers of cortical cells, a uniseriate endodermis and a central vascular cylinder. Cytologically, the cortical and endodermal cells are similar except for the presence of tubular-like invaginations of the plasmalemma, especially near the plasmodesmata, in the former. The vascular cylinder consists of a uniseriate pericycle surrounding 6–9 phloem strands occurring on alternating radii with an equal number of xylem bundles. The center of the root contains a single, late maturing metaxylem vessel element. Each phloem strand consists of one protophloem sieve element, two companion cells and 1–3 metaphloem sieve elements. The protophloem element and companion cells are contiguous with the pericycle. Metaphloem sieve elements are contiguous with companion cells and are separated from tracheary elements by xylem parenchyma cells. The protoplasts of contiguous cells of the root are joined by various numbers of cytoplasmic connections. With the exception of the pore-plasmodesmata connections between sieve-tube members and parenchymatic elements, the plasmodesmata between various cell types are similar in structure. The distribution of plasmodesmata supports a symplastic pathway for organic solute unloading and transport from the phloem to the cortex. Based on the arrangement of cell types and plasmodesmatal frequencies between various cell types of the root, the major symplastic pathway from sieve elements to cortex appears to be via the companion and xylem parenchyma cells.     

水松的次生韧皮部解剖及其系统位置的讨论

在光学显微镜和扫描电子显微镜下观察,水松茎次生韧皮部的主要特征为：韧皮部由轴向系统和径向系统组成。轴向系统由筛胞、韧皮薄壁组织细胞、蛋白细胞和韧皮纤维组成,径向系统由韧皮射线组成。在横切面上,轴向系统的各组成分子以单层切向带交替有规律的排列,其排列顺序为：筛胞－韧皮薄壁组织细胞－韧皮纤维-筛胞。筛胞的径向壁上嵌埋有草酸钙结晶,韧皮纤维仅一种类型,韧皮射线同型、单列。根据水松茎次生韧皮部的解剖研究,并与杉科其它各属的有关资料进行比较,我们认为：水松属与水杉属和落羽杉属有较近的亲缘关系。     

Anatomy of Secondary Phloem of Glyptostrobus in Relation to Its Systematic Position

A anatomical characters of secondary phloem in Glyptostrobus pensilis (Staunt.)Koch were observed by means of both light and scanning electron microscopy(SEM). The secondary phloem is composed of axial and radial systems. In the axial systems, the phloem consists of sieve cells, phloem parenchyma cells, albuminous cell and phloem fibers. In the radial systems, it consists of phloem rays. The alternate arrangement of different cells in cross section results in tangential bands. The sequence of radial arrangement follows the pattern of sieve cells, phloem parenchyma cells, sieve cells and phloem fibers, sieve cells. Many crystals of calbium oxalate are embedded in the radial walls of seive cells. The phloem fibers are of only one type. The phloem rays are homogeneous, uniseriate. According to the anatomical characters of secondary phloem of Glyptostrobus pensilis (Staunt.)Koch and comparison with the other genera of Taxodiaceae, Glyptostrobus, Metasequoia and Taxodium have close relationships.     

Anatomy of Gymnosperms Endemic to China,II. Taiwania flousiana Gaussen (Taxodiaceae)

Taiwania Hayata contains two species: T.flousiana Gaussen and T. cryptomerioides Hayata, both endemic to China. T. flousiana was investigated with both light and scanning electron microscopes in respect to shoot apex, external and internal surfaces of leaf cuticle, primary leaf, juvenal and mature leaves, young stem, secondary phloem and wood of stem, etc, It is shown that the shoot apex consists of the following five regions: (1) the apical initials; (2) the protoderm, (3) the subapical moher cells;. (4) the peripheral meristem, and (5) the pith mother cells. The periclinal and anticlinal division of the apical initials takes place with approximately equal frequency. The juvenal leaf is nearly triangular or crescent-shaped in cross section and belongs to the leaf type II. The mature leaf is quadrangular in cross section (the leaf type I). There are a progressive series of changes in size and shape of the leaf cross section. The stoma of the mature leaf is amphicyclic and occasionally tricyclic. The crystals in the juvenal leaf cuticle are more abundant than those in the mature leaf cuticle. The transfusion tissue conforms to the Cupressus type. The structure of juvenal leaf is the nearest to that in Cunninghamia unicanaliculata D. Y. Wang et H. L. Liu, while the mature leaf is similar to that of the Cryptomeria. Sclerenchymatous cells of the hypodermis in the young stem comprise simple layers and are arranged discontinuously. No primary fibers are found in the primary phloem. Medullary sheath is present between the primary xylem and the pith. There are some sclereids in the pith. The secondary phloem of the stem consists of regularly alternate tangential layers of cells in such a sequence: sieve cells, phloem parenchyma cells, sieve cells, phloem fibers, sieve cells. The phloem fiber may be divided into thick-walled and thin-walled phloem fiber. The crystals of calcium oxalate in the radial walls of sieve cells are abundant. Homogeneous phloem rays are uniseriate or partly biseriate, 1-48 (2-13) cells high, and of 26-31 strips per square mm. Growth rings of the wood in Taiwania are distinct. The bordered pits on the radial walls of early wood tracheids are usually uniseriate, occasionally paired and opposite pitting. Wood parenchyma is present, and its cells contain brown resin substances. Their end walls are smooth, lacking nodular thickenings. Wood rays are homogeneous. Cross-field pits are cupressoid. Resin canals are absent. Based on the anatomy of Taiwania and comparison with the other genera of Taxodiaceae, the authors consider the establishment of Taiwaniaceae not reasonable, but rather support the view that the genus is better placed between Cuninghamia and Arthrotaxis in Taxodiaceae.     

Ultrastructural indications for coexistence of symplastic and apoplastic phloem loading in Commelina benghalensis leaves

The ultrastructural ontogeny of Commelina benghalensis minor-vein elements was followed. The mature minor vein has a restricted number of elements: a sheath of six to eight mestome cells encloses one xylem vessel, three to five vascular parenchyma cells, a companion cell, a thin-walled protophloem sieve-tube member and a thick-walled metaphloem sieve-tube member. The protophloem sieve-tube member (diameter 4–5 m; wall thickness 0.12 m) and the companion cell originated from a common mother cell. The metaphloem sieve-tube member (diameter 3 m; wall thickness 0.2 m) developed from the same precursor cell as the phloem parenchyma cells. Counting the plasmodesmatal frequencies demonstrated a symplastic continuum from mesophyll to the minor-vein phloem. The metaphloem sievetube member and the phloem parenchyma cells are the termini of this symplast. The protophloem sieve-tube member and companion cell constitute an insulated symplastic domain. The symplastic route, mesophyll to metaphloem sieve tube, appears to offer a path for symplastic loading; the protophloem sieve tube may be capable of accumulation from the apoplast. A similar two-way system of loading may exist in a number of plant families. Plasmodesmograms (a novel way to depict cell elements, plasmodesmatal frequencies and vein architecture) of some other species also displayed the anatomical requirements for two routes from mesophyll to sieve tube and indicate the potential coexistence of symplastic and apoplastic loading.     

ULTRASTRUCTURE OF THE NACREOUS LEPTOIDS (SIEVE ELEMENTS) IN THE POLYTRICHACEOUS MOSS ATRICHUM UNDULATUM

The physiological phloem equivalents, leptoids, of the polytrichaceous moss Atrichum undulatum appear to be similar to the nacreous sieve elements that occur in many higher plants. These leptoids are elongated cells with nacreous thickenings on their radial and tangential walls. Their oblique end walls, which lack such thickenings, are traversed by numerous pores through which the plasmalemma, endoplasmic reticulum, and cytoplasm are continuous between adjacent leptoids of a longitudinal file. These end walls closely resemble the simple sieve areas of the sieve elements found in Polypodium vulgare. The leptoid sieve pores have a median expanded area and frequently are occluded by small amorphous protein plugs at each end. Also, callose was observed as electron-luscent areas both on the faces of the end walls and as a thin cylinder surrounding the lateral area of each pore. Amorphous and granular cytoplasmic contents of the leptoids appear to be morphologically similar to the slime (P-protein) found in the sieve-tube elements of many angiosperms. Differentiating leptoids are characterized by the formation of numerous membrane-bound protein bodies in close association with polysomes and endoplasmic reticulum. As the leptoid matures, the contents of the protein bodies become dispersed in the cytoplasm. Ultrastructurally and ontogenetically the leptoids in the gametophores of A. undulatum appear almost identical to the sieve elements of P. vulgare and therefore should be considered sieve elements rather than phloem-like equivalents.     

VASCULAR SYSTEM OF LODICULES OF DACTYLIS GLOMERATA L.

The vascular system for the two lodicules in a floret of Dactylis glomerata L. was studied in serial sections. The floret stele contained a few modified tracheary elements and xylem transfer cells enveloped by a phloem of squat sieve-tube members and intermediary cells. A single sieve tube and associated phloem parenchyma exited the right and left sides of the stele and upon nearing the base of each lodicule branched and formed the minor veins of the lodicule. The minor veins underwent limited branching and anastomosing to form a small three-dimensional system which described an arc during its ascent in the adaxial portion of each lodicule. The sieve tubes in the minor veins extended halfway up the lodicule and contained short sieve-tube members with transverse, slightly oblique, or lateral simple sieve plates. The associated phloem parenchyma cells were intermediary cells, companion cells, and less intimate parenchyma cells. Intermediary cells terminated the minor veins and touched the distal ends of the terminal sieve-tube members, which lacked distal sieve plates. Although the transverse area of the sieve-tube members remained constant up the lodicule, the transverse area of the associated phloem parenchyma fluctuated.     

Anatomy of the Secondary Phloem and the Crystalliferous Phloem Fibers in the Stem of Torreya grandis

The structure of the secondary phloem and the development of the crystaleiferous phloem fibers in the stem of Torrey grandis were observed under the ligth microscope and SEM. The secondary phloem is composed of sieve cells, phloem parenchyma cells, crystalliferous phloem fibers and stone cells in the longitudinal system, and the uniserite homogeneous phloem rays consisting of parenchyma cells only in the radial system. In the cross section, there are 3–9 sieve cells in radial rows forming discontinuous tangential layers, the crystalliferous phloem fibers often in a single discontinuous tangential layer and the stone cells dispersed in rangential layer of phloem parenchyma. The developmental process of crystalliferous phloem fibers is as follows: initial cells appeared in the end of April and were well differentiated in the first week of May. Some crystals were deposited in the primary wall, while others were free in the cell. At the end of May, the secondary wall of most crysalliferous phloem fibers started to be thickened. With the thickening of the secondary wall, all the crystals were embedded in the wall from June to August From the end of September to the early days of October, the crystalliferous phloem fibers reached their full maturation. It is shown by microchemical identification and EDAX analysis that the crystals embedded in the wail of crystalliferous phloem fibers are calcium oxalate crystals.     

THE CAMBIUM AND SEASONAL DEVELOPMENT OF THE PHLOEM IN PYRUS MALUS

Evert , R. F. (U. Wisconsin, Madison.) The cambium and seasonal development of the phloem in Pyrus malus. Amer. Jour. Bot. 50(2): 149–159. Illus. 1963.—The cambium in apple consists of several layers of cells at all times, and practically all cambial cells divide periclinally one or more times before undergoing differentiation. The cambial initials do not seem to be in a uniform, uniseriate layer. Judged by collections made during 2 seasons (August, 1958–October, 1960), the seasonal cycle of phloem development is as follows. Early in April, cells in the outer margin of the cambial zone begin to differentiate into sieve elements. At approximately the same time, activity (division) commences throughout the cambial zone. By the end of July or early August, sieve-element differentiation is completed. Cessation of function begins in either late September or in October with the formation of definitive callose on the sieve areas of sieve elements in the outer margin of the functional phloem. By late November, all sieve elements are devoid of contents and most of their companion cells collapsed. Phloem differentiation precedes xylem differentiation by approximately a month and a half; xylem and phloem differentiation cease almost simultaneously; and fiber-sclereid development is coincident with the period of maximal xylem differentiation.     

Secondary phloem anatomy of Cycadeoidea (Bennettitales)

Secondary phloem anatomy of several species of Cycadeoidea is described from trunks in the Wieland Collection, Peabody Museum of Natural History. The trunks were collected from the Lakota Formation, Lower Cretaceous, Black Hills of South Dakota. Secondary phloem is extensively developed and consists of alternating, tangential bands of fibers and sieve elements, with rare phloem parenchyma. Uniseriate rays, 2-22 cells high, occur between every one to three files of the axial system. Fibers are long, more than 1200 μm, approximately 26.6-34.2 μm in diameter, and have slit-like apertures on the lateral walls. Sieve elements range from 16-25 μm in diameter and are up to 500 μm long. Elliptical sieve areas appear on both end and radial walls and measure 10 μm across; minute spots, which may represent sieve pores, are present within the sieve areas. Secondary phloem of North American Cycadeoidea is similar in organization (alternating tangential bands) and cell types (sieve cells, fibers, axial parenchyma) to that known in other extant and fossil cycadophytes and some seed ferns. The unusual pattern of cell types and thickness of secondary phloem is discussed in the context of plant habit, phloem efficiency, and potential phylogenetic importance.     

On the occurrence of nuclei in mature sieve elements

Summary The secondary phloem of 3 species of the Taxodiaceae and 13 species of woody dicotyledons was examined for the occurrence of nuclei in mature sieve elements. Nuclei were found in all mature sieve cells of Metasequoia glyptostroboides, Sequoia sempervirens and Taxodium distichum, and in some mature sieve-tube members in 12 of the 13 species of woody dicotyledons. Except for nuclei of sieve cells undergoing cessation of function, the nuclei in mature sieve cells of M. glyptostroboides, S. sempervirens and T. distichum were normal in appearance. The occurrence and morphology of nuclei in mature sieve-tube members of the woody dicotyledons were quite variable. Only 3 species, Robinia pseudoacacia, Ulmus americana and Vitis riparia, contained some mature sieve elements with apparently normal nuclei.This research has been supported by National Science Foundation grants GB-5950 and GB-8330.     

Wood Anatomy and the Development of Interxylary Phloem of Ipomoea hederifolia Linn. (Convolvulaceae)

In Ipomoea hederifolia Linn., stems increase in thickness by forming successive rings of cambia. With the increase in stem diameter, the first ring of cambium also gives rise to thin-walled parenchymatous islands along with thick-walled xylem derivatives to its inner side. The size of these islands increases (both radially and tangentially) gradually with the increase in stem diameter. In pencil-thick stems, that is, before the differentiation of a second ring of cambium, some of the parenchyma cells within these islands differentiate into interxylary phloem. Although all successive cambia forms secondary phloem continuously, simultaneous development of interxylary phloem was observed in the innermost successive ring of xylem. In the mature stems, thick-walled parenchyma cells formed at the beginning of secondary growth underwent dedifferentiation and led to the formation of phloem derivatives. Structurally, sieve tube elements showed both simple sieve plates on transverse to slightly oblique end walls and compound sieve plates on the oblique end walls with poorly developed lateral sieve areas. Isolated or groups of two to three sieve elements were noticed in the rays of secondary phloem. They possessed simple sieve plates with distinct companion cells at their corners. The length of these elements was more or less similar to that of ray parenchyma cells but their diameter was slightly less. Similarly, in the secondary xylem, perforated ray cells were noticed in the innermost xylem ring. They were larger than the adjacent ray cells and possessed oval to circular simple perforation plates. The structures of interxylary phloem, perforated ray cells, and ray sieve elements are described in detail.     

Anatomical Studies on Secondary Phloem of Pinus bungeana

This paper deals with structures of the secondary phloem of Pinus bungeana Zucc. The sieve cells lived for only one growing season. Most of them formed in spring and summer and then died in the end of winter. However, some of them formed in autumn and died eventually in the end of next spring. Two types of albuminous cells: type A and type B were seen in radial plates and rays, which possessed the following common characteristics, there were unilateral sieve area connections between these and the sieve cells. These cells had larger nuclei, denser cytoplasm with abundant mitochondria and rich RNA-protein. Their death closely followed that of the sieve cells. Type A albuminous cells differed from type B in that the former collapsed before the contents of sieve cells accompanied with it dissppeared. But, type B did not collapse until complete disappearance of the contents of sieve cells. The cytological characteristics of albuminous cells, the relationship between radial plates and rays, and possible physiological significance are also disscussed.     

Immunocytochemical localisation of phloem lectin from Cucurbita maxima using peroxidase and colloidal-gold labels

Antibodies were raised against lectin purified from the sieve-tube exudate of Cucurbita maxima. Immunocytochemistry, using peroxidase-labelled antibodies and Protein A-colloidal gold, was employed to determine the location of the lectin within the tissues and cells of C. maxima and other cucurbit species. The anti-lectin antibodies bound to P-protein aggregates in sieve elements and companion cells, predominantly in the extrafascicular phloem of C. maxima. This may reflect the low rate of translocation in these cells. Under the electron microscope, the lectin was shown to be a component of P-protein filaments and was also found in association with the sieve-tube reticulum which lines the plasmalemma. The anti-lectin antibodies reacted with sieve-tube proteins from other species of the genus Cucurbita but showed only limited reaction with other genera. We suggest that the lectin serves to anchor P-protein filaments and associated proteins to the parietal layer of sieve elements.Abbreviation SDS-PAGE sodium dodecyl sulphate-polyacrylamide gel electrophoresis     

PHLOEM ANATOMY OF THE CARBONIFEROUS COENOPTERID FERNS ANACHOROPTERIS AND ANKYROPTERIS

Phloem histology in the petioles of two genera of Pennsylvanian ferns is detailed from coal balls collected at various localities in North America. Both Ankyropteris and Anachoropteris have primary phloem that completely surrounds the central xylem trace and is separated from it by a parenchymatous sheath. Ankyropteris contains very narrow (about 13.5 μm diam) sieve elements and a few strands of phloem parenchyma. End walls are either horizontal or slightly oblique and sieve areas as well as scattered individual pores have been observed. Anachoropteris phloem contains two different sizes of sieve elements. Small sieve elements that surround the C-shaped trace are similar to those seen in Ankyropteris. Larger elements (approximately 50–120 μm in diam) are present only within the C-shaped trace, and are elongate (up to 2.5 mm) with very oblique end walls. Sieve areas on these large cells are conspicuous, 5–8.5 μm in diam and aggregated into groups. The cell wall within each sieve area appears to be composed of criss-crossed fibrillar material. Phloem anatomy in these two ferns is compared to that previously described in other Carboniferous vascular cryptogams, as well as that known from extant plants.     

Ultrastructure of Sieve Elements in Secondary Phloem of Dalbegia odorifera During Leaf-Bearing and Leaf-Absent Period

Ultrastructures of sieve elements of secondary phloem of 1–2 year old branchlet of tropical deciduous tree Dalbegia odorifera T. Chen growing on Hainan Island were studied under transmission electron microscope and a comparation was made between the sieve elements in leaf-bearing and leaf-absent period. During the leaf-bearing period, there was a tailed spindleshaped P-protein body in each mature sieve element. The main part of the P-protein body con sisted of a disordered fine fiber mass with two crystalline tails. The sieve elements had horizontal end walls with simple sieve plate. The inner layers of the wall near the sieve plate appeared intumescent, protruding into the sieve element lumen. During the leaf-absent period, a functional phloem remained about the same thickness as that during the leaf-bearing period. The sieve elements in the leaf-absent period contained normal protoplasts and the P-protein and the sieve plate pores had the same structures as those during the leaf-bearing period. More starch grains and vesicles were found in sieve elements in the leaf-absent period.     

Structural Evidence for Plastid Inclusions as a Possible 'Sealing' Mechanism in the Phloem of Monocotyledons

Sieve elements in monocotyledons possess unique plastids. Structuralevidence indicates that when mature sieve-tube members are mechanicallyperturbed the plastids release proteinaceous, quasi –crystalline inclusions. The inclusions become lodged in sieve-platepores and appear to seal portions of phloem sieve tubes.     

(Questions) on phloem biology. 2. Mass flow, molecular hopping, distribution patterns and macromolecular signalling

This review speculates on correlations between mass flow in sieve tubes and the distribution of photoassimilates and macromolecular signals. Since micro- (low-molecular compounds) and macromolecules are withdrawn from, and released into, the sieve-tube sap at various rates, distribution patterns of these compounds do not strictly obey mass-flow predictions. Due to serial release and retrieval transport steps executed by sieve tube plasma membranes, micromolecules are proposed to “hop” between sieve element/companion cell complexes and phloem parenchyma cells under source-limiting conditions (apoplasmic hopping). Under sink-limiting conditions, micromolecules escape from sieve tubes via pore-plasmodesma units and are temporarily stored. It is speculated that macromolecules “hop” between sieve elements and companion cells using plasmodesmal trafficking mechanisms (symplasmic hopping). We explore how differential tagging may influence distribution patterns of macromolecules and how their bidirectional movement could arise. Effects of exudation techniques on the macromolecular composition of sieve-tube sap are discussed.