THE FINE STRUCTURE OF DIFFERENTIATING XYLEM ELEMENTS

Differentiating xylem elements of Avena coleoptiles have been examined by light and electron microscopy. Fixation in 2 per cent phosphate-buffered osmium tetroxide and in 6 per cent glutaraldehyde, followed by 2 per cent osmium tetroxide, revealed details of the cell wall and cytoplasmic fine structure. The localized secondary wall thickening identified the xylem elements and indicated their state of differentiation. These differentiating xylem elements have dense cytoplasmic contents in which the dictyosomes and elements of rough endoplasmic reticulum are especially numerous. Vesicles are associated with the dictyosomes and are found throughout the cytoplasm. In many cases, these vesicles have electron-opaque contents. "Microtubules" are abundant in the peripheral cytoplasm and are always associated with the secondary wall thickenings. These microtubules are oriented in a direction parallel to the microfibrillar direction of the thickenings. Other tubules are frequently found between the cell wall and the plasma membrane. Our results support the view that the morphological association of the "microtubules" with developing cell wall thickenings may have a functional significance, especially with respect to the orientation of the microfibrils. Dictyosomes and endoplasmic reticulum may have a function in some way connected with the synthetic mechanism of cell wall deposition.     

扁圆封印木（相似种）茎干的解剖特征

贵州省水城矿区晚二叠世煤核中扁圆封印木（相似种Ｓｉｇｉｌｌａｒｉａ ｃｆ．ｂｒａｒｄｉｉＢｒｏｎｇｎ．）茎干的主要解剖特征如下：管状中柱,具多边形薄壁细胞组成的髓。初生木质部成环带状,外缘呈规则的齿槽状,向心式发育。次生木质部显束状特征,横切面管胞为方圆至长方形,纵切面为梯状壁增厚,并具流苏纹。射线１—２列细胞宽,数个至十余个细胞高。叶迹起源于初生木质部外缘的槽中,中始式,但以向心发育为主。     

对开蕨属导管的发现

通过对开蕨的解剖学研究,发现在根的次生木质部中,存在单穿孔倾斜乃至横的穿孔板的孔纹导管,螺纹加厚明显,是蕨类植物中一种新类型的导管。     

The role of microtubules in vessel member differentiation inColeus

Summary The role of microtubules in tracheary element formation in cultured stem segments ofColeus has been investigated through the use of the antimicrotubule drug, colchicine. Colchicine treatment of the cultured stem segments produced a dual effect on xylem differentiation. If applied at the time of stem segment isolation or shortly thereafter, wound vessel member formation is almost completely blocked. However, if colchicine is applied after the third day of culture, it does not inhibit differentiation, but instead large numbers of xylem elements are formed which have highly deformed secondary walls. Both effects are related to colchicine's specific affinity for microtubules. In the first case it is shown that colchicine blocks mitosis, presumably by destroying the spindle apparatus, and thus inhibits divisions which are prerequisite for the initiation of xylem differentiation. While, if colchicine is applied after the necessary preparative divisions have taken place, it destroys specifically the cortical microtubules associated with the developing bands of secondary wall, thus causing aberrant wall deposition.Light and electron microscopic analysis of drug-treated cells reveals that the secondary wall becomes smeared over the surface of the primary wall and does not retain the discrete banded pattern characteristic of secondary thickenings in untreated cells. Examination of colchicine-treated secondary walls in KMnO₄ fixed material shows that in the absence of microtubules the cellulose microfibrils lose their normal parallel orientation and are deposited in swirls and curved configurations, and often lie at sharp angles to the axis of the secondary wall band. Microtubules, thus, appear to play a major role in defining the pattern of secondary wall deposition and in directing the orientation of the cellulose microfibrils of the wall. Factors in addition to microtubules also act in controlling the secondary wall pattern, since we observe that even in the absence of microtubules secondary thickenings of two adjacent xylem elements are deposited directly opposite one another across the common primary wall.     

A comparative ultrastructural study of pit membranes with plasmodesmata associated thickenings in four angiosperm species

Recent micromorphological observations of angiosperm pit membranes have extended the number and range of taxa with pseudo-tori in tracheary elements. This study investigates at ultrastructural level (TEM) the development of pseudo-tori in the unrelated Malus yunnanensis, Ligustrum vulgare, Pittosporum tenuifolium, and Vaccinium myrtillus in order to determine whether these plasmodesmata associated thickenings have a similar developmental pattern across flowering plants. At early ontogenetic stages, the formation of a primary thickening was observed, resulting from swelling of the pit membrane in fibre-tracheids and vessel elements. Since plasmodesmata appear to be frequently, but not always, associated with these primary pit membrane thickenings, it remains unclear which ultrastructural characteristics control the formation of pseudo-tori. At a very late stage during xylem differentiation, a secondary thickening is deposited on the primary pit membrane thickening. Plasmodesmata are always associated with pseudo-tori at these final developmental stages. After autolysis, the secondary thickening becomes electron-dense and persistent, while the primary thickening turns transparent and partially or entirely dissolves. The developmental patterns observed in the species studied are similar and agree with former ontogenetic studies in Rosaceae, suggesting that pseudo-tori might be homologous features across angiosperms.     

The Secondary Xylem Anatomy of 6 Desert Plants of Caragana

The structures of secondary xylem of 6 species of Caragana, which grow in desert regions of Northwest of China, are described in details. The main quantitative characters are compared with species. And a key to the identification of wood structures of 6 species is given. The main similarities of secondary xylem of these 6 species are as follows: vessels per mm2 very numerous, percentage of multiple vessels high; vessel elements very short, perforations simple and in almost horizontal end walls; intervessel bordered pits alternate and vestured, and vessels with spiral thickenings. Libriform fibres are very short, and usually with thick end walls, and with simple pits. Rays are heigh to very low, and with multiseriate and uniseriate, and with heterogeneous type Ⅰ and Ⅱ. In addition, there are differences in other characters, e.g. vessel distribution, amount of axial parenchyma and distribution, ray frequency, crystals present or absent, and crystal distribution, if present. These differences can be used as the anatomical characters to identify the wood structures of the 6 species. In this article we also discuss the relation between the structure of xylem elements and the environmental influences.     

Occurrence of Helical Thickenings on the Vessel Element Walls of Dicotyledonous Woods

One-hundred and forty taxa belonging to 38 families were examinedfor helical thickenings on their vessel element walls usingscanning electron microscopy. Helical thickenings covering theentire vessel element wall occur in seventeen taxa and in ninetaxa they are localized. The thickenings are present in somevessel elements only except in members of the Meliaceae. Thelatter have prominent thickenings on the narrow vessel elementwalls and fine thickenings or rarely, no thickenings on thewider vessel element walls. In the Meliaceae they occur as discontinuoushelices which merge with the vessel element walls at intervals.The systematic and diagnostic value of the thickenings is limitedbecause of their wide, morphological variations. There is noevidence for physiological or ecological significance of helicalthickenings. Their frequent presence in temperate and subtropicalregions requires an analysis of their probable adaptive significance. Scanning electron microscopy, wood anatomy, vessel element wall, helical thickenings     

Development of mestome sheath cells in leaves ofAegilops comosa var.thessalica

Summary The development of mestome sheath cells ofAegilops comosa var.thessalica was studied by electron microscopy. Anatomical and cytological observations show that this grass belongs to the C₃ or non-Kranz plants. In the asymmetrically thickened walls of mestome sheath cells a suberized lamella is present. This lamella is deposited asynchronously. In the midrib and the large lateral bundles it appears first in the outer and inner walls and usually later in the radial walls. In the small lateral bundles its appearance is delayed in the inner walls of those cells situated on the xylem side. At maturity the suberized lamella is observed in all cell walls; however, in the small lateral bundles it is partly or totally absent from the walls of some cells situated on the xylem side. Tertiary wall formation is asynchronous as well, for it generally follows the deposition pattern of the suberized lamella.During the development of the mestome sheath cells microtubules show marked changes in their number and orientation, being fewer and longitudinal during suberin deposition. Dictyosomes are very active and may be involved in primary and tertiary wall formation. Endoplasmic reticulum cisternae are abundant and partly smooth, while plasmalemmasomes may function to reduce the plasmalemma extension. However, cytoplasmic structures that are clearly involved in suberin synthesis could not be identified.Suberized lamellae react strongly with silver hexamine. This is probably due to post-fixation with osmium tetroxide.On the basis of structural characteristics the mestome sheath may be regarded as an endodermis (cf., alsoFahn 1974). The significance of this view for water and assimilate exchange between the mesophyll and the bundle is discussed.This report represents a portion of a doctoral dissertation.     

八属木兰科植物木材导管分子的比较解剖

本文比较描述了我国木兰科木莲属、华盖木属,长蕊木兰属,拟单性木兰属,合果木属,观光木属,香木兰属和鹅掌楸属等8属22种植物次生质部的导管分子,这些导管分子的长度和宽度都有差异,木莲属的所有种中都有具梯状穿孔板的导管分子,但在少数种中偶然可见到单穿孔,在其它属中,也都有具梯状穿孔板的导管分子,仅在观光木属中偶见的单穿孔,木莲属大多数种均无螺纹加厚,仅桂南木莲一种除外,另外,除了合果木属和鹅掌楸无螺纹加厚外,其余均有此种加厚,各属的导管分子还存在其它一些差异。     

CERCIDIUM FLORIDUM SEED COAT,LIGHT AND ELECTRON MICROSCOPIC STUDY

Scott , Flora Murray , B. G. Bystrom , and E. Bowler . (U. California, Los Angeles.) Cercidium floridum seed coat, light and electron microscopic study. Amer. Jour. Bot. 49(8): 821–833. Illus. 1962.—The structure of the seed coat of the desert tree Cercidium is typical of the family Leguminosae, but the inner integument is mucilaginous. The characteristic palisade cells of the epidermis and the many-armed cells of the mucilaginous zone are discussed in detail. The minute wax rodlets on the surface of the young seed fuse later into a film of wax. The epidermal palisade cells are polygonal in transection and the walls are strengthened by cellulose flanges. The palisade cells are enclosed in a thin suberin-like sheath which connects with the film which lines all air-filled intercellular spaces in the outer integument. Plasmodesmata extend from the protoplast through the subcuticular zone to the cuticle and also interconnect with adjacent cells through numerous pits in the interflange areas of the main wall. The cells of the inner integument are many-armed. Intercellular spaces at all stages of growth observed are occluded with mucilage; nevertheless, they are lined with a suberin-like film similar in reaction to that of the air-filled intercellular spaces of the outer integument. The distribution of wall materials and ergastic substances was determined by microchemical tests. For electron microscopic study, the epidermis and the mucilaginous zone were fragmented ultrasonically, after chemical treatment when necessary. Cuticular fragments, frequently polygonal in outline, are dense to the electron beam at photographic illumination. When exposed to the electron beam, full-strength, volatile components are driven off and condense on other wall fragments or on substrate. Unique structures occur in the subcuticular zone, termed here, collar, cone and paddle structure. The basic material of the cell wall as a whole is cellulose impregnated with matrix materials, mainly non-cellulosic polysaccharides. In addition, lipid-like substances are conspicuous particularly in the flanges of the palisade cells. Under the electron beam, full-strength, these substances volatilize and leave the flanges with a blistered outline except in the region of the light line. The volatile substances may be absent from this zone or the microfibrillar structure may prevent distortion. The microfibrillar pattern throughout the length of the flanges is dominantly parallel, but a change to reticulate arrangement occurs in the light-line region. Numerous pits mark the paths of the plasmodesmata in the subcuticular zone and also through flange and interflange areas of the wall. The dominant microfibrillar pattern in the mucilage cells is reticulate in young and old cells, except for helical orientation in extending arms. Pits occur on arm ends but are infrequent elsewhere. The loose microfibrillar structure presumably allows continuous secretion of mucilage.     

Anatomical Characteristics of the Stem of Sigillaria cf. brardii from Coal Balls in Guizhou Province,China

The stem specimens of Sigillaria cf. brardii were collected from the coal balls of Upper Permian in Shuicheng Coal Mines in Guizhou Province. The main anatomical characteristics of Sigillaria cf. brardii are described as follows: The stem is siphonostelic, with pith composed entirely of polygonal parenchyma cells, there are secondary walls in some pith cell cavities these secondary walls show the characters of cell division. Surrounding the pith is the continuous cylindrical primary xylem which consists entirely of tracheids. The outermost, and part are the protoxylem elements show spiral secondary thickenings. In cross section, the outer edge of exarch primary xylem appears regularly sinuous, with trace of mesarch leaf originating from the furrows. The centripetal metaxylem is characterized by scalariform wall thickenings on the tracheids, and delicated strands of secondary wall materials extending between abjacent bars, these structures are called fimbris, or williamson striations, and are characteristic in lepidodendrids. The secondary xylem consists of tracheids and vascular rays. The tracheids, too, have scalariform wall thickenings and fimbris. The rays are one-to twocell width and several to more than ten cells in height.     

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.     

生长在太白山上的紫萼丁香木材的生态解剖

生长在秦岭太白山北坡的紫萼丁香(Syringa oblata var.giraldii Rehd.)的木材结构随着海拔高度的增加而变化。其年轮宽度,导管分子长度、直径;纤维管胞长度、宽度;射线高度等值逐渐增加。导管频率、射线频率、单孔率值则逐渐减少。导管、纤维管胞的壁减薄,其次生壁上的螺纹加厚由明显到不明显。     

SOME ULTRASTRUCTURAL OBSERVATIONS ON ENDODERMAL CELL DEVELOPMENT IN ZEA MAYS ROOTS

The fine structure of primary, secondary, and tertiary stages of Zea endodermal cell development was investigated. The casparian strip formed in situ in the anticlinal walls and remained at a fixed point relative to the endodermis-pericycle boundary. The only protoplasmic structure that had a constant spatial association with the developing strip was the plasmalemma. Plasmodesmata appeared to be more numerous on the tangential walls than on radial walls; only rarely were they located in the casparian strip. The suberized lamella developed on inner and outer tangential walls before it appeared on the radial walls. No cytoplasmic organelles were found to have any particular spatial association with this layer. The suberized lamella was about 0.04 μm thick except near plasmodesmata and along the adaxial margin of the casparian strip, where it was thicker. Occasionally it failed to form along the abaxial margin of the strip. The adherent affinity between plasmalemma and casparian strip was lost after the strip was covered by suberized lamella. The secondary wall became asymmetrically thickened by differential deposition of successive lamellae. A thin layer of secondary wall material extended across the floor of each pit. Pit cavities often contained mitochondria, and plasmodesmata were restricted to the pits. The plasmodesmata were constricted where they entered the thin layer of secondary wall material and where they penetrated the suberized lamella. The various stages of cell development tended to be asynchronous. No passage cells were observed. Endodermal cell development in Zea closely resembles that described for barley.     

Testing hypotheses that link wood anatomy to cavitation resistance and hydraulic conductivity in the genus Acer

? Vulnerability to cavitation and conductive efficiency depend on xylem anatomy. We tested a large range of structure-function hypotheses, some for the first time, within a single genus to minimize phylogenetic 'noise' and maximize detection of functionally relevant variation. ? This integrative study combined in-depth anatomical observations using light, scanning and transmission electron microscopy of seven Acer taxa, and compared these observations with empirical measures of xylem hydraulics. ? Our results reveal a 2 MPa range in species' mean cavitation pressure (MCP). MCP was strongly correlated with intervessel pit structure (membrane thickness and porosity, chamber depth), weakly correlated with pit number per vessel, and not related to pit area per vessel. At the tissue level, there was a strong correlation between MCP and mechanical strength parameters, and some of the first evidence is provided for the functional significance of vessel grouping and thickenings on inner vessel walls. In addition, a strong trade-off was observed between xylem-specific conductivity and MCP. Vessel length and intervessel wall characteristics were implicated in this safety-efficiency trade-off. ? Cavitation resistance and hydraulic conductivity in Acer appear to be controlled by a very complex interaction between tissue, vessel network and pit characteristics.     

Spiral Thickenings of Vessel Elements in Magnoliaceae in China

Spiral thickenings in the inner walls of vessels were studied in 10 genera 69 species of Magnoliaceae in China under scanning electron microscopic observation. Spiral thickenings were found in some species. The majority of these species showed complete thickenings of the inner wall, however, in some of which the spiral thickenings were less obvious at both ends. In others, these spiral thickenings existed only in the middle portion, uniformly or irregularly distributed.     

MODIFIED REMNANTS OF CENTRIPETAL XYLEM IN THE LEAF TRACE OF CEPHALOTAXUS

A study of the leaf traces at the nodes in various species and varietal forms of Taxus, Torreya, Amentotaxus and Cephalotaxus reveals, only in Cephalotaxus, an unusual type of parenchymatous tissue associated with the xylem of the leaf trace. The cells of this tissue occur in one to three layers, have abundant cytoplasm and conspicuous nuclei. The thin walls of these cells are devoid of pits and show spiral or spiral-reticulate thickenings. These thickenings, although readily taking the counterstain, reveal the presence of lignin as determined by the phloroglucin test and by fluorescence microscopy. This tissue is always internal—that is, adaxial—to the protoxylem of the trace. From the node it accompanies the trace for a short distance into the leaf base, where it gives way centrally to the typical leaf parenchyma and laterally to the transfusion tissue on the flanks of the bundle. In the basipetal direction it is in contact with the inner face of the annular protoxylem of the stele, eventually disappearing in the typical pith parenchyma. This tissue occasionally reveals instances of well-lignified tracheid-like centripetal elements. On the basis of the characteristics of this tissue, it is suggested that its origin lies in former centripetal xylem. The significance of this tissue to the evolution of the stele and the systematic position of Cephalotaxus is discussed.     

RETICULATE THICKENINGS IN SOME SPECIES OF JUGLANS

Reticulate thickenings in Juglans nigra, J. major, J. microcarpa, and J. californica (including J. hindsii) appear, under the transmitted light microscope, to be a cluster of very large or “gash-like” vessel-ray pits. The use of scanning electron microscopy shows that these so-called pits are thickenings on the inside wall of the vessel.     

Water transport in xylem conduits with ring thickenings

Helical or annular wall thickenings are not only present in protoxylem, but may also he a feature of the tracheids or vessel elements of secondary wood. The frequency of their occurrence tends to be a function of climatic factors and conduit diameter. In order to obtain a functional explanation for these structures, the hydrodynamic behaviour of xylem conduits with various patterns of annular wall thickenings was investigated numerically using a commercial CFD (Computational Fluid Dynamics) package. The fluid flow phenomena are presented in detail. The calculations show that the developing pressure gradient of the structures with corrugated walls is in each case lower than that of a smooth pipe with a diameter corresponding to the distance between two opposite thickenings. Furthermore, complex flow patterns with circulation zones between the thickenings develop which are dependent on the geometry of the wall. It may be hypothesized that these circulation zones influence the lateral water flow. The results are discussed with regard to the relationships between the water conduction function of the xylem and ecological factors.     

Comparative Anatomical Studies on the Vessel Elements of Eight Genera in Magnoliaceae

This paper deals mainly with comparative studies on vessel elements of secondary xylem in Chinese Manglietia, Manglietiastrum, Alcimandra, Parakmeria, Paramichelia, Tsoongiodendron, Aromadendron and Liriodendron. There are some differences between the genera in vessel element length and width. Manglietia species all have scalariform perforation plates, only a few also have simple perforations. The other genera have scalariform perforation plates. only one (Tsoongiodendron odorum Chun) has simple perforations in addition. There are no spiral thickenings in Manglietia except for one species [Manglietia chingii Dandy (M. temuipes Dandy)] . Paramichelia and Liriodendron have no spiral thichenings, but they are present in the other genera. In addition, some other differences in vessel elements between these 8 genera are also discussed.