Suberized tyloses in trees: An ultrastructural and cytochemical study

The nature of the wall layers observed in suberized tyloses was studied in Populus basalmifera L., Ulmus americana L. and Quercus rubra L. As the suberin layers were present only in tyloses that had completed their expansion, most of the results concern mature tyloses. The cyto- and immunocytochemical tests were conducted, respectively, with an exoglucanase having a binding affinity for β(1→4)-D-glucans, the subunits of cellulose, and with two monoclonal antibodies specific for un-esterified and esterified pectic molecules. In the three species, labelling for pectic compounds was intense over the external layer of tyloses but usually more dispersed or nearly absent over the layer corresponding to a primary wall that was, however, intensely labelled for β(1→4)-D-glucans. The outer wall layer, comparable to a middle lamella in mature tyloses, was continuous with similar material that appeared to be secreted by the tylosis. This material was particularly abundant in pit chambers, in void spaces between the tylosis and the vessel wall, particularly at the junction of the vessel and two adjacent cells, and close to the rim of vessel perforation plates. In P. balsamifera, a single suberized layer or occasionally a succession of suberized and cellulose-containing layers was observed internal to the tylosis primary wall. In U. americana, the wall of tylosis was similar to that of P. balsamifera except that, at times, a secondary-wall-like layer was formed and only a single suberized layer was observed. In Q. rubra, the suberized layer was always observed internal to the tylosis secondary wall. Simple pits were also constantly noted in Q. rubra tyloses. The occasional occurrence of a cellulosic layer internally to the suberized layer was observed in the three species. Histochemical tests revealed that lignin was also an important component of the tylosis wall. The tyloses frequently contained phenolic compounds in close association with the suberized layers. The significance of the formation of suberized tyloses in trees is discussed.     

A suberized perimedullary reaction zone in Populus balsamifera novel for compartmentalization in trees

 Following artificial inoculation of nonhost Populus balsamifera with Ophiostoma ulmi, structural defensive tissues were formed in the xylem. Among these tissues there was a perimedullary sheath of cells, located adjacent to the invaded xylem, that originated from the dedifferentiation of perimedullary and xylem parenchyma cells. Histochemical tests revealed that this sheath was intensively suberized. A band of lignified cells was frequently detected on both sides of this suberized tissue. The formation of such a tissue at the pith margin represents a new type of anatomical barrier in relation to compartmentalization processes described for trees. Ultrastructural examination showed that the wall of cells forming this zone was generally composed of a compound middle lamella, a suberized secondary wall and a tertiary wall layer. Using colloidal gold conjugated to monoclonal antibodies against pectin and to an exoglucanase for cellulose, only limited labelling was obtained for pectin whereas labelling for cellulose was abundant in the compound middle lamella and the tertiary wall layer. In a few fibres close to this suberized zone, the latter probe also made it possible to distinguish the occasional presence of several alternating wall layers mainly composed of either suberin or cellulose. In Salix sp., another tree species belonging to the Salicaceae, this type of suberized reaction zone was also observed. The new reaction zone is similar in structure and location to a suberized barrier formed in nonwoody carnation (Dianthus caryophyllus) plants in the defense against vascular fungi. Received: 30 July 1996 / Accepted: 7 January 1997     

Histological analysis in shoot organogenesis from hypocotyl explants of<Emphasis Type="Italic"> Kandelia candel</Emphasis> (Rhizophoraceae)

In vitro culture of hypocotyl explants from Kandelia candel, a common mangrove species, on hormone-free Murashige and Skoog (MS) medium resulted in shoot formation. Since the hypocotyls showed good potential for in vitro shoot multiplication, the process of bud primordium formation was analyzed from a histological viewpoint. A wound periderm first appeared at the top, exposed cut surface of the explants. The wound-induced meristem continued to divide giving rise to suberized cells oriented towards the cut surface. After formation of the suberized cell layers, the meristem and its inner derivatives differentiated into multilayered, uniformly packed parenchyma cells. Bud primordia differentiated from the dense cytoplasmic cells of the wound-induced meristem just beneath the suberized layer near the severed vascular bundles. Each explant produced several visible shoot buds. Furthermore, histological sections revealed that additional bud primordia were present within the explant just underneath the suberized cells and that these bud primordia appeared to be arrested in their development. The fact that additional bud primordia were present within the explant suggests that further manipulation of the explant is helpful to maximize the potential of this system.     

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.     

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.     

Response of xylem parenchyma by suberization in some hardwoods after mechanical injury

Summary Wound responses of xylem parenchyma by suberization were investigated in some hardwoods by light and electron microscopy. Suberized ray and axial parenchyma cells form a distinct boundary around the wound in all investigated species. Vessels and fibres within and close behind the suberized area appeared more or less occluded; vessels in Fagus, Quercus, and Populus contained suberized tyloses, those in Betula and Tilia contained amorphous and fibrillar deposits. A common mechanism for suberin deposition in the parenchyma cells became evident. Cisternae of the endoplasmic reticulum were apparently involved in suberization. Suberin compounds are extruded by cytoplasmic vesicles, which fused with the plasma membrane, in order to release their content. The suberin layer exhibited the typical lamellated structure; cytoplasmic continuity between suberized cells by plasmodesmata was maintained through the suberin layer. Fagus revealed the most intense suberized area as compared with the other species. Within the reaction zone of Fagus and Quercus, some individual ray and axial parenchyma cells exhibited a subdivision into 2 or 3 compartments prior to suberization. Subdivision was achieved by the formation of a primary wall-like layer. Subsequently, the compartments became individually suberized. Wounding during winter did not induce suberization. Also, samples wounded and kept under water during the vegetation period showed no response. The role of suberization in the effectivity of wound-associated compartmentalization is discussed.     

Ultrastructure and Development of Oil Idioblasts in Annona muricata L.

The ultrastructure and development of oil idioblasts in theshoot apex and leaves in Annona muricata L. are described, andthree arbitrary developmental stages are distinguished: cellsin which no additional cell wall layers have been depositedagainst the initial primary cell wall, possessing an electron-translucentcytoplasm and distinct plastids which lack thylakoids (stage1); cells in which a suberized layer has been deposited againstthe primary wall (stage 2, the cytoplasm resembles that of thepreceding stage), and cells in which an additional inner walllayer has been deposited against the suberized layer, whichincreases in thickness with development (stage 3). In this stagean oil cavity is formed, surrounded by the plasmalemma, andattached to a bell-like protrusion of the inner wall layer,the cupule. A complex membranous structure occurs next to thecupule. Smooth tubular endoplasmic reticulum (ER), appearingas linearly arranged tubules, and groups of crystalline bodieswith an almost hexagonal outline are present. The final stagewas further subdivided into three subgroups (a, b, c) basedon the extent of the oil cavity, its contents, and the compositionof the cytoplasm, and increasing thickness of the inner walllayer. The oil is probably synthesized in the plastids, releasedinto the cytoplasm, and then passed through the plasmalemmasurrounding the oil cavity. Oil idioblasts, Annona muricata L., suberized layer, inner wall layer, oil cavity, cupule, smooth tubular ER, crystalline bodies     

ULTRASTRUCTURE OF THE CORALLINACEAE (RHODOPHYTA) II. VEGETATIVE CELLS OF LITHOTHRIX ASPERGILLUM1

The ultrastructure of the calcareous red coralline alga Lithothrix aspergillum Gray and the development of the various tissue types has been studied. The sub-apical meristematic tissue alternately produces genicular or intergenicular cells. The genicular cells rapidly elongate and their cell walls thicken and become denser as more fibrillar wall material is laid down within the cell wall. These cells contain little cytoplasm and few organelles. The inter genicular cells which elongate only slightly during development have a small vacuole and many free starch grains in the cytoplasm. The peripheral cells in each inter genicular layer remain meristematic and form a cortical cell layer over the genicular cells. These cortical cells and the apical meristematic cells are covered by small epidermal cells which have extensive cell wall ingrowths between the chloroplasts. The inter genicular cells are calcified. Although the CaCO₃ is laid down within the cell walls, there is always a thin layer of CaCO₃-free organic cell wall material between the plasmalemma and the CaCO₃ impregnated wall. Only the distal tips of the genicular cells are calcified. In old genicular tissues of Lithothrix, secondary deposits of CaCO₃ of unknown crystallography are also found in the spaces between the cell walls. Thus there appear to be at least two mechanisms of calcification in this alga.     

菰适应湿地环境的解剖和屏障结构特征研究

菰(Zizania latifolia)是一种多年生挺水植物,为了探讨该植物根、茎和叶的解剖结构、组织化学及其质外体屏障的通透性生理。该文利用光学显微镜和荧光显微镜,对菰的根、茎、叶进行了解剖学和组织化学研究。结果表明:(1)菰不定根解剖结构由外而内分别为表皮、外皮层、单层细胞的厚壁机械组织层、皮层、内皮层和维管柱;茎结构由外而内分别为角质层、表皮、周缘厚壁机械组织层、皮层、具维管束的厚壁组织层和髓腔。叶鞘具有表皮和具维管束皮层,叶片具有表皮,叶肉和维管束。(2)不定根具有位于内侧的内皮层及其邻近栓质化细胞和外侧的外皮层组成的屏障结构;茎具内侧厚壁机械组织层,外侧的角质层和周缘厚壁机械组织层组成的屏障结构,屏障结构的细胞壁具凯氏带、木栓质和木质素沉积的组织化学特点,叶表面具有角质层。(3)菰通气组织包括根中通气组织,茎、叶皮层的通气组织和髓腔。(4)菰的屏障结构和解剖结构是其适应湿地环境的重要特征,但其茎周缘厚壁层和厚壁组织层较薄。由此推测,菰适应湿地环境,但在旱生环境中分布有一定的局限性。     

Organization of mature external glands on the trap and other organs of the bladderwortUtricularia monanthos

Summary The mature dome-shaped glands which cover the outer surfaces of the trap, leaves, anchor and runner stolons inU. monanthos are described using conventional and some high voltage transmission electron microscopy. The glands occur as scattered ordinary external glands and as a compact clump of vestibule glands at the entrance to the doorway. Each gland rests on a basal epidermal cell and consists of a single pedestal and terminal cell. Vestibule and leaf glands differ slightly from the other glands mainly in the structure of the outer wall of the terminal cell. Nuclear crystals are prominent and the cytoplasm of the pedestal and terminal cells contains tubular structures usually aggregated near the nucleus. The pedestal cell is a transfer cell with short wall protuberances on the outer wall, conspicuous mitochondria and a heavily impregnated lateral wall.The terminal cell often has an outer wall that is greatly thickened and a protoplast that may degenerate early. In the most developed cells the protoplast remains active for a long period and the outer wall is differentiated into several layers. The outermost layer is cuticularized consisting of an open meshwork of deposits. In leaf glands a local polysaccharide mass is usually developed within the cuticularized region. The inner non-impregnated region of the outer wall may show four layers. In vestibule glands fewer layers are present and the wall shows prominent lamellations. Some ordinary external glands differentiate a sponge-like substructure within the inner wall.The ultrastructure and function of the glands are discussed. We support the concept that mature external glands are responsible for secreting water, with those on traps being particularly active during the resetting of the organ. Our work provides a structural basis for recent suggestions by other workers that the mechanism of secretion probably involves establishing a standing osmotic gradient within the gland.     

Ultrastructure of the suberized styloid crystal cells in Agave leaves

Summary Styloid calcium oxalate crystal idioblasts of Agave americana L. are suberized. Where the crystals do not touch the cell wall directly they are enclosed in a suberinic sheath which is connected with the suberinic wall layer. No polysaccharides are laid down as a tertiary wall layer, nor could any polysaccharides be found in the crystal sheath. These results contradict those of Arnott (1973) but agree fully with those of Rothert and Zalenski (1899).     

Ultrastructural aspects of kranz anatomy inDigitaria sanguinalis andSetaria viridis (poaceae)

Structural differentiation of Kranz anatomy has been investigated in leaf cross sections of two C-4 Poaceae:Digitaria sanguinalis andSetaria viridis. The study mainly focused on cellular and interfacial features of bundle sheath (BS) and mesophyll (MS) cells of the C-4 structure. Prominent BS, spaced by only two MS cells apart, were surrounded concentrically by a layer of MS cells. BS cells ofS. viridis had centrifugally arranged relatively large chloroplasts containing much starch, but the chloroplasts had agrana to rudimentary grana. Structural and size dimorphisms, when starch was present, were detected between BS and MS chloroplasts. Loosely arranged MS cells had peripherally displaced smaller chloroplasts containing little to none starch. BS chloroplasts ofD. sanguinalis were similar to those ofS. viridis, but had very little starch and well-developed long agranal stroma lamella. Features of MS cells were similar in both species, but well-defined peripheral reticulum (PR) was easily recognized in MS chloroplasts ofS. viridis. Virtually no PR was developed in BS chloroplasts examined. BS cells contained more mitochondria and microbodies, but no structural dimorphism was noticed. The electron-dense suberized lamella were often observed between BS and MS cells, especially in the primary wall of BS cells. It was most frequently found at the BS and MS cell interfaces and terminated in radial walls of the adjacent BS cells. Prominent pits with plasmodesmata (pd) were seen in the walls of both cells. There also were numerous pd in outer tangential walls of the BS cells. The number of pd ranged from 20 to 60. The pd trasversed a segment of cell wall much thinner than the adjacent wall. The current cellular data have been compared to the ultrastructural features known in leaves of other C-4 plants, especially NADP-ME species.     

夹竹桃科2种引种植物分泌结构的解剖学研究

为了解夹竹桃科(Apocynaceae)植物乳汁管的发生发育,对爱之蔓(Ceropegia woodii)和百万心(Dischidia ruscifolia)营养器官中的分泌结构进行了显微观察。结果表明,爱之蔓和百万心营养器官中均有无节分枝乳汁管的分布,茎皮层中的乳汁管大部分具有明显的分枝,叶中乳汁管具明显分枝,分布与走向多与叶脉维管组织平行。另外,爱之蔓营养器官中的分泌结构除乳汁管外,还有分泌腔。这为夹竹桃科植物的系统分类研究提供了解剖学依据。     

Ontogeny of external glands in the bladderwortUtricularia monanthos

Summary The development of external glands on traps and stolons ofU. monanthos has been studied using transmission electron microscopy. During early differentiation of the epidermis some cells remain narrow and develop a protuberance which subsequently divides into a terminal and a pedestal cell, with the remainder of the original cell forming the basal epidermal cell of the gland. The lateral wall of the pedestal cell soon becomes densely impregnated throughout its thickness, and this is followed by the formation of discontinuous cuticular deposits within the primary wall of the terminal cell. The outer wall of the terminal cell then usually undergoes extensive secondary wall thickening beginning with the formation of ingrowths which for a period characterize the cell as a transfer cell. Later, at the stage when traps begin capturing prey, these ingrowths are overlain by further layers of secondary wall material. Concomitantly, in the pedestal cell, wall ingrowths become fully differentiated on the outer transverse wall and persist throughout the remaining life of the gland.The function of external glands during early ontogeny is discussed. At the stage when the terminal cell is differentiated as a transfer cell it is suggested that the gland is mainly responsible for absorbing solutes from the external medium. Once traps commence capturing prey the gland may become modified for a rôle in water secretion, facilitated by the differentiation of the pedestal cell as a transfer cell, and by the formation of a thick outer wall in the terminal cell.     

Immunolocalization of beta-D-glucans,pectins, and arabinogalactan-proteins during intrusive growth and elongation of nonarticulated laticifers in Asclepias speciosa Torr

Nonarticulated laticifers are latex-containing cells that elongate indefinitely and grow intrusively between the walls of meristematic cells. To identify biochemical mechanisms involved in the growth of nonarticulated laticifers, we have analyzed the distribution of various polysaccharides and proteoglycans in walls of meristematic cells in contact with laticifers, nonadjacent to laticifers, and in laticifer walls. In the shoot apex of Asclepias speciosa, the levels of callose and a (1-->4)-beta-galactan epitope are lower in meristematic walls in contact with laticifers than in nonadjacent walls. In contrast, we did not detect a decline in xyloglucan, homogalacturonan, and arabinogalactan-protein epitopes upon contact of meristematic cells with laticifers. Laticifer elongation is also associated with the development of a homogalacturonan-rich middle lamella between laticifers and their neighboring cells. Furthermore, laticifers lay down walls that differ from those of their surrounding cells. This is particularly evident for epitopes in rhamnogalacturonan I. A (1-->5)-alpha-arabinan epitope in this pectin is more abundant in laticifers than meristematic cells, while the opposite is observed for a (1-->4)-beta-galactan epitope. Also, different cell wall components exhibit distinct distribution patterns within laticifer walls. The (1-->5)-alpha-arabinan epitope is distributed throughout the laticifer walls while certain homogalacturonan and arabinogalactan-protein epitopes are preferentially located in particular regions of laticifer walls. Taken together, our results indicate that laticifer penetration causes changes in the walls of meristematic cells and that there are differences in wall composition within laticifer walls and between laticifers and their surrounding cells.     

Fine structure of isolated and non-isolated potato tuber periderm

Cell walls of the periderm of native potato tuber (Solanum tuberosum L. cv. Primura) consist of a primary wall, a suberized secondary wall and a tertiary wall. With a mixture of pectinase and cellulase intact periderm membranes can be isolated. Isolation does not affect fine structure. It is suggested that the lignin in the middle lamellae and primary walls prevents the enzymes from digesting pectinaceous materials and cellulose. In specimens fixed with OsO₄, the suberized walls appear as alternating electrondense and electron-lucent lamellae. This lamellar architecture is not altered by extraction with chloroform. Therefore, the current view that the electronlucent lamellae consist of soluble lipids (waxes) can no longer be maintained. It is argued that the lamellation is a property of the suberin itself, and the suberized wall consists of alternating layers of suberins differing in polarity. A hypothesis of suberin assembly from sub-units is advanced and the subunits are shown for the first time.     

Ultrastructure of leaves in C4 Cyperus iria and C3 Carex siderosticta

The ultrastructural aspects ofCyperus iria leaves showing the C₄ syndrome and the typical C₃ species,Carex siderosticta, in the Cyperaceae family were examined.C. iria exhibited the chlorocyperoid type, showing an unusual Kranz structure with vascular bundles completely surrounded by two bundle sheaths. The cellular components of the inner Kranz bundle sheath cells were similar to those found in the NADP-ME C₄ subtype, having centrifugally arranged chloroplasts with greatly reduced grana and numerous starch grains. Their chloroplasts contained convoluted thyla-koids and a weakly-developed peripheral reticulum, although it was extensive mostly in mesophyll cell chloroplasts. The outer mestome bundle sheath layer was sclerenchymatous and generally devoid of organelles, but had unevenly thickened walls. Suberized lamellae were present on its cell walls, and they became polylamellate when traversed by plasmodesmata. Mesophyll cell chloroplasts showed well-stacked grana with small starch grains. InC. siderosticta, vascular bundles were surrounded by the inner mestome sheath and the outer parenchymatous bundle sheath with intercellular spaces. The mestome sheath cells degraded in their early development and remained in a collapsed state, although the suberized lamellae retained polylamellate features. Plastids with a crystalline structure, sometimes membrane-bounded, were found in the epidermal cells. The close interveinal distance was 35–50 μm inC. iria, whereas it was 157–218 μm inC. siderosticta. These ultrastructural characteristics were discussed in relation to their photosynthetic functions.     

The distribution of plasmodesmata in the phloem ofHevea brasiliensis in relation to laticifer loading

Summary Cell to cell connections, including plasmodesmata and perforations, were examined in the non-conducting secondary phloem ofHevea brasiliensis. Samples were taken from trunks of numerous trees, from several clones, and prepared for thin sectioning and transmission or scanning electron microscopy and as optical sections for fluorescence microscopy. Numerous plasmodesmata were found clustered in primary pit-fields between the ray and axial parenchyma cells. Between the laticifers and adjacent parenchyma sheath cells, structures corresponding to functional plasmodesmata were not observed. But some unusual structural features were occasionally seen in these walls. These observations are discussed in relation to the possible function of the cell types, and to the loss of latex on the tapping ofHevea. It is suggested that the loading of the laticifer might first require a symplastic pathway for the transport of metabolites, at the end of which the assimilates must enter the apoplast. A transmembrane active transport system then transfers the metabolites in the laticifer. The presumable role of parenchyma cells in the loading of laticifers is emphasized.     

Distribution and structure of the plasmodesmata in mesophyll and bundle-sheath cells of Zea mays L.

In leaf blades of Zea mays L. plasmodesmata between mesophyll cells are aggregated in numerous thickened portions of the walls. The plasmodesmata are unbranched and all are characterized by the presence of electron-dense structures, called sphincters by us, near both ends of the plasmodesmatal canal. The sphincters surround the desmotubule and occlude the cytoplasmic annulus where they occur. Plasmodesmata between mesophyll and bundle-sheath cells are aggregated in primary pit-fields and are constricted by a wide suberin lamella on the sheath-cell side of the wall. Each plasmodesma contains a sphincter on the mesophyll-cell side of the wall. The outer tangential and radial walls of the sheath cells exhibit a continuous suberin lamella. However, on the inner tangential wall only the sites of plasmodesmatal aggregates are consistently suberized. Apparently the movement of photosynthetic intermediates between mesophyll and sheath cells is restricted largely or entirely to the plasmodesmata (symplastic pathway) and transpirational water movement to the cell walls (apoplastic pathway).Abbreviation ER endoplasmic reticulum     

Localization of cell wall polysaccharides in nonarticulated laticifers ofAsclepias speciosa Torr.

Summary Asclepias speciosa Torr, has latex-containing cells known as nonarticulated laticifers. In stem sections of this species, we have analyzed the cell walls of nonarticulated laticifers and surrounding cells with various stains, lectins, and monoclonal antibodies. These analyses revealed that laticifer walls are rich in (1→4) β-D-glucans and pectin polymers. Immunolocalization of pectic epitopes with the antihomogalacturonan antibodies JIM5 and JIM7 produced distinct labeling patterns. JIM7 labeled all cells including laticifers, while JIM5 only labeled mature epidermal cells and xylem elements. Two antibodies, LM5 and LM6, which recognize rhamnogalacturonan I epitopes distinctly labeled laticifer walls. LM6, which binds to a (l→5) α-arabinan epitope, labeled laticifer walls more intensely than walls of other cells. LM5, which recognizes a (1→4) β-D-galac-tan epitope, did not label laticifer segments at the shoot apex but labeled more mature portions of laticifers. Also the LM5 antibody did not label cells at the shoot apical meristem, but as cells grew and matured the LM5 epitope was expressed in all cells. LM2, a monoclonal antibody that binds to β-D-glucuronic acid residues in arabinogalactan proteins, did not label laticifers but specifically labeled sieve tubes. Sieve tubes were also specifically labeled byRicinus communis agglutinin, a lectin that binds to terminal β-D-galactosyl residues. Taken together, the analyses conducted showed that laticifer walls have distinctive cytochemical properties and that these properties change along the length of laticifers. In addition, this study revealed differences in the expression of pectin and arabinogalactan protein epitopes during shoot development or among different cell types.