Development and Structure of Phloem in the Petiole of Lagenaria siceraria (Mol.) Standl. and Momordica charantia L.

Light microscopic studies of the petioles of Lagenaria sicerariareveal that the external phloem of each bicollateral vascularbundle develops earlier than the internal phloem, and that thesieve elements of the external phloem are arranged in the outerand inner zones. Each sieve element of L. siceraria and Momordicacharantia is vertically associated with a maximum of six andtwo companion cells respectively. Discrete granular bodies seenin the cytoplasm of young sieve elements develop into globular,oval, or elongated slime bodies. Enlargement and fusion of slimebodies, and the subsequent dispersal of slime occur in the parietalcytoplasm. The dispersal of slime coincides with degradationof the nucleus and perforation of the pore sites. Before nucleardisorganization, the sieve-element nucleolus is extruded. Slimeafter its immediate dispersal appears amorphous and uniformlydistributed in the sieve elements. Plugs exhibit varying degreesof condensation of slime near the sieve plates. Certain maturesieve elements in the external phloem of L. siceraria have ovalbodies which we consider reaggregated or undispersed slime.Evidence has been obtained that a central cavity occurs in afew, almost mature, sieve elements wherein the cytoplasm includingthe slime is peripheral.     

Preservation of the Tonoplast in Metaphloem Sieve Elements of Embryonic Roots of Zea mays L.

Decortication of embryonic roots of 4- to 5-day-old Zea seedlingsand subsequent chemical fixation permitted comparison of cutand uncut developing sieve elements In a decorticated root wheresieve tubes are not severed, metaphloem sieve elements in latestates of development and some mature sieve elements exhibita highly vacuolate condition When roots are cut or diced inthe course of fixation intact vacuoles are not observed in latestages of sieve-element ontogeny The degree of callose formationat sites of developing sieve-plate pores and in the pores ofmature sieve elements varies greatly with both decorticationand non-decortication treatments Nuclei were not observed insieve elements at the electron microscope level, but they wereseen at the light microscope level in serial sections of sieveelements in the late to mature developmental stages representedAlthough the occurrence and distribution of plastids, mitochondria,endoplasmic reticulum, dictyosomes and nbosomes also vanes insieve elements of decorticated roots, disruption or surgingof sieve-element contents is greater for sieve tubes that aresevered during fixation treatment A discussion is presentedrelating effects of trauma on observed developmental stagesand sieve-element structure Zea mays L, maize, corn, phloem, Sieve elements, tonoplast, ultrastructure     

Structure and Development of Sieve Elements in the Root of Isoetes muricata Dur.

Root tissues of Isoetes muricata Dur. were fixed in glutaraldehydeand postfixed in osmium tetroxide for electron microscopy. Veryyoung root sieve elements can be distinguished from contiguousparenchyma cells by the presence of crystalline and/or fibrillarproteinaceous material in dilated cisternae of rough endoplasmicreticulum (ER). Similar crystalline-fibrillar material accumulatesin the perinuclear space. During differentiation, the portionsof ER enclosing this proteinaceous substance become smooth surfacedand migrate to the cell wall. Along the way many of them formmultivesicular bodies which fuse with the plasmalemma, dischargingtheir contents toward the wall. Nuclear degeneration is pycnotic.At maturity, the sieve element contains a degenerate, filiformnucleus, plastids, and mitochondria. In addition, the wall ofthe mature sieve element is lined by a plasmalemma and a parietalnetwork of smooth ER. Sieve-area pores are present in both endand lateral walls of mature sieve elements. Whereas a singlecluster of pores occurs in each end wall, the pores of the lateralwalls are solitary and few in number.     

Another View of the Ultrastructure of Cucurbita Phloem

The primary phloem of young internodes of Cucurbita maxima wasstudied with the electron microscope. Phloem parenchyma cellsare highly vacuolated and contain nuclei, endoplasmic reticulum,ribosomes, mitochondria, chloro-plasts, and occasional dictyosomes.As compared with parenchyma cells, the most distinctive featuresof companion cells are their extremely dense cytoplasm, lowdegree of vacuolation, lack of chloroplasts, and numerous sieve-elementconnexions. Companion cells contain plastids with few internalmembranes. At maturity the enucleate sieve element is linedby a plasmalemma, one or more cistema-like layers of endoplasmicreticulum, and a membrane which apparently delimits the parietallayer of cytoplasm from a large central cavity. In OsO_4–-andglutaraldehyde-fixed elements, the central cavity is traversedby numerous strands, which run from cell to cell through thepores of sieve plates and lateral sieve areas, and which arederived ontogenetically from the slime bodies of immature cells.Numerous normal-appearing mitochondria are present in the parietallayer of cytoplasm. The pores of sieve plates and lateral sieveareas are lined with cytoplasm. The ultrastructural detailsof young sieve elements differ little from those of other youngnucleate cells. During sieve-element development, the sieveelement increases in vacuolation. At the same time, slime bodiesdevelop in the cytoplasm. With glutaraldehyde fixation, thesebodies often exhibit a double-layered limiting membrane. Asthe sieve element continues to differentiate, the slime bodiesincrease in size and the parietal layer of cytoplasm becomesvery narrow. Presently, the slime bodies begin to disperse andtheir contents fuse. This phenomenon occurs in the parietallayer of cytoplasm, while the latter is still delimited fromthe large central vacuole by a distinct tonoplast. The initiationof slime-body dispersal more or less coincides with perforationof the pore sites, and many pores are traversed by slime earlyin their development. Before slime-body dispersal, all dictyosomesand associated vesicles disappear from the cytoplasm. Eventually,the tonoplast diappears and the slime becomes distributed throughoutthe central cavity in the form of strands. Nuclei and ribosomesdisappear before breakdown of the tonoplast. Sieve elementsare connected with companion cells and parenchyma cells by plasmodesmata.     

The Ultrastructure of Protophloem Sieve Elements in Leaves of Aegilops comosa var. thessalica

The differentiation and obliteration of protophloem sieve elementsin leaves of the grass Aegilops comosa var. thessalica havebeen studied by electron microscopy. These elements differentiatesimilarly to metaphloem sieve elements of the same plant andother monocotyledons. Plasmalemma, smooth endoplasmic reticulum(ER), mitochondria, P-type plastids and sometimes nuclear remnantsconstitute the protoplasmic components at maturity, all areperipherally distributed. The differentiation of end walls intosieve plates and the presence of sieve areas on the lateralwalls indicate that protophloem sieve elements are componentsof sieve-tube. They may be functional for a brief period butsoon after their maturation they are compressed and finallyobliterated by the stretching of actively-growing surroundingcells. The protoplasmic components of mature elements degenerateand are destroyed during obliteration of the sieve elements. Aegilops comosa var. thessalica, protophloem, sieve elements, differentiation, ultrastructure     

Variations in the level of enzyme activity and immunolocalization of calcium-dependent protein kinases in the phloem of different cucumber organs

Calcium-dependent protein kinases (CDPKs) constitute a unique family of enzymes in plants that are characterized by a C-terminal calmodulin (CaM)-like domain. Through protein kinase assays, we have examined the levels of cucumber calcium-dependent kinase (CsCDPK) activity in various organs of cucumber seedlings and plants. The activity of CsCDPK was highest in cucumber plant leaves followed by seedling roots and hypocotyls; however, cucumber plant flowers, seedling cotyledons, and hooks had levels that were barely detectable. The CsCDPKs were immunolocalized using polyclonal antibodies that are highly specific against a part of the kinase domain of a calcium-dependent protein kinase (CsCDPKS) in the phloem sieve elements (SEs) in various organs of cucumber. In addition, this study indicates the presence of CsCDPKs in organelle-like bodies associated with the plasma membrane of sieve elements in mature stems and roots as well as in the storage bodies of immature seeds. These findings are discussed in terms of the likely roles played by CDPKs in the signal transduction pathways for Ca2+-regulated phloem transport of assimilates from leaves to various organs during growth and development of cucumber seedlings and plants.     

Transcellular Strands in Sieve Tubes; What Are They?

We show that sieve elements of Nymphoides peltata (S. G. Gmel.)O. Kuntze contain strands which are bundles of P-protein filaments.We observe the strands under the light microscope (differential-interferencecontrast), and in the scanning electron microscope which showssome of them to be arranged as a parietal network. We find bundlesof filaments which correspond to these strands in sections ofembedded sieve elements in the transmission electron microscope,and also in freeze-fracture replicas of sieve elements in vascularbundles frozen intact while translocating carbon-14. Not allthe strands are necessarily transcellular; some may end in theparietal layer just to the inside of the plasmalemma where theyappear to come in contact with membranes, possibly of endoplasmicreticulum. The filaments in the strands have the same bandedappearance as filaments in the sieve pores. We are unable tofind any membrane or other special boundary round the strands;we propose they should be called ‘filamentous strands’.We suggest that the filaments are aggregated into strands bythe Bernoulli effect when fluid flows through sieve elements.We suggest that the strands may be formed by flow during translocationas well as by flow due to injury.     

STRUCTURE AND DEVELOPMENT OF SIEVE ELEMENTS IN THE LEAF OF ISOETES MURICATA

Leaf tissue of Isoetes muricata Dur. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. The very young sieve elements can be distinguished from contiguous parenchyma cells by their distinctive plastids and the presence of crystalline and fibrillar proteinaceous material in dilated cisternae of the rough ER. During differentiation, the portions of ER enclosing this proteinaceous substance become smooth surfaced and migrate to the cell wall. Along the way they apparently form multivesicular bodies which then fuse with the plasmalemma, discharging their contents to the outside. At maturity, the sieve element contains an elongate nucleus, which consists of dense chromatin material, and remnants of the nuclear envelope. In addition, the mature sieve element is lined by a plasmalemma and a parietal, anastomosing network of smooth ER. Both plastids and mitochondria are present. P-protein is lacking at all stages of development. Tonoplasts are. not discernible in mature sieve elements. The end walls of mature sieve elements contain either plasmodesmata or sieve pores or both, but only plasmodesmata occur in the lateral walls.     

Aphid stylectomy reveals an osmotic step between sieve tube and cortical cells in barley roots

The aim of the present study was to quantify osmotic pressuresdirectly in the translocation pathway, from leaf to growingroot tip, in order to understand the forces driving solutesfrom a source to a sink. Solutes move through the translocationpathway down an osmotically derived turgor gradient. Accordinglyaphid stylectomy and single cell sampling techniques have beencombined to examine the osmotic pressure of root phloem andgrowing root cells. Sieve tube sap was obtained from shootsand, for the first time, roots of barley seedlings using aphidstylectomy. Vacuolar sap was also obtained from a variety ofcells in leaf and root tissues using single cell sampling methods.Osmotic pressure of sieve tube sap from roots and shoots wasmeasured at high temporal resolution (within min) and over longperiods of time (up to 24 h). Osmotic pressure did not changesignificantly in the minutes immediately following excision,suggesting that confidence can be placed in the assumption thatstylet exudate is representative of sieve tube sap in vivo.There were no differences in the osmotic pressure of sieve tubesap from shoots (1.240.26 MPa, n = 10) or roots (1.420.15MPa, n = 13). However, osmotic pressure of sap from root corticalcells (0.710.09, n = 12) was about 0.7 MPa lower than thatof the sieve elements from roots, this difference may be maintainedby consumption of incoming solutes at the root tip. Resultsare discussed in the context of pressure driven flow in thephloem and symplastic contact between root tip cells and sievetube. It is hoped that the approach described here will provideimportant insights into the nature of the relationship betweenroot cell extension and assimilate supply through the phloem. Key words: Phloem, sieve tube, aphid, root, barley, osmotic pressure, translocation     

P PROTEIN IN THE PHLOEM OF CUCURBITA : II. The P Protein of Mature Sieve Elements

During maturation of sieve elements in Cucurbita maxima Duchesne, the P-protein bodies (slime bodies) usually disperse in the tonoplast-free cell. In some sieve elements the P-protein bodies fail to disperse. The occurrence of dispersal or nondispersal of P-protein bodies can be related to the position of the sieve elements in the stem or petiole. In the sieve elements within the vascular bundle the bodies normally disperse; in the extrafascicular sieve elements the bodies often fail to disperse. Extrafascicular sieve elements showing partial dispersal also occur. The appearance of the sieve plate in fixed material is related to the degree of dispersal or nondispersal of the P-protein bodies. In sieve elements in which complete dispersal occurs the sieve plate usually has a substantial deposit of callose, and the sieve-plate pores are filled with P protein. In sieve elements containing nondispersing P-protein bodies the sieve plate bears little or no callose, and its pores usually are essentially "open." The dispersed P-protein components may aggregate into loosely organized "strands," which sometimes extend vertically through the cell and continue through the sieve-plate pores; but they may be oriented otherwise in the cell, even transversely.     

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.     

OBSERVATIONS ON METAPHLOEM IN THE VEGETATIVE PARTS OF PALMS

Metaphloem was studied in available vegetative parts of 374 species in 164 genera of palms. Sieve elements usually have compound sieve plates except in the subfamilies Lepidocaryoideae and Nypoideae. Sieve elements in roots usually have oblique to very oblique end walls, whereas in stems and leaves they have transverse to oblique walls. Within a phloem strand the degree of compounding of a sieve plate is directly correlated with element diameter. Plastids are normally present in functioning, enucleate sieve elements. Small quantities of “slime” substances have been detected in young sieve elements in stems and petioles of a few species. Many sieve plates in functioning sieve elements lacked callose in materials quick-killed in liquid nitrogen or chilled acetic-alcohol. Definitive callose is confined to sieve elements just before their obliteration. Sieve tubes in leaf and stem are usually ensheathed by contiguous parenchyma cells while those in root have very few contiguous parenchyma cells. Two types of contiguous parenchyma cells can be distinguished by difference in cytoplasmic density, especially with the electron microscope. Cells with denser cytoplasm are interpreted as companion cells. Lignified contiguous parenchyma cells are occasionally present in metaphloem of petioles. The possible diagnostic and taxonomic features of metaphloem are discussed.     

The Branch Roots of Zea. 3, Vascular Connections and Bridges for Nutrient Recycling

Vascular connections between branch roots and nodal roots offield-grown maize were studied by optical and electron microscopy.Their extent and openness were evaluated by locating dyes andlatex particles pulled into the connections by gentle vacuum.The connecting complex is very extensive both around and alongthe main root. It includes rearranged and modified vasculartissues in the base of the branch within the parent cortex,small diameter tracheary elements and sieve tubes which connectthe branch vascular system with the vasculature of the mainroot, and also interconnect the components of the latter systemwithin those portions of the main-root vascular conduits towhich the connections are made. We have named this complex theRoot Vascular Plexus. Sites of direct contact of sieve tubeswith tracheary elements in the vascular plexus are postulatedas the sites of transfers from phloem to xylem of sugar andamino acids that have been detected in xylem exudates from maizeroots. The postulate is extended to account for phloem-xylemexchange in roots of other plants where nutrient recycling hasbeen found. It is suggested that pit membranes within the vascularplexus prevent air embolism entering main roots from the branches.Copyright1993, 1999 Academic Press Branch roots, phloem-xylem exchange, root vascular plexus, embolism prevention     

On Arborescence in Gahnia clarkei and Gahnia sieberiana

Specimens of Gahnia sieberiana from Brisbane, Queensland, andof Gahnia clarkei from near Orbost, Victoria, were collectedand examined both morphologically and anatomically. The speciesgrow in wet areas and are of interest because they representthe largest arborescent species known in the Cyperaceae. Stemdiameters up to 120 mm and stems up to 10 m long have been observed.Such long stems tend to be supported by nearby vegetation. Althoughfresh stems are tough and woody, they are brittle. Branchingof the stems is sympodial, and numerous branches are producedby plants growing in exposed habitats. There is less branchingin plants from shaded habitats. Basal shoots may also occur.Adventitious roots develop basally on most plants, but withG. sieberiana, some adventitious roots form near the shoot apexand grow in and around leaf bases. Anatomical features of interestare an endodermoid layer composed of sclereids with elongate,undulated, outer tangential walls that are lignified and suberized,short vessel elements with horizontal to oblique simple perforationplates, and relatively short sclereids surrounding vessel elementsin the vascular bundles. Some vascular bundles are bipolar.The presence of short vessel elements here is in marked contrastto the longer tracheary elements in other arborescent monocotyledons. Arborescence, stem anatomy, Cyperaceae, Gahnia, saw sedge, Monocotyledon, bipolar bundles, morphology, endodermoid layer     

Sieve-Element Ontogeny in the Aerial Shoot of Equisetum hyemale L.

The aerial shoots of Equisetum hyemale L. var. affine (Engelm.)A. A. Eat. were examined with the electron microscope as partof a continuing study of sieveelement development in the lowervascular plants. Young E. hyemale sieve elements are distinguishablefrom all other cell types within the vascular system by thepresence of refractive spherules, proteinaceous bodies whichdevelop within dilated portions of the endoplasmic reticulum(ER). Details of cell wall thickening differ between protophloemand metaphloem sieve elements. Following cell wall thickeningthe ER increases in quantity and aggregates into stacks. Shortlythereafter, nuclear degeneration is initiated. During the periodof nuclear degeneration some cytoplasmic components-dictyosomes,microtubules and ribosomes-degenerate and disappear, while organellessuch as mitochondria and plastids persist. The latter undergostructural modifications and become parietal in distribution.Eventually the massive quantities of ER are reduced, leavingthe lumen of the cell clear in appearance. At maturity the plasmalemma-linedsieve element contains a parietal network of tubular ER, aswell as mitochondria, plastids, and refractive sphemh At thistime many of the spherules are discharged into the region ofthe wall. Sieveelement pores occur in both lateral and end walls.At maturity many pores are traversed by large numbers of ERmembranes. The metaphloem sieve elements of the mid-internodalregions apparently are sieve-tube members. The connections betweenmature protophloem sieve elements and pericycle cells are associatedwith massive wall thickenings on the pericyclecell side.     

Solute concentrations of the phloem and parenchyma cells present in squash callus

Abstract. Glutaraldehyde fixation was used to determine the solute concentrations in the various cell types present in tissue cultures of squash ( Cucurbita pepo ). Small pieces of callus were plasmolyzed in a graded series of mannitol solutions and fixed in 20 kg m⁻³ glutaraldehyde adjusted to be isosmotic with the particular plasmolysing solution. The callus samples were further processed using standard electron microscopy techniques. Using this procedure, mature sieve elements that form in squash callus have an osmotic potentional of -2.4MPa. The osmotic potential of the callus sieve elements was comparable to values reported for the sieve tube members of the phloem in intact plants. This ability of callus sieve elements to develop high internal hydrostatic pressures demonstrates that they are capable of phloem loading. However, the osmotic potentials of the surrounding parenchymatous cells and companion cells were only –1.15 and –1.5 MPa, respectively. In contrast to the companion cells of the phloem in intact plant tissues, the osmotic potential of the callus companion cells indicated that they were not directly involved in phloem loading. Several immature sieve elements containing distinct nuclei and vacuoles were observed in the callus granules. These immature sieve elements were plasmolyzed in weaker mannitol solutions (below 0.6kmol m⁻³) than the enucleate sieve elements (1.01 kmol m⁻³ mannitol). The low solute concentrations in immature sieve elements indicated that the ability to load sugars occurs concomitantly with the maturation of the sieve element protoplast.     

The distribution of P-Protein in mature sieve elements of celery

Summary The extent of blocking of sieve-plate pores caused by release of cell turgor was investigated by fixing and processing for electron microscopy a long length of celery (Apium graveolens L.) phloem. Differences in distribution of P-protein within the pores were observed between those cells near the two cut ends, and the central cells.To assess the effect of chemical fixation on the distribution of P-protein, strands of celery phloem (fixed or unfixed, and not treated with cryoprotectants) were frozen in Freon 12 and then freeze-substituted. In sieve elements from unfixed tissue there were a greater number of sieve plates displaying partially open pores.Direct freezing of unprotected phloem tissue in Freon 12 resulted in the formation of ice crystals within the lumen of the sieve elements. Freezing of tissue at rates fast enough to avoid the formation of damaging ice crystals resulted in sieve-plate pores having an unoccluded central channel with a peripheral lining of P-protein. In the lumen of the sieve elements the P-protein filaments occurred as discrete bundles ca. 0.5 m in diameter, and as a parietal layer varying in thickness from 0.1 to 0.5 m.     

STRUCTURE AND DEVELOPMENT OF THE SIEVE ELEMENT IN THE STEM OF LYCOPODIUM LUCIDULUM

Stem tissue of Lycopodium lucidulum Michx. was fixed in glutaraldehyde and postfixed in osmium tetroxide for electron microscopy. Although their protoplasts contain similar components, immature sieve elements can be distinguished from parenchymatous elements of the phloem at an early stage by their thick walls and correspondingly high population of dictyosomes and dictyosome vesicles. Late in maturation the sieve-element walls undergo a reduction in thickness, apparently due to an “erosion” or hydrolysis of wall material. At maturity, the plasmalemma-lined sieve elements contain plastids with a system of much convoluted inner membranes, mitochondria, and remnants of nuclei. Although the endoplasmic reticulum (ER) in most mature sieve elements was vesiculate, in the better preserved ones the ER formed a tubular network closely appressed to the plasmalemma. The sieve elements lack refractive spherules and P-protein. The protoplasts of contiguous sieve elements are connected with one another by pores of variable diameter, aggregated in sieve areas. As there is no consistent difference between pore size in end and lateral walls these elements are considered as sieve cells.     

Changes in the Endoplasmic Reticulum During Sieve Element Differentiation in Triticum aestivum

The changes in structure of the endoplasmic reticulum (ER) andits associations with other cell components have been studiedin differentiating protophloem sieve elements of root tips ofTriticum aestivum. In the young sieve elements single ER cisternaebearing ribosomes are dispersed in the cytoplasm. As differentiationprogresses ER increases in amount while a small proportion ofit aggregates into stacks or becomes associated with the nuclearenvelope and the mitochondria. These modifications occur inthe last two sieve elements containing ribosomes and coincidewith most dramatic changes in the degenerating nucleus. Stacksconsist of relatively few ER cisternae and may be encounteredfree in the cytoplasm or applied to the nuclear envelope. Electron-densematerial accumulates between the contiguous cisternae of thestacks. ER-attached ribosomes persist even in nearly maturesieve elements, but their pattern of arrangement becomes changed.The structural evidence indicates that only a few highly degradedER elements are retained in fully mature sieve elements. Triticum aestivum, root protophloem, sieve elements, endoplasmic reticulum, differentiation     

Observations on Companion Cells and Specialized Phloem Parenchyma Cells of Nelumbo nucifera Gaertn

The phloem of very young petioles of Nelumbo nucifera Gaertn.(Nelumbium speciosum Willd.) was studied with the light microscope.The elongated, mature sieve elements contain slime, plugs, strands,and numerous plastids. Some sieve elements remain nucleatedfor a brief period even after the sieve plates are well developed.The companion cells numbering 8–14 undergo disintegrationbefore the elongation of the ontogenetically related sieve elementis completed. They are uninucleate to begin with but later becomebinucleate and finally degenerate and obliterate. The variousstages in their ontogeny and disintegration are described. Ofthe very few specialized phloem parenchyma cells present, someare associated with sieve elements. They have slime body-likestructures, and plastid-like bodies which group together andeventually disintegrate.