Band Of Microtubules Research Articles

Cellular morphogenesis and the formation of marginal bands in amphibian splenic erythroblasts.

The spleen of Ambystoma mexicanum (axolotl) larvae develops as a closed sac containing differentiating nucleated erythrocytes, and is typically isolated from the general circulation for about 10 days post-hatching. Beginning 3-4 days posthatching, it can be removed intact for examination of the morphology and cytoskeletal structure of the erythropoietic cells. In the smallest (earliest) spleens, spheroidal cells predominate, while older ones contain a preponderance of cells exhibiting the flattened elliptical morphology typical of all non-mammalian vertebrate erythrocytes. Most striking in the splenic erythroid population are cells with singly or doubly pointed morphology. Though common in the developing spleen and circulation of young larvae, pointed cells are less frequently encountered in the circulation of older larvae, indicating that they are intermediate stages in the differentiation of spheroids to flattened ellipsoids. This is supported by structural observations on cytoskeletons prepared from the splenic cells. Incomplete singly and doubly pointed marginal bands of microtubules are observed, many of which contain a pair of centrioles within or close to a pointed end, suggestive of organizing center function. The observations are consistent with a sequence of changes in cell morphology from spherical to doubly pointed to singly pointed to flattened ellipse, causally linked to stages of marginal band biogenesis.

Microtubules Associated with Simultaneous Cytokinesis of Coenocytic Microsporocytes

Microtubule arrays associated with simultaneous cytokinesis in the coenocytic microsporocytes of Lonicera japonica and Impatiens sultani were studied by indirect immunofluorescence. The future division planes are not predicted prior to meiosis by either a preprophase band of microtubules or cytoplasmic lobing. Cleavage planes appear to be determined by position of the four haploid nuclei and the development of postmeiotic microtubule systems. Perpendicular second division spindles in Lonicera result in tetrahedrally arranged tetrads while parallel spindles in Impatiens result in tetragonal arrangement. Immediately following meiosis bands of microtubules, the secondary spindles, develop between both sister and nonsister nuclei. These arrays give way to systems of microtubules that radiate equally from each of the four nuclei in the coenocytic sporocyte. Simultaneous cytokinesis is initiated by centripetal wall deposition at the periphery of the sporocyte and proceeds along planes marked by interaction of the opposing arrays of nuclear-based microtubules.

MICROTUBULES ASSOCIATED WITH SIMULTANEOUS CYTOKINESIS OF COENOCYTIC MICROSPOROCYTES

Microtubule arrays associated with simultaneous cytokinesis in the coenocytic microsporocytes of Lonicera japonica and Impatiens sultani were studied by indirect immunofluorescence. The future division planes are not predicted prior to meiosis by either a preprophase band of microtubules or cytoplasmic lobing. Cleavage planes appear to be determined by position of the four haploid nuclei and the development of postmeiotic microtubule systems. Perpendicular second division spindles in Lonicera result in tetrahedrally arranged tetrads while parallel spindles in Impatiens result in tetragonal arrangement. Immediately following meiosis bands of microtubules, the secondary spindles, develop between both sister and nonsister nuclei. These arrays give way to systems of microtubules that radiate equally from each of the four nuclei in the coenocytic sporocyte. Simultaneous cytokinesis is initiated by centripetal wall deposition at the periphery of the sporocyte and proceeds along planes marked by interaction of the opposing arrays of nuclear‐based microtubules.

The Fine Structure of the Amebocyte in the Blood ofLimulus polyphemus. II. The Amebocyte Cytoskeleton: A Morphological Analysis of Native, Activated, and Endotoxin-Stimulated Amebocytes

This study evaluates the structural organization of the cytoskeleton in native, activated, and endotoxin-activated amebocytes of the horseshoe crab, Limulus polyphemus. The discoid shape of native amebocytes appears to be maintained by a three-part arrangement consisting of a microtubule marginal band in a two-dimensional plane, an extensive actin-membrane array, and three-dimensional cortical actin array. This arrangement is disrupted during amebocyte activation in the absence of endotoxin, as the cortical actin array becomes diffuse and the cells form many pseudopodia. However, under these conditions the marginal band remains intact and continues to be located at the periphery of these cells. Amebocytes activated by bacterial endotoxin display a different cytoskeletal arrangement. After exposure to endotoxin, the cells displace their entire cortical actin array and marginal band to a more centralized location surrounding the nucleus.

Microtubule organization during the development of the mitotic apparatus in cultured mesophyll protoplasts of higher plants – an immunofluorescence microscopic study

Microtubule organization in the course of the interphase‐mitosis transition, karyokinesis and cytokinesis in cultured mesophyll protoplasts of Medicago sativa L. cv. Regen S and Nicotiana tabacum L. cv. Wisconsin 38 was investigated during the early culture stages. A prominent circular band of microtubules, the prophase band of microtubules (PB) was seen in the cell cortex during prophase in most cells. The band appeared during very early prophase and disappeared during late prophase. Other prophase microtubule arrays were nuclear‐envelope associated microtubule fluorescence (perinuclear fluorescence, PNF) and groups of microtubules radiating from the nuclear envelope into the cytoplasm. These radiating microtubutes (perinuclear radiating microtubules, PNR) were particularly long and prominent in the PB plane appearing to link the PNF with the PB network. The possible functions of the PNR are discussed. The role of the PB is discussed in the light of the unorganized nature of these cells.

ULTRASTRUCTURE OF THE GAMETE OFPEDIASTRUM DUPLEX(CHLOROPHYCEAE)1

ABSTRACTGametes of Pediastrum duplex Meyen were investigated ultrastructurally, with emphasis on the flagellar apparatus. The cells are naked, biflagellate, and measure approximately 2.5 × 8 μm. Distinguishing them from zoospores is their possession of an eyespot and mating structure (the apical cap), and their lack of the peripheral band of cytoplasmic microtubules involved in colony formation. Four featurs of the flagellar apparatus are especially noteworthy: (1) the basal bodies are directly opposed and (2) are interconnected via their cores, (3) the central portion of the distal fiber is elaborated into an unusual ribbed structure which overlies the striated microtubule‐associated component (SMAC) of the two‐membered rootlets, and (4) the X‐rootlets are dissimilar in microtubular number. The smaller X‐rootlet consists of four microtubules in a three over one (3/1) configuration, whereas the larger has been found to be either 5 / 1, 6 / 1 or 7 / 1. The former rootlet extends past the nucleus whereas the latter extends down the opposite side of the cell, passing near the eyespot. The first two of these flagellar apparatus features have been previously noted in other motile cells of the Hydrodictyaceae. Although not specifically mentioned, published micrographs suggest the presence of the latter two as well, which may indicate that all four flagellar apparatus features are characteristics of all motile cells in the Hydrodictyaceae.

Preprophase bands in a suspension culture of the monocot Spartina pectinata

Continuous suspension cultures of the marsh grass Spartina pectinata grow as either unorganized colonies or files of cells. Immunofluorescence of tubulin revealed microtubule (MT) structures similar to those encountered in meristematic cells, including cortical microtubule (MT) bands in some interphase cells and in all prophase cells. These MT bands were judged to be preprophase bands (PPBs) on the basis of their temporal appearance in the cell cycle and their position and orientation relative to division planes. Although PPBs are widely thought to be associated with organized tissues and polarized divisions, there are reports of PPBs in suspension cultures of four dicot species. This is the first report of a PPB in suspension cultures of a monocot species.

Preprophasic microtubule systems and development of the mitotic spindle in hornworts (Bryophyta)

Studies of monoplastidic mitosis in hornworts (Bryophyta) using transmission electron microscopy and indirect immunofluorescence staining of microtubules have revealed that two mutually perpendicular microtubule systems predict division polarity in preprophase. Events of cytoplasmic reorganization in preparation for division occur in the following order: migration of the single plastid to a position perpendicular to the division site, constriction of the plastid where its midpoint intersects the division site, development of an axial system of microtubules parallel to the elongating plastid isthmus, and appearance of an atypical preprophase band of microtubules (PPB). The PPB is asymmetrical with a tight band of microtubules on the side over the plastid isthmus and a broad band of widely spaced microtubules over the nucleus. The axial system contributes directly to development of the spindle. In prometaphase, the axial system separates at the equator and additional microtubule bundles project from polar regions, creating two opposing halfspindles. The PPB is still present during asymmetrical organization of the spindle and microtubules extending from the broad portion of the PPB to poles appear to be incorporated into the developing spindle. Dynamic changes in the microtubular cytoskeleton demonstrate (1) intimate relationship of plastid and nuclear division, (2) contribution of preprophase/prophase microtubule systems to spindle development in monoplastidic cells, and (3) dynamic reorientation of microtubules from one system to another.

Preprophase bands of microtubules and the cell cycle: Kinetics and experimental uncoupling of their formation from the nuclear cycle in onion root-tip cells

We have studied the timing of preprophase band (PPB) development in the division cycle of onion (Allium cepa L.) root-tip cells by combinations of immunofluorescence microscopy of microtubules, microspectrophotometry of nuclear DNA, and autoradiography of [(3)H]thymidine incorporation during pulse-chase experiments. In normally grown onion root tips, every cell with a PPB had the G2 level of nuclear DNA. Some were in interphase, prior to chromatin condensation, and some had varying degrees of chromatin condensation, up to the stage of prophase at which the PPB-prophase spindle transition occurs. In addition, autoradiography showed that PPBs can be formed in cells which have just finished their S phase, and microspectrophotometry enabled us to detect a population of cells in G2 which had no PPBs, these presumably including cells which had left the division cycle. The effects of inhibitors of DNA synthesis showed that the formation of PPBs is not fully coupled to events of the nuclear cycle. Although the mitotic index decreased 6-10-fold to less than 0.5% when roots were kept in 20 μg·ml(-1) aphidicolin for more than 8 h, the percentage of cells containing PPBs did not decrease in proportion: the number of cells in interphase with PPBs increased while the number in prophase decreased. Almost the same phenomena were observed in the presence of 100 μg·ml(-1) 5-aminouracil and 40 μg·ml(-1) hydroxyurea. In controls, all cells with PPBs were in G2 or prophase, but in the presence of aphidicolin, 5-aminouracil or hydroxyurea, some of the interphase cells with PPBs were in the S phase or even in the G1 phase. We conclude that PPB formation normally occurs in G2 (in at least some cases very early in G2) and that this timing can be experimentally uncoupled from the timing of DNA duplication in the cell-division cycle. The result accords with other evidence indicating that the cytoplasmic events of cytokinesis are controlled in parallel to the nuclear cycle, rather than in an obligatorily coupled sequence.

The role of F-actin in determining the division plane of carrot suspension cells. Drug studies

Abstract Following the report that a network of F-actin is associated with the nucleus throughout the division cycle, we have examined the involvement of F-actin in determining the division plane of carrot suspension cells. This was achieved by treating cells with drugs and then staining the unfixed cells with rhodaminyl lysine phallotoxin in detergent extraction buffer. In interphase, actin cables radiate from the nucleus but at the cortex become more or less transversely arranged in the pattern already known for cortical microtubules. Concentration of the cortical F-actin into a band at preprophase draws most of the nucleus-associated actin into a transvacuolar disc, thereby forming the phragmosome within which mitosis and cytokinesis occur. In addition to this transversely aligned structure, F-actin is also associated with the spindle poles during mitosis but these filaments tend to align at right angles to the phragmosomal actin. F-actin therefore defines transverse and longitudinal vectors as division approaches. Depolymerization of F-actin with cyto-chalasin D can cause the spindle axis to reorientate such that the pole-pole axis comes to lie, abnormally, parallel with the phragmosome. The cytokinetic apparatus (the phragmoplast) develops centrifugally within the phragmosome. There has been considerable speculation on the nature of the elements that guide the phragmoplast to the cortical site previously occupied by the preprophase band of microtubules. This study demonstrates that F-actin bridges the leading margin of the outgrowing phragmoplast to the opposing cortex. Radial actin strands therefore provide a ‘memory’ of the predetermined division plane whose perimeter had been marked at preprophase by a band composed of microtubules and F-actin. This relationship was perturbed with the herbicide, chloroisopropylphenyl carbamate. Preprophase bands of actin appear to form normally in herbicidetreated cells. However, cytokinesis does not occur within this predicted plane since the drug perturbs the mitotic spindle, forming three nuclei which become separated by Y-shaped, actin-containing phragmoplasts. Cytoplasmic actin strands connect the edges of the phragmoplast to the cortex. It is suggested that the irregular distribution of F-actin, which radiates from the herbicide-altered mitotic apparatus, provides alternative paths for outgrowth of the abnormal phragmoplasts. Caffeine is known to cause failure of cell plate formation. But apart from inducing cytoplasmic ‘starbursts’ of F-actin in interphase cells it does not appear to have any effect on F-actin-containing division structures. It is concluded that the formation of a transvacuolar phragmosome, spindle alignment and the ‘correct’ outgrowth of a planar cytokinetic apparatus to the predetermined boundary of the division site all involve F-actin.

Regulation of the Plane of Cell Division in Vacuolated Cells I. The Function of Nuclear Positioning and Phragmosome Formation

In small leaf explants from Nautilocalyx lynchii (Hook, f.) Sprague (Gesneriaceae) the vacuolated epidermal cells divide at a more or less predictable time and most often periclinally. A phragmosome (PS) and a band of microtubules (BMT) are formed before mitosis. To deduce the importance of the PS and other premitotic structures for the realization of a new cell wall in a predetermined plane, the cells were studied under normal conditions and after treatments that inhibit PS formation. PS formation was preceded by migration of the nucleus to the centre of the cell and an increase in the number of cytoplasmic strands. Fully developed PSs and BMTs coincided in time and occupied the same position at the cell periphery. The beginning of the formation of a PS always preceded the onset of prophase, but there was no strict correlation between successive stages of prophase and PS formation. Colchicine (COL), oryzalin (OR), and cytochalasin B (CB) inhibited the formation of a PS as well as nuclear positioning and induced abnormally placed cell walls in many of the cells that had been inhibited before mitosis. The abnormally placed cell walls were apparently caused by failing positioning of mitotic spindles resulting in the beginning of cytokinesis at an abnormal site in the cell and insufficient guidance of cell plate plus phragmoplast during cytokinesis. The occurrence of some sharply bent cell walls suggested the existence of a specialized cortical region still present after prolonged COL treatment and a function of such a cortical region to guide the phragmoplast. These results suggest that in vacuolated cells the cytoplasmic strands are important to position the nucleus and that a combination of cytoplasmic strands, PS, and cortical division site is needed to locate a new cell wall at a predetermined place in the cell.

Tubulin, but not microtubules, is the substrate for tubulin:tyrosine ligase in mature avian erythrocytes.

Chicken erythrocytes, which contain a marginal band of microtubules, were used to study the influence of the aggregation state of tubulin on the post-translational incorporation of tyrosine into the alpha-tubulin subunit. We found that the incorporation of [14C]tyrosine occurs almost exclusively into the nonassembled tubulin pool. The marginal band was practically not labeled. The low incorporation into microtubules was not due to the lack of tubulin with acceptor capacity since after cold-induced disassembly, an additional amount of [14C]tyrosine could be incorporated. 14C-Tyrosinated tubulin of the nonassembled pool could not be incorporated into microtubules of the marginal band after prolonged incubation at 37 degrees C or when the marginal band was regenerated after cold-induced depolymerization. In erythrocytes, tubulin:tyrosine ligase behaved as a soluble entity when the cells were lysed under microtubule-preserving conditions.

Microtubule patterns during meiosis in two higher plant species

A monoclonal antibody to higher plant tubulin was used to trace microtubule (MT) structures by immunofluorescence throughout mitosis and meiosis in two angiosperms,Lycopersicon esculentum andOrnithogalum virens. Root tip cells showed stage specific MT patterns typical of higher plant cells. These included parallel cortical interphase arrays oriented perpendicular to the long axis of the cell, preprophase band MTs in late interphase through prophase, barrelshaped spindles, and finally phragmoplasts. Pollen mother cell divisions exhibited randomly oriented cortical MT arrays in prophase I, pointed spindles during karyokinesis, and elongate phragmoplasts. A preprophase band was not observed in either meiotic division. MT initiation sites were seen as broad zones associated with the nuclear envelope.

FLAGELLAR ROOTS AND THE RESERVOIR CYTOSKELETON OF COLACIUM LIBELLAE (EUGLENOPHYCEAE)1

ABSTRACTThe three flagellar roots of Colacium Ehrenberg give rise to the three microtublar bands of the reservoir cytoskeleton. The dorsal root (DR) originates at the basal body (bb1) of the emergent flagellum. It is initiated on the left side of the cell, runs toward the right side under the posterior end of the reservoir and thence anteriorly in a spiral path over the dorsal surface of the reservoir until it terminates on the left side of the eyespot. Along its length, it appears to initiate a dorsal band (DB) which forms the major dorsal portion of the reservoir cytoskeleton—the dorsal microtubules (DMT). Two roots originate at the basal body (bb2) of the non‐emergent flagellum. The ventral root (VR) runs up the left side of the cell and initiates the band of microtubules which forms part of the presumptive vestigial cytopharynx. Therefore, it forms the reinforcing microtubules (MTR) of Colacium. The intermediate root (IR) forms the para‐reservoir microtubules (PMT). Flagellar root correlation with the reservoir cytoskeletal bands strengthens their homologies with the bodonid bands and further supports the hypothesis that the euglenoids are derived from the kinetoplastid flagellates.

The Floral Nectaries of <italic>Hibiscus rosa-sinensis</italic><subtitle>1. Development of the Secretory Hairs</subtitle>

Floral nectaries of Hibiscus rosa-sinensis occur on the lower inner side of the fused sepals and each one consists of numerous (50000–55000) secretory hairs, occupying a cylinder-like zone completely lining the inner side of the sepals. Each hair originates from a single protodermal mother cell and, at maturity, it is built up of a basal cell, a stalk, 35–40 intermediate cells and a tip secretory cell. Development of protodermal cells into secretory hairs is asynchronous, the first cells to initiate development being those situated in the lowermost part of the cylindrical zone, and development progressing upwards. Volume increase of protodermal mother cells initiating development is accompanied by cell polarization manifested by organelle displacement towards the apical region. Secretory hairs are formed through a sequence of transverse and, later on, anticlinal divisions. Divisions of apical cells are preceded by well defined pre-prophase microtubule bands, which foreshadow the plane of the forthcoming division and predict with accuracy the sites of parental walls where the new cell plate fuses at cytokinesis. Stalks consist of either one or two cells. Two-celled stalks occur in 40 per cent of secretory hairs and derive from a transverse division of one stalk cell; the wall formed is always deposited parallel to the proximal and distal walls, but never to the lateral ones. The significance of this mode of division is discussed in relation to the fact that lateral walls are entirely impregnated with a cutin-like material that blocks apoplastic movement of solutes.

Induction, polarity and spatial control of cytokinesis in some abnormal subsidiary cell mother cells ofZea mays

Cytokinesis in the subsidiary cell mother cells (SMCs) ofZea mays leaves grown in the presence of 5 mM of caffeine solution is usually partially inhibited. A continuous wall strip, resembling a portion of the subsidiary cell (SC) wall, is laid down in the preprophase microtubule band (PMB) cortical zone. Sometimes, the incomplete SC (“SC”) wall grows centripetally in the absence of a phragmoplast and the gap becomes smaller or closes. The “SC” nucleus escapes through the “SC” wall gap into the larger SMC compartment and may fuse with the other nucleus. The aberrant SMCs (a-SMCs) pass through another division cycle, reattempting to produce a SC. A typical PMB is found in the “SC” space, in the site of the previous PMB. Moreover, in some preprophase SMCs, the cytoplasm adjacent to the “SC” wall is traversed by a small number of microtubules. The preprophase nuclei are partly or totally separated from the PMB by the perforated “SC” wall and may lie far from the latter. Usually, one mitotic spindle is assembled. The cycling paired polarized nuclei appear to synchronize and their chromosomes line up together on a single metaphase plate. Although the mitotic spindle axis is diversely oriented, one of its poles tends to be stabilized in the proximity of the “SC” wall gap. These divisions separate abnormal cells. Most or all the cell plate edges fuse with wall regions far from the PMB cortical zone. However, when some of them approach the “SC” wall strips, they are “attracted” and intersect their rims. In rare occasions the cell plate, invading the “SC” space is guided by the PMB cortical zone to create a SC-like curved wall portion, in absence of a daughter nucleus. Observations show that the cell plate arrangement in redividing aberrant SMCs is not subjected to a strict spatial control. The disorder of polarization sequence generated by the “SC” wall ring and especially the perturbation of the spatial (and functional?) relationship between PMB-PMB cortical zone and the nucleus—mitotic spindle is a causal factor of the variable cell plate arrangements.

Pre‐prophase band of microtubules, absent from tip‐growing moss filaments, arises in leafy shoots during transition to intercalary growth

AbstractIn the presence of cytokinin, undetermined side branch initials of the moss, Physcomitrella patens, arc induced to form buds and then leafy shoots rather than to develop as tip‐growing filaments. This represents a transition between the two modes of plant cell expansion–tip growth and uniform intercalary growth. The organization of microtubules in filaments is different from that in leafy shoots and can be traced back to the influence of phytohormones on side branch initials. Microtubules either focus at a particular region (as in tip‐growing cells) or in the presence of high levels of cytokinin form swollen bud initials in which microtubules are more diffusely organized. Higher levels of cytokinin are capable of destabilizing tip microtubules in caulonemal filaments. Although caulonemata are not normally target cells, this implies that cytokinin may exert its morphogenetic effects by altering microtubule organization.In tip‐growing filaments, interphase microtubules trace a meandering course through the cytoplasm towards the tip and are not for the main part associated with the plasma membrane as are cortical arrays. There is no pre‐prophase band of microtubules to indicate the future division plane, even though the oblique division plane is known to be precisely controlled relative to environmental factors. This microtubule cycle contrasts with cells of the leafy shoots that develop from buds: in these, the interphase array is cortical, consisting of flat‐pitched microtubular helices that do not focus upon a growing tip. It is now shown that pre‐prophase hands occur at this stage.The absence of bands does not readily correlate with imprecise control of the division plane. Instead, it is proposed that the ability to form pre‐prophase bands depends upon the arrangement of microtubules in the preceding interphase array. Ways in which bands might be formed are discussed and the generality of these ideas is tested by observations on higher plant cells.

Microtubule reorganization, cell wall synthesis and establishment of the axis of elongation in regenerating protoplasts of the algaMougeotia

Microtubule reorganization and cell wall deposition have been monitored during the first 30 hours of regeneration of protoplasts of the filamentous green algaMougeotia, using immunofluorescence microscopy to detect microtubules, and the cell-wall stain Tinopal LPW to detect the orientation of cell wall microfibrils. In the cylindrical cells of the alga, cortical microtubules lie in an ordered array, transverse to the long axis of the cells. In newly formed protoplasts, cortical microtubules exhibit some localized order, but within 1 hour microtubules become disordered. However, within 3 to 4 hours, microtubules are reorganized into a highly ordered, symmetrical array centered on two cortical foci. Cell wall synthesis is first detected during early microtubule reorganization. Oriented cell wall microfibrils, co-aligned with the microtubule array, appear subsequent to microtubule reorganization but before cell elongation begins. Most cells elongate in the period between 20 to 30 hours. Elongation is preceded by the aggregation of microtubules into a band intersecting both foci, and transverse to the incipient axis of elongation. The foci subsequently disappear, the microtubule band widens, and microfibrils are deposited in a band which is co-aligned with the band of microtubules. It is proposed that this band of microfibrils restricts lateral expansion of the cells and promotes elongation. Throughout the entire regeneration process inMougeotia, changes in microtubule organization precede and are paralleled by changes in cell wall organization. Protoplast regeneration inMougeotia is therefore a highly ordered process in which the orientation of the rapidly reorganized array of cortical microtubules establishes the future axis of elongation.

Is the preprophase band of microtubules a marker of organization in suspension cultures?

To date it has been accepted that preprophase bands of microtubules (PPBs) either do not precede cell division or do so inconsistently in suspension cultures, the assumption being that such cultures proliferate in an “unorganized” state in which placement of cell plates is not regulated by the PPB system that is widespread in organized tissues. Using indirect immunofluorescence microscopy with antitubulin, the relative frequency of occurrence of PPBs in enzymatically separated cells from root tips and suspension cultures of carrot and tobacco, was quantified by taking the ratio of the number of PPBs: phragmoplast. This ratio was termed the “PPB index”.

Prophase bands of microtubules occur in protoplast cultures of Vicia hajastana Grossh

Circumnuclear bands of microtubules (MT) have been found in the prophase of mitoses in cultured protoplasts of Vicia hajastana. The timing of the appearance and disappearance of the prophase band of MT (PB) relative to the stage of mitosis was studied using simultaneous staining of MT by immunofluorescence and DNA by Hoechst 33258. These protoplasts regenerate into unorganized tissue. Pre-prophase bands of MT have previously been found only in highly organized tissues of higher plants. The role of PB in cell division is discussed.