CELL DIVISION IN BULBOCHAETE. II. HAIR CELL FORMATION1

Most vegetative cells of Bulbochaete, and all those of Oedogonium, possess an apical, circular discontinuity in the structure of their secondary wall. Rupture of the wall at this precise site permits expansion of the ring during cell division and release of the zoospore following zoosporogenesis. Certain cells of Bulbochaete (always the apical daughter cell of a division pair) lack this type of discontinuity. Instead, the apical wall is thinned out on one side, so that the cell bulges asymmetrically. In the middle of the bulge is a wall discontinuity which extends only part way around the cell. The wall will rupture here, too, for zoospore release, but if a cell having such a wall, divides, it invariably does so asymmetrically, with one pole of the spindle located in the bulge. Cytokinesis then cuts off a small, colorless daughter cell. The wall ruptures at the discontinuity, and this daughter cell emerges through the slit and differentiates into a hair. The creation of hairs in such cells commences with the deposition of a pad of primary wall lining the bulge. Golgi bodies are involved in its secretion, but not in that of a secondary wall layer which forms next in the premitotic cell and covers the primary wall. The cell becomes polarized; the nucleus migrates toward this region as the chloroplast moves aside. After the asymmetric mitosis, a curved phycoplast cuts off the hair cell nucleus and prevents the chloroplast from moving back into the future hair, whose cytoplasm soon loses much of its affinity for heavy metal stains. Following rupture of the parental wall, the phycoplast moves some distance past the limits of the newly deposited secondary wall layer and then forms a cross wall under the hair. The secondary wall of the hair is not continuous with the secondary wall structure of the parental cell; the circular discontinuity that arises around the base of the bulging parental wall is then perpetuated and accentuated as the hair's secondary wall thickens. This wall weakening becomes the dislocation that will predetermine the site of the ring and consequently the direction of cell expansion in the next normal division of the cell subtending the hair. Abnormal ring formation and the creation of terminal twin hairs have also been examined. The lip of the growing hair contains a characteristic organization of membranes and other components which may be related to the organization of the hair's numerous longitudinally oriented microtubules. These results are discussed in terms of the morphology of the wall in the Oedogionales generally. The creation of the special wall morphology that leads to hair cell formation is considered to be ontogenetically related to a similar wall morphology that is involved in formation of the fertilization pore of the oogonium.     

CELL DIVISION IN SPIROGYRA. I. MITOSIS

Dividing cells of Spirogyra sp. were examined with both the light and electron microscopes. By preprophase many of the typical transverse wall micro-tubules disappeared while others were seen in the thickened cytoplasmic strands. Microtubules appeared in the polar cytoplasm at prophase and by prometaphase they penetrated the nucleus. They were attached to chromosomes at metaphase and early anaphase, and formed a sheath surrounding the spindle during anaphase; they were seen in the interzonal strands and cytoplasmic strands at telophase. The interphase nucleolus, containing 2 distinct zones and chromatinlike material, fragmented at prophase; at metaphase and anaphase nucleolar material coated the chromosomes, obscuring them by late anaphase. The chromosomes condensed in the nucleoplasm at prophase, moving into the nucleolus at prometaphase. The nuclear envelope was finally disrupted at anaphase during spindle elongation; at telophase membrane profiles coated the reforming nuclei. During anaphase and early telophase the interzonal region contained vacuoles, a few micro-tubules, and sometimes eliminated n ucleolar material; most small organelles, including swollen endoplasmic reticulum and tubular membranes, were concentrated in the polar cytoplasm. Quantitative and qualitative cytological observations strongly suggest movement of intact wall rnicrotubules to the spindle at preprophase and then back again at telophase.     

CELL DIVISION AND THE ROLE OF THE PRIMARY WALL IN THE FILAMENTOUS DESMID ONYCHONEMA LAEVE (CHLOROPHYTA)1

Cell division and the role of the primary wall in filament formation in the desmid Onychonema laeve Nordst. were investigated by transmission and scanning electron microscopy. In addition, sequential chemical extractions and enzyme treatments were performed, on cell walls of intact filaments. Interphase cells are deeply constricted, consisting of two semicells, each elliptical in front view and circular in side view. In addition to two short lateral spines, each semicell has two apical processes that originate on opposite sides at an angle of about 15° from the central axis and overlap the adjacent cell. Division is initiated as in other desmids by a slight separation of the semicells and development of a girdle of primary wall material at the isthmus. In O. laeve the girdle of primary wall expands to form a spherical vesicle (termed a division vesicle) between the separating semicells. Nuclear division and septum formation occur in this vesicle when it is nearly the full diameter of the filament. Morphogenesis of the apical processes begins with completion of the septum, before the secondary wall appears. At maturity each apical process is surrounded by a thick layer of both secondary and primary wall, except that its capitate tip protrudes through the shroud of primary wall. Sequential treatment with hot ammonium oxalate, 4% NaOH, 17.5% NaOH and 10% chromic acid or various enzyme solutions did not cause filament breakage. SEM and TEM views of O. laeve after these treatments show intact secondary walls and intact primary wall material covering and connecting the apical processes of adjacent cells. It is the persistence of the primary wall between cells and around the apical processes that maintains the long, unbranched filamentous morphology of Onychonema laeve.     

ROLE OF SULFUR IN THE CELL DIVISION OF CHLORELLA, WITH SPECIAL REFERENCE TO THE SULFUR COMPOUNDS APPEARING DURING THE PROCESS OF CELL DIVISION. I.

1. The role of sulfur in the cell division of Chlorella was studiedby following the fate of the sulfur supplied to the sulfur-deficientcells using ³⁵S as a tracer.
2. The sulfur-deficient cells whichwere unable to perform celldivision were made capable of divisionby the provision of ³⁶S-labeledsulfate under non-photosynthesizingconditions. Soon after theprovision of sulfate the labeledsulfur went rapidly into thecold perchloric acid (PCA)-solublefraction of algal cells,almost entirely in the form of sulfateand/or some other inorganicsulfur substance (s). With the lapseof time, more or less remarkablechanges occurred in the patternof ³⁵S-distribution in differentfractions of cell material.It was noticed that, at the onsetof cell division, a sulfur-containingpeptide-nucleotide compound(s)(SPN), which has been reportedearlier, appeared in a largequantity in the cold PCA-solublefraction, and that its quantitydecreased gradually during thesubsequent process of cell division,suggesting that the compoundwas transformed into some othersubstance (s), presumably withits nucleotide moiety going intonucleic acids and the peptidemoiety going into some essentialproteins.
3. Another noteworthyphenomenon observed during the process ofcell division wasthe incorporation of ³⁶S in a group of hotPCA-soluble substances.These sulfur substances were revealedto be sulfur-containingnucleotidic compounds, which might possiblybe some essentialcomponents of, or substances in close relationto, deoxypentosenucleic acid (DNA).

(Received March 1, 1960; )     

CELL DIVISION IN CHLAMYDOMONAS MOEWUSII1

Cell division in Chlamydomonas moewusii is described. The cells become immobile with flagellar abscission prior to mitosis. The basal bodies migrate toward the nucleus and become intimately associated with the nuclear membrane which is devoid, of ribosomes where adjacent to the basal bodies. The basal bodies replicate at preprophase. The nucleolus fragments at this stage. By prophase the basal body pairs have migrated, to the nuclear poles. Spindle fibers become prominent in the nucleus. The nuclear membrane does not fragment. The nucleus assumes a crescent-form by metaphase. Polar fenestrae are absent. Kinetochores appear at anaphase. An interzonal spindle elongates as the chromosomes move to the nuclear poles. Daughter nuclei become abscised by an ingrowth of nuclear membrane, leaving behind a separated, degenerating interzonal spindle. Ribosomes reappear on the outer nuclear membrane at late telophase. Nucleoli reform early in cytokinesis. The cleavage furrow, associated microtubules, and endoplasmic reticulum comprise the phycoplast. Cytokinesis proceeds rapidly after the completion of telophase. The basal body-nucleus relationship becomes reorganized into the typical interphase condition late in cytokinesis. Specific and predictable organelle rearrangements during mitosis have been described. Cell division in C. moewusii is compared with other algae, especially C. reinhardi.     

CELL DIVISION IN COSMARIUM BOTRYTIS1

Cell division in Cosmarium is described. Premitotic cells are very dense; the semicells, previously appressed to one another, separate slightly during entry into prophase. This separation coincides with deposition of a girdle of new wall material around the isthmus, where the 2 semicells are joined. Micro-tubules, abundant around the isthmus wall during interphase, all disappear during prophase; meanwhile, other microtubules proliferate outside the nuclear envelope. By metaphase, the nucleolus has dispersed, although remnants of it persist. The nuclear envelope breaks up, but some membranes coat metaphase chromosomes. The spindle, while being typical, is somewhat multipolar; microtubules, usually associated with elements of endoplasmic reticulum, are oriented toward numerous regions in the poles. During anaphase and telophase, these spindle tubules become increasingly directed toward a few discrete foci, and they persist after telophase. Meanwhile, the septum grows from the girdle of wall material to bisect the cell. After cytokinesis, some microtubules reappear near the isthmus, but only adjacent to the older, non-expanding semicell wall. Cell expansion then takes place, during which the nucleus, ensheathed in a complex microtubular system, moves into the forming semicell. Later, the chloroplast follows the nucleus and its 2 pyrenoids elongate and divide. When semicell expansion is complete, the chloroplast cleaves adjacent to the isthmus. The nucleus, now apparently not associated with microtubules, concurrently moves back into the isthmus. Continuous deposition of primary wall material accompanies cell expansion. Wall materials are apparently secreted as aggregates (perhaps derived from the contents of vesicles) adjacent to the plasmalemma, whose fibrous components become increasingly oriented in the outer layers of the wall by stretching. Late in semicell formation, this deposition ceases and during further expansion, the semi-cell develops a pattern of warts and ridges. Secondary wall deposition under the primary wall then follows, matching this pattern of ornamentation. In addition, numerous plugs of amorphous material capped by specialized regions of the plasmalemma, traverse the entire thickness of the secondary wall which becomes further thickened at these particular sites. The amorphous plugs presumably are eroded away later to form the mucilage pores of the vegetative cells. The wall microtubules gradually become more symmetrically arrayed around the isthmus as this new secondary wall thickens. These observations are discussed in comparison with other work on morphogenesis in desmids.     

ULTRASTRUCTURE AND DIFFERENTIATION IN CLADOPHORA GLOMERATA. I. CELL DIVISION

Cladophora glomerata is a coenocytic, fresh-water green alga in which mitosis and cytokinesis occur independently. The mitotic spindle is centric, closed, and develops from two half-spindles which form from amorphous but well-defined MTOCs at each pole. The nucleolus is only partially dispersed during mitosis and structured kinetochores are evident on the chromosomes. Anaphase separation of chromosomes is asynchronous and results from spindle elongation plus shortening of the chromosome-to-pole distance. Neither a phycoplast nor a phragmoplast is present during cytokinesis. Microtubules are associated with the septum but whether they participate actively in its ingrowth is not clear. Two types of vesicles are associated with the growing septum. The membrane at its leading edge is thicker and more densely stained than elsewhere. The ultrastructure of nuclear and cell division in C. glomerata is sufficiently different from the data on other green algae that conclusions about phylogeny must await further study, especially of other coenocytic green algae.     

PHOTOCONTROL OF THE ORIENTATION OF CELL DIVISION IN ADIANTUM. I. EFFECTS OF THE DARK AND RED PERIODS IN THE APICAL CELL OF GAMETOPHYTES

When protonemata of Adiantum capillus-veneris L. which had been grown filamentously under continuous red light were transferred to continuous white light, the apical cell divided transversely twice, but the 3rd division was longitudinal. An intervening period of darkness lasting from 0 to 90 hr either between the 1st and the 2nd cell division or between the 2nd and the 3rd one did not affect the number of protonemata in which the 3rd cell division was longitudinal. The insertion of red light instead of darkness greatly decreased the percentage of 1st longitudinal divisions occurring at the 3rd division, and increased the number of transverse divisions. Fifty percent reduction of induction of 1st longitudinal division was caused by ca. 50 hr exposure to red light between 1st and 2nd division and by ca. 20 hr between 2nd and 3rd division, and total loss was induced by an exposure of ca. 100 hr or longer to red light in the former and by ca. 40 hr longer in the latter. Thus, by using an appropriate intervening dark period or exposure to red light, the orientation and timing of cell division could be controlled in apical cell of the fern protonemata.     

THE CONTROL OF THE ORIENTATION OF CELL DIVISIONS IN FERN GAMETOPHYTES

Time-lapse observations of filamentous fern gametophytes were used to evaluate whether the plane of cell division is referable to the plane of minimal surface area before and during the transition to two-dimensional growth. Cell dimensions of the apical cell were related to the length/width ratios associated with minimal area in the transverse plane vs. longitudinal plane, by modeling the apical cell as a hemisphere subtended by a cylinder. Our working hypothesis predicts that filamentous growth is perpetuated by an apical cell geometry that makes the transverse division plane the orientation of minimal surface area, whereas the transition to two-dimensional growth (longitudinal division of the apical cell) occurs once the longitudinal plane becomes the position of minimal surface area. The predictions of this hypothesis are fulfilled regardless of variations in light intensity and light quality, the presence of regulators of metabolism, or whether the experimental perturbation causes a corresponding selective inhibition of the transition to two-dimensional growth. Thus, the control of the plane of cell division in this system seems to depend on thermodynamic considerations of surface area. Furthermore, we favor the conclusion that the role of the genome in the transition to two-dimensional growth involves its influence on apical cell dimensions rather than the induction of specific genes for specific morphogenetic mRNAs.     

THE DINOFLAGELLATE PELLICULAR WALL LAYER AND ITS OCCURRENCE IN THE DIVISION PYRRHOPHYTA1

Forty-five species of dinoflagellates were surveyed for the presence of a pellicular layer in the amphiesma or cell covering. Such a layer was found in 15 of the 20 genera studied. Half the pellicles tested were resistant to acetolysis and may contain a sporopollenin-like material similar to that of some dinoflagellate cyst walk. Most organisms which formed pellicles were capable of reinforcing this layer with cellulose. Pellicles of Heterocapsa niei (Loeblich) Morrill & Loeblich and Scrippsiella trochoidea (Stein) Loeblich were studied with the electron microscope. Evidence is presented indicating that dividing cells of S. trochoidea from new walls while still enclosed in the parental pellicular layer.     

A STUDY OF CELL WALL AND FLAGELLA FORMATION DURING CELL DIVISION IN THE SCALY GREEN ALGA,SCHERFFELIA DUBIA (CHLOROPHYTA)1

The thecate green flagellate Scherffelia dubia (Perty) Pascher divides within the parental cell wall into two progeny cells. It sheds all four flagella before cell division, and the maturing progeny cells regenerate new walls and flagella. By synchronizing cell division, we observed mitosis, cytokinesis, cell maturation, flagella extension, and cell wall formation via differential interference contrast microscopy of live cells and serial thin‐section EM. Synthesis of thecal and flagellar scales is spatially and temporally strictly separated. Flagellar scales are collected in a pool during late interphase. Before prophase, Golgi stacks divide, flagella are shed, the parental theca separates from the plasma membrane, and flagellar scales are deposited on the plasma membrane near the flagellar bases. At prophase, Golgi bodies start to synthesize thecal scales, continuing into interphase after cytokinesis. During cytokinesis, vesicles containing thecal scales coalesce near the cell posterior, forming a cleavage furrow that is initially oriented slightly diagonal to the longitudinal cell axis but later becomes transverse. After the progeny nuclei have moved into opposite directions, resulting in a “head to tail” orientation of the progeny cells, theca biogenesis is completed and flagellar scale synthesis resumes. Progeny cells emerge through a hole near the posterior end of the parental theca with four flagella of about 8 μm long. The precise timing of flagellar and thecal scale synthesis appears to be an evolutionary adaptation in a scaly green flagellate for the thecal condition, necessary for the evolution of the phycoplast and thus multicellularity in the Chlorophyta.     

ROLE OF DICTYOSOMES IN WALL FORMATION DURING CELL DIVISION OF CHLORELLA VULGARIS

The behavior of dictyosomes in wall formation during cell division of Chlorella vulgaris follows a definite pattern. During formation of the partition membrane they migrate into the equatorial plane and pair. There is a close spatial relationship between the dictyosomes and the partition membrane which, itself, may be derived from the fusion of dictyosomal vesicles. Dictyosomes also may participate significantly in the deposition of new wall material.     

ULTRASTRUCTURE OF CELL DIVISION AND REPRODUCTIVE DIFFERENTIATION OF MALE PLANTS IN THE FLORIDEOPHYCEAE (RHODOPHYTA): CELL DIVISION IN POLYSIPHONIA1

Cell division in the marine red algae Polysiphonia harveyi Bailey and P. denudata (Dillwyn) Kutzing was studied with the electron microscope. Cells comprising the compact spermatangial branches of male plants were used exclusively because of their small size, large numbers and the ease with which the division planes can be predetermined. Some features characterizing mitosis in Polysiphonia confirm earlier electron microscope observations in Membranoptera, the only other florideophycean algae in which mitosis has been studied in detail. Common to both genera are a closed, fenestrated spindle, perinuclear endoplasmic reticulum, a typical metaphase plate arrangement of chromosomes, conspicuous, layered kinetochores, chromosomal and non-chromosomal microtubules, and nucleus associated organelles (NAOs) known as polar rings (PRs) located singly in large ribosome-free zones of exclusion at division poles in late prophase. However, other features, unreported in Membranoptera, were observed consistently in Polysiphonia. These include the presence of PR pairs in interphase-early prophase cells, the attachment of PRs to the nuclear envelope during all mitotic stages, the migration of a single PR to establish the division axis, a prominent, nuclear envelope protrusion (NEP) at both division poles at late prophase, the prometaphase splitting of PRs into proximal and distal portions, and the reformation of post-mitotic nuclei by the separation of an elongated interzonal nuclear midpiece at telophase. During cytokinesis, cleavage furrows impinge upon a central vacuolar region located between the two nuclei and eventually pit connections are formed in a manner basically similar to that reported for other red algae. Diagrammatic sequences of proposed PR behavior during mitosis are presented which can account for events known to occur during cell division in Polysiphonia. Mitosis is compared with that reported in several other lower plants and it is suggested that features of cell division are useful criteria to aid in the assessment of phylogenetic relationships of red algae.     

TOPOGRAPHY OF CELL DIVISION IN THE STRUCTURALLY COMPLEX DINOFLAGELLATE GENUS ORNITHOCERCUS1

The dinophysoid dinoflagellates (to which Ornithocercus belongs) achieve growth both by increase in size of individual wall elements and, during rapid lateral expansion associated with cell division, by the formation of a semimeridional band of material termed the megacytic zone (MZ). The MZ maintains mother cell wall integrity during complete cytokinesis of the cell body and enclosure with new wall elements. The lists, extensive wing-like extensions of the wall, can only be reformed after dissolution of the MZ. Beginning near the ventral region (which is the last region of the wall to be duplicated), the MZ dislocates and its material is apparently resorbed. The last region of attachment is invariably dorsal and in several, but not all species, the daughter cells may remain attached during early list formation by a special remnant of the MZ, termed here the dorsal megacytic bridge (DMB). After full separation of daughter cells remnants of the DMB persist for an unknown but presumably short period. The topography of this process, involving radical ontogenetic alterations in the appearance of the daughter cells and some wall surfaces, is illustrated here by the scanning electron microscope. In addition 2 aberrant types of division are shown, one of which results in a double individual, termed a geminoid.     

ON THE CYTOLOGY OF ANTHERIDIUM FORMATION IN THE FERN SPECIES ONOCLEA SENSIBILIS. I. CYTOLOGY OF CELL PLATE AND CELL WALL FORMATION

In the four-celled antheridium of the fern species Onoclea sensibilis a central spermatogenous cell is enveloped by a jacket of three cells. Starting from the base, the jacket comprises the cup-shaped basal cell, the ring cell—both of which encircle the spermatogenous cell—and the cap cell. The lower wall of the spermatogenous cell has the configuration of a funnel; its upper wall is dome shaped. The choice of whole antheridia for study instead of sectioned ones has, for the first time, made it possible to study the formation of the uniquely shaped antheridial cell plates step by step. The cell plate antecedent of the funnel wall has the configuration of a funnel. This conclusion conflicts with Davie's contention that this cell wall is oriented transversely at first and acquires funnel-shape secondarily. The present studies further show that the funnel cell plate forms from base to rim. This finding contrasts with a report that in another fern species this cell plate begins to form on one side of the initial and then proceeds circularly around it. The base of the funnel cell plate attaches to the basal wall of the antheridium initial in a separate event. The genesis of the dome-shaped upper wall of the spermatogenous cell is described for the first time.     

CELL DIVISION IN SPIROGYRA. II. CYTOKINESIS

Cytokinesis in cells of Spirogyra sp. was studied with both light and electron microscopes. Early formation of the cross wall was achieved by annular ingrowth of a septum; the cross wall was completed by a phragmoplast containing Golgi vesicles, longitudinally aligned microtubules, and associated electron-dense material. Spirogyra may represent an intermediate stage in the evolution of the phragmoplast seen in higher plants.     

AN ULTRASTRUCTURAL STUDY OF CELL DIVISION IN THE CAMBIUM

The fine structure of dividing cambial cells of Ulmus americana and Tilia americana has been studied in material fixed in glutaraldehyde followed by osmium tetroxide. The cambia examined consisted of 7–9 rows of unexpanded fusiform cells, all of which had similar ultrastructural components. The fine structure and sequence of events of mitosis and cytokinesis in the dividing cambial cells apparently are similar to those of dividing cells in root tips and leaves. Of special interest was the observation that during cytokinesis, a broad cytoplasmic plate or phragmosome precedes the developing phragmoplast and cell plate through the dividing cambial cell. Smooth and coated vesicles derived from dictyosomes are associated with cell plate formation in these cells, smooth vesicles primarily with earlier stages of plate formation, and coated vesicles in later stages.