DNA SYNTHESIS,CELL DIVISION,AND CELL DIFFERENTIATION DURING LEAF DEVELOPMENT OF XANTHIUM PENNSYLVANICUM

Leaf growth consists of two basic processes, cell division and cell enlargement. DNA synthesis is an integral part of cell division and can be studied with autoradiographic techniques and incorporation of some labeled precursor. Studies were made on the synthesis of nuclear DNA through incorporation of ³H-thymidine in various parts of the lamina during the entire course of leaf development of Xanthium pennsylvanicum. The time course analysis of DNA synthesis was correlated with cell division and rates of cell enlargement. Significant differences in ³H-thymidine incorporation were found in various parts of the lamina. Cell division and DNA synthesis were highest in the early stages of development. Since no ³H-thymidine was incorporated after cessation of cell division (LPI 2.8) in the leaf lamina, it appears that DNA synthesis is not needed for enlargement and differentiation of Xanthium cells. Rates of cell enlargement were negligible in the early development and reached their maximum after cessation of mitoses, between plastochron ages (LPI) 3 and 4. Cells matured between LPI's 5 and 6. Enzymatic activity was correlated with cell division and cell differentiation at various stages of leaf development.     

EFFECTS OF RED AND FAR-RED RADIATION ON CELL DIVISION AND ELONGATION IN THE STEM OF PINTO BEAN SEEDLINGS

Nine-day-old Pinto bean seedlings, Phaseolus vulgaris L., were treated daily for 12 days with supplementary red or far-red radiation following daily exposure to 8 hr of daylight. Stem elongation was nearly exponential from the 4th to the 12th day of treatment and was about 2.9 times greater under far-red radiation. Cell division and cell elongation were promoted essentially equally by far-red radiation. There was virtually no difference in either the rate of leaf initiation or the duration of growth of internodes under the supplementary radiation.     

STEM ELONGATION OF XANTHIUM PLANTS PRESENTED IN TERMS OF RELATIVE ELEMENTAL RATES

Vegetative Xanthium plants grown under noninductive conditions were marked along the stem with India ink and photographed during three successive days. The relative elemental rates of stem elongation [d(dX/dt)/dX] were estimated for 18 plants between 15 and 18 plastochrons. On the average, only the 8.0 cm terminal part of the stem was elongating in this group of plants. Young internodes were elongating at constant relative elemental rates ([d(dX/dt)/dX] was about 0.2 days^–1); nodal portions of the stem beteween two young internodes were not elongating. Internodes longer than 2 cm displayed an acropetal pattern of elongation in which the basal part of an internode stopped elongating and matured first and the apical portion last. The pattern of elongation of the stem could be best approximated to a set of cascading waterfalls with declining plateaus in the direction of the water flow. The acropetal pattern of individual internode elongation observed in Xanthium was similar to those reported for Helianthus and Phaseolus internode growth.     

LIGHT-DEPENDENT INHIBITION OF CELL DIVISION AND RHIZOID ELONGATION IN GEMMAE OF THE FERN,VITTARIA GRAMINIFOLIA

Gametophytes of the shoe-string fern Vittaria graminifolia produce linear, six-celled propagules called gemmae. The terminal cells of each gemma elongate into primary rhizoids in culture, and the inner body cells divide asymmetrically to produce prothallial or rhizoid initials. The initiation of both asymmetric cell division and rhizoid elongation is delayed by light intensities greater than 2 w/m². The maximal rates of cell division and rhizoid elongation are unaltered. A 24-hr pulse of high light intensity delays cell division and rhizoid elongation to the same extent, whenever applied during the first 3 d of culture. The model we propose for cell division hypothesizes the existence of a preparatory phase of finite duration prior to mitosis that is sensitive to light intensity. If a cell is irradiated by light intensities greater than 2 w/m² while in the preparatory phase, its entrance into mitosis is delayed. A similar model is proposed for the initiation of rhizoid elongation. Despite the fact that both cell division and rhizoid elongation are dependent on photosynthesis, direct measurements of CO₂-uptake rates show that the inhibitory effects of high light intensities are not due to an inhibition of photosynthesis.     

GROWTH HORMONE AND CARTILAGE CELL DIVISION IN HYPOPHYSECTOMIZED RATS

The effects of growth hormone on cell division in the growth cartilage of normal and hypophysectomized rats have been studied by autoradiography with tritiated thymidine. Following hypophysectomy the percentage labelling fell to 1.5-2%, about one fifth of that found in normal animals. Following injection of 100-1600 μg of bovine growth hormone, the percentage labelling increased after a delay of 8-16 hr, the delay, the rate of rise and the maximum reached being dose dependent. Single or repeated doses of growth hormone produced no significant change in normal young rats.     

INHIBITION OF CELL PROLIFERATION AND EXPANSION IN VITRO BY THREE STEM GROWTH RETARDANTS

Three inhibitors of stem elongation, Amo, CCC, and Phosfon, inhibit cell division and expansion in tissues cultured in vitro. However, contrary to the case in intact plants, gibberellic acid does not prevent the retardant-induced inhibition in vitro. Supplementary auxin is also without effect in preventing the inhibition. Thus, the effect of the retardants cannot be simply that of inhibiting gibberellin or auxin synthesis. With respect to growth, carrot, chrysanthemum, and geranium tissues are equally sensitive to all 3 retardants, whereas tobacco tissues are considerably more resistant to Amo and apparently unaffected by CCC.     

CALCIUM DELAYS THE ONSET OF CELL DIVISION AND RHIZOID ELONGATION IN GEMMAE OF THE CALCIFUGE FERN,VITTARIA GRAMINIFOLIA

Vittaria graminifolia gametophytes reproduce asexually by linear six-celled propagules known as gemmae. It has been shown previously that asymmetric cell division and rhizoid elongation in gemmae are inhibited by millimolar concentrations of Ca^{+ +}. The present paper shows that millimolar Ca^{+ +} delays the onset of cell division at a point prior to mitosis, without affecting the maximal rate of cell division. Ca^{+ +} is most effective in delaying cell division when it is present during the first 24 hours of culture, 2 or 3 days before the initiation of cell division. Millimolar Ca^{+ +} inhibits rhizoid elongation by delaying its onset, while also reducing its maximal rate. Ca^{+ +} is also most effective in delaying rhizoid elongation during the first 24 hours of culture. Culture of gemmae on Ethylglycol-bis-(aminoethyl ether)-N,N,N‘,N‘-tetraacetic acid-buffered media shows that the maximum frequency of cell division occurs at pCa 7, while for the initiation of rhizoid elongation, it occurs at pCa 5.     

ROLE OF ASYMMETRIC CELL DIVISION IN PTERIDOPHYTE DIFFERENTIATION. II. EFFECT OF CA2+ ON ASYMMETRIC CELL DIVISION,RHIZOID ELONGATION,AND ANTHERIDIUM DIFFERENTIATION IN VITTARIA GEMMAE

Gametophytes of Vittaria graminifolia reproduce vegetatively by means of gemmae. Each gemma consists of a linear array of six cells: four body cells and a knob-shaped terminal cell at each end. When gemmae are shed from the gametophyte onto Knop's mineral medium, the two terminal cells do not divide, but elongate to form primary rhizoids. The body cells undergo asymmetric cell division, and the smaller daughter cells differentiate into either secondary rhizoids or prothalli. When gibberellic acid is included in the medium, antheridia are formed as a result of asymmetric cell division instead of vegetative structures. We studied the effect of Ca²⁺ on asymmetric cell division, rhizoid elongation, and antheridium formation in gemmae cultured on Knop's mineral medium and variations of Knop's medium. Ca²⁺ inhibited the onset of cell division and rhizoid elongation, but was required for differentiation of antheridia. Treatments which lowered the Ca²⁺ content of gemmae (EGTA and dilute HCl extraction, culture on verapamil-containing and Ca²⁺-deficient medium) caused an early onset of cell division and rhizoid elongation. The stimulation of growth was most pronounced when gemmae were deprived of Ca²⁺ during the first 24 hr of culture. The proportion of cell divisions which differentiated into antheridia in response to GA was greatly reduced when the Ca²⁺ status of gemmae was lowered with verapamil and Ca²⁺-EGTA buffers.     

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.