CELL CYCLE DURATION IN THE MERISTEM OF NEPHROLEPIS BISERRATA STOLONS: THE ROLE OF THE APICAL CELL

The stolons of Nephrolepis biserrata (sw.) Schott are thin axes that grow rapidly (from 2 to 4 mm per day) in the controlled conditions applied. In the cylindro-conical meristem, three histological zones are defined. Cell cycle duration was determined for each zone by autoradiographic methods after incorporation of tritiated thymidine and confirmed by the colchicine-induced metaphase-accumulation technique. The apical cell and its derivatives (Zone 1) are mitotically more active (cell cycle duration: 80 hr) than the cells of the subapical zones (2 and 3), where cell cycle lengths are 142 hr and 95 hr respectively. These data, compared to previous results, give evidence for the main role played by the relative rate of division of the apical cell compared to that of lateral cells in the organization and the shape of the meristem of pteridophytes. Moreover, the apical cell appears to be unique in having a differentiated cytological aspect not usually associated with an intensely proliferating cell.     

THE ROOT APICAL MERISTEM OF OSMUNDA REGALIS

Roots of the Osmundaceae differ from most ferns in having more than one apical cell. The size of the apical initial group, which includes cells that are considered to be apical cells, varies directly with root diameter in Osmunda regalis L. Mitotic indices were 6.63% for apical cells, 7.45% for the entire apical group, 6.25% for distal derivatives, and 7.15% for developing cortical cells. Cytophotometric measurements of Fuelgen-stained nuclei indicated no endopolyploidy in the populations of cells studied. These findings demonstrate that there is no quiescent center in the roots of O. regalis.     

IN VITRO DEVELOPMENT OF THE ISOLATED SHOOT APICAL MERISTEM OF ANGIOSPERMS

Development of complete plants was achieved from isolated shoot apical meristems of Nicotiana tabacum L., Daucus carota L., Nicotiana glauca Grah., Tropaeolum majus L., and Coleus blumei Benth. The explants consisted of only meristematic dome tissue with no visible leaf primordia. A simple nutrient medium composed of the Murashige and Skoog salt mixture, 100 mg/liter myo-inositol, 0.4 mg/liter thiamin-HCl, 1-2 mg/liter IAA, 30 g/liter sucrose, and 1% agar was adequate. Histologically there occurred principally tissue enlargement during the first 3-6 days, followed by appearance of bipolar organization in 6-9 days and formation of a well-defined root apex and initiation of first leaf primordium by 12 days.     

CELL DIVISION KINETICS IN THE SHOOT APICAL MERISTEM OF CERATOPTERIS THALICTROIDES BRONG. WITH SPECIAL REFERENCE TO THE APICAL CELL

The duration of mitosis and the cell cycle were determined for defined cell populations of the shoot apical meristem of Ceratopteris thalictroides Brong. by using the colchicine-induced metaphase accumulation technique. The results indicate that the apical cell is mitotically active and cycles at an apparently greater frequency than the cells of subjacent populations. Duration of mitosis was similar for all cells of the meristem. These results are correlated with mitotic indices of control apices, the geometry of the apex, and the mean number of cells in the meristem. Shoot apices from adult plants were examined to determine mitotic indices within the meristem; mitotic activity was again noted for the apical cell. These results contradict recent proposals that the pteridophyte apical cell serves as a unicellular quiescent center which lacks histogenic potential and offer experimental support for the classical concept of apical cell function in those fern shoot meristems which terminate in a single apical cell.     

THE PICEA ABIES SHOOT APICAL MERISTEM IN CULTURE. II. DEPOSITION OF POLYSACCHARIDES AND LIGNIN-LIKE SUBSTANCES BENEATH CULTURES

Excised shoot apical meristems of Picea abies seedlings grow and develop primordial leaves when cultured on Millipore (mixed esters of cellulose) filter membranes lying on a simple, defined medium gelled with agarose. When the cultures are removed from the membranes, each leaves a spot of altered light transmission, spectral characteristics, hygroscopicity, and chemical reactivity. These spots are the manifestation of deposition in the membrane pore space of polysaccharides, lignin-like components, and probably other substances. Deposition of water-insoluble, Schiff's reagent-positive substances can be detected in the filter membranes after only 3–6 hr exposure to a meristem and continues for 10–15 days or longer. Precursors of the insoluble deposition materials can diffuse through at least nine layers of Millipore membrane before deposition at a site remote from living cells. Placement of a dialysis membrane between the meristem and the Millipore membrane prevents detectable deposition in the latter. The observations are consistent with the hypothesis that apical meristems can synthesize and export mobile precursors of cell wall components as well as any substances necessary to promote their condensation or polymerization into insoluble materials at remote sites. The system may be useful in studying synthesis of cell wall components and investigating the functional role of growth regulators in shoot apical development.     

A HISTOCHEMICAL STUDY OF THE SEEDLING SHOOT APICAL MERISTEM OF PINUS LAMBERTIANA

Vernalized seeds of Pinus lambertiana were scarified and planted in perlite. At 5, 8, 10, 13 and 16 days after planting, seedlings were selected for morphological examination and histochemical study. The shoot apical meristem consisted of a relatively homogeneous population of cells at 5 days. Cytohistological zonation was observed in the meristem by the eighth day and needle primordia initiation began at this time. Acid phosphatase (AP) activity was high in the extreme tip of the apex at 5 days. At 8 days AP activity was intense in the peripheral zone but weak in the apical initial and central mother cell zones. The apical meristem of the 10–16-day-old seedlings exhibited high AP activity in the peripheral zone only during the early stages of needle primordia initiation. The distribution of cytoplasmic and nuclear protein-bound SH was correlated with cytohistological zonation. Protein-bound SH was distributed relatively uniformly at 5 days, but by the eighth day the 4 cytohistological zones contained differential quantities. Succinic dehydrogenase (SD) activity was observed throughout the apex at 5 days, but by the eighth day the apical initial and central mother cell zones exhibited differentially greater levels of SD activity. Irradiation with 500 R of X-rays at 7 days after planting completely inhibited needle primordia initiation and disrupted the cytohistological zonation of the apex. Correlated with the inhibition of needle primordia initiation was the loss of SD activity in the apical initial and central mother cell zones. Irradiation also resulted in the gradual loss of protein-bound SH from the cytoplasm of the apical initial, central mother cell and peripheral zone.     

MITOTIC ACTIVITY IN THE ROOT APICAL MERISTEM OF AZOLLA FILICULOIDES LAM., WITH SPECIAL REFERENCE TO THE APICAL CELL

The meristematic activity of the apical cell and its derivatives (merophytes) in the unbranched, determinate roots of Azolla filiculoides Lam. was investigated. The plane of division of the apical cell indicates that it is the initial of each merophyte. The division plane of each newly formed merophyte is strictly periclinal to the root surface and provides confirmation that the immediate derivatives of the apical cell cannot be the ultimate root initials. The frequency of cell division as determined by the mitotic index, and by the duration of the cell cycle as determined by the colchicine method, confirmed the meristematic activity of the apical cell. As roots increase in length, the duration of the cell cycle in the total meristem increases, with the apical cell possessing the longest cell cycle, whereas the immediate derivatives maintain approximately the same cycle duration as in shorter roots. In determinate Azolla roots, cell division appears to play a major role up to a certain root length, then increase in length is produced mainly by cell elongation.     

A MORPHOMETRY STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). II. THE MATURE,ZONATE SHOOT APICAL MERISTEM

The shoot apical meristems of adult Echinocereus engelmannii plants are zonate and have a tunica, central mother cells, a peripheral zone, and a pith-rib meristem. An ultrastructural, stereological study showed that each zone has its own distinct ultrastructure, but that the differences between the zones are quite small, both on a protoplasmic basis and on a cytoplasmic basis. Furthermore, the ultrastructure present in the adult apices differed only slightly from that which had been found in seedling apices, demonstrating a long-term stability of structure. The standard deviations found in the sample were small, indicating little variability from one plant to the next and suggesting that there are little or no cyclic changes during the plastochron or a 24-hr photoperiod. The ultrastructures found in the shoot apical meristems differed significantly and markedly from mature tissues of the same plants.     

QUANTITATIVE STUDIES OF THE ROOT APICAL MERISTEM OF EQUISETUM SCIRPOIDES

An investigation was made of the meristematic activity of the apical cell, its immediate derivatives (merophytes), and of other selected cell populations of the root of Equisetum scirpoides Michx. The plane of the first division of a derivative of the apical cell is radiallongitudinal, which provides evidence that merophytes immediately adjacent to the apical cell cannot be the ultimate root initials. The apical cell is as active mitotically in roots 20–40 mm long as it is in roots that are 0.25–1 mm in length. The mitotic activity of the apical cell and of other cell populations was determined from the mitotic index, and from determination of the durations of the cell cycle and of mitosis of the apical cell by using the colchicine method of metaphase accumulation. Microspectrophotometric measurements of DNA content indicated that there was no consistent increase in DNA (endopolyploidy) in the apical cell or in the other meristematic cells as roots increased in length. Conclusion: there is no evidence that the apical cell becomes quiescent or undergoes endopolyploidy as a root increases in length.     

香榧营养苗端细胞组织分区的超微结构观察

本文研究了榧树(Torreya grandis)成熟植株在季节生长中营养苗端的超微结构变化。各区域细胞的主要区别特征为:顶端原始细胞与亚顶端细胞相接的细胞壁较厚,液泡多分布于细胞游离面,质体中淀粉粒较小;亚顶端细胞壁较厚,液泡较大,质体中淀粉粒较大而多;周缘区细胞质体多不具淀粉粒,液泡也较小,胞间连丝丰富;肋状区细胞被大量的含淀粉质体及液泡占据了大部分空间,胞间连丝丰富。在季节变化的四个时期中,各区域细胞的亚显微结构特征亦不相同。休眠期各区细胞淀粉质体较发达,细胞壁较厚,液泡大;叶扩展期淀粉质体减少或消失;芽鳞形成期出现大量小液泡;新的顶芽形成期液泡增加,核糖体含量较高。讨论了各区域细胞核形态与其细胞活跃性的关系。     

THE APICAL MERISTEM OF SPLACHNIDIUM RUGOSUM (PHAEOPHYTA)1

The meristem of Splachnidium rugosum consists of a central apical cell surrounded by a region of actively dividing cells, many of which bear hairs. Conceptacle initials are scattered throughout the surface layer of the meristematic region. Conceptacle initials and apical hairs differentiate adjacent to the apical cell. The apical cell and the conceptacle initials are distinctive, pear-shaped cells possessing similar cytological features that are consistent with significant metabolic activity. They have a nucleus surrounded by dictyosomes, a stellate chloroplast, mitochondria, and numerous vesicles and physodes. When the apical cell is damaged as a result of experimental manipulation, growth ceases. It is inferred that the apical cell controls cell division in the meristematic region and also the differentiation of conceptacle initials and apical hairs. The apical meristems of Splachnidium and species of the Fucales have several important features in common, including the growth-regulatory role of the apical cell and the process of conceptacle initiation. The taxa may possibly have a common evolutionary origin. The problematic and unresolved taxonomic status of Splachnidium is discussed.     

AN ELECTRON MICROSCOPIC EVALUATION OF CYTOHISTOLOGICAL ZONATION IN THE SHOOT APICAL MERISTEM OF PINUS BANKSIANA

Shoot apical meristems of jack pine (Pinus banksiana) were examined by light and electron microscopy. Cytohistological zonation was evident when meristems were fixed in Craf IV, embedded in paraffin, and stained with Chlorazol Black E. When meristems were fixed for electron microscopy the cytoplasm of the apical initials and central mother cells each contained numerous lipid bodies and their nuclei contained little, if any, heterochromatin. The cytoplasm of the peripheral zone was rich in ribosomes. The nuclei of the peripheral zone and rib meristem were heterochromatic. Thus, the lack of heterochromatin in the nuclei and the dissolution of lipids in the cytoplasm of the apical initials and central mother cells appeared to contribute most to the organization and appearance (cytohistological zonation) of the shoot apex when standard histological techniques are used.     

THE ROLE OF LIGHT IN HISTOGENESIS AND DIFFERENTIATION IN THE SHOOT OF PISUM SATIVUM. I. THE APICAL REGION

Thomson , B. F., and P. M. Miller . (Connecticut Coll., New London.) The role of light in histogenesis and differentiation in the shoot of Pisum sativum. I. The apical region. Amer. Jour. Bot. 49(3): 303–310. Illus. 1962.—Seedlings of Pisum sativum grown under constant conditions and kept in total darkness or exposed daily to red or white light were harvested at the same plastochron age and examined histologically to determine what specific aspects of histogenesis and differentiation are affected by light. The tissue organization of the shoot apex is the same in all light conditions to a point below the 2 youngest leaf primordia. The first detectable difference is a slight thickening of the internode in light due to more and larger cells. The first effect on longitudinal growth appears below the fourth youngest primordium and consists of an increase of internode length in light-grown plants. This is associated with a greater distance between the apex and the first mature protoxylem. The distance from apex to the first pith, provascular strands, and protophloem and the distances between the 4 youngest leaf primordia are not affected by light.     

HORMONAL REQUIREMENTS OF EXCISED DIANTHUS CARYOPHYLLUS L. SHOOT APICAL MERISTEM IN VITRO

Whereas a medium containing kinetin alone enabled a few Dianthus caryophyllus L. apical meristem dome explants to develop into rooted plants, the highest frequency of plants was obtained in one containing supplements of both IAA and kinetin. In an unsupplemented medium, continued development required that explants have 2 pairs of primordial and a pair of expanding leaves. Kinetin alone caused production of many new leaves, but the development was significantly less than when it was furnished in combination with IAA. IAA given alone caused meristem explants to develop primarily callus, roots, and a few leaves. Gibberellin and abscisic acid were without promotive effects on leaf and shoot formation. A balance of hormonal substances, synthesized in young leaf structures and relocated to the meristem, is proposed as the fundamental mechanism that regulates new leaf initiation in the shoot apex.     

THE ROOT APICAL MERISTEM OF ASPLENIUM BULBIFERUM: STRUCTURE AND DEVELOPMENT

The root apical meristem of Asplenium bulbiferum Forst. f. has a prominent four-sided pyramidal cell with its base in contact with the rootcap. Derivatives (merophytes) that contribute to the main body of the root are produced from the three proximal faces of the apical cell. The rootcap has its origin from the fourth (distal) face of the apical cell. The first division in a proximal merophyte is periclinal to the root surface, separating an outer cell and an inner cell. The outer cell is the origin of the outer part of the cortex and the epidermis; the larger inner cell is the origin of the inner cortex, endodermis, pericycle, and vascular tissue. After the establishment of the basic number of cells in a unilayered merophyte, the cells undergo transverse divisions forming longitudinal files of cells. The mitotic index of the apical cell indicates that it is not a quiescent cell. Also, the first plane of division in a newly formed merophyte dictates that the apical cell is the originator of merophytes.