THE ONTOGENY OF CARBONIFEROUS ARTICULATES: THE APEX OF SPHENOPHYLLUM

Twig apices of Sphenophyllum lescurianum, S. constrictum, and two new Sphenophyllum taxa are described in transverse and longitudinal section from middle and upper Pennsylvanian age specimens. In all of the species the single apical cell has the shape of a tetrahedron, with a triangular upper surface and three internal cutting faces. Segment cells are produced from each of the cutting surfaces in a dextrorse or sinistrorse direction, depending upon the species. The central portion of each segment cell contributes to the initiation of the procambium, while the remaining outer portion undergoes a vertical and subsequent horizontal division to form segment cells. Segment cells are aligned in vertical tiers beneath the respective apical cell cutting faces, with the individual leaves positioned directly beneath a tier of segment cells. Leaf primordia are first observed as a series of surface undulations below the apex, with an intercalary meristem located directly beneath each primordium. The vegetative apical organization of Sphenophyllum is demonstrated to be very similar to the type of organization found at the stem tips of Catamites and Equisetum.     

DIFFERENTIATION OF LATERAL SHOOTS AS THORNS IN ULEX EUROPAEUS

Ulex europaeus is a much-branched shrub with small, narrow, spine-tipped leaves and axillary thorn shoots. The origin and development of axillary shoots was studied as a basis for understanding the changes that occur in the axillary shoot apex as it differentiates into a thorn. Axillary bud primordia are derived from detached portions of the apical meristem of the primary shoot. Bud primordia in the axils of juvenile leaves on seedlings develop as leafy shoots while those in the axils of adult leaves become thorns. A variable degree of vegetative development prior to thorn differentiation is exhibited among these secondary thorn shoots even on the same axis. Commonly the meristems of secondary axillary shoots initiate 3–9 bracteal leaves with tertiary axillary buds before differentiating as thorns. In other cases the meristems develop a greater number of leaves and tertiary buds as thorn differentiation is delayed. The initial stages in the differentiation of secondary shoot meristems as thorns are detected between plastochrons 10–20, depending on vigor of the parent shoot. A study of successive lateral buds on a shoot shows an abrupt conversion from vegetative development to thorn differentiation. The conversion involves the termination of meristematic activity of the apex and cessation of leaf initiation. Within the apex a vertical elongation of cells of the rib meristem initials and their immediate derivatives commences the attenuation of the apex which results in the pointed thorn. All cells of the apex elongate parallel to the axis and proceed to sclerify basipetally. Back of the apex some cortical cells in which cell division has persisted longer differentiate as chlorenchyma. Although no new leaves are initiated during the extension of the apex, provascular strands are present in the thorn tip. Fibrovascular bundles and bundles of cortical fibers not associated with vascular tissue differentiate in the thorn tip and are correlated in position with successive incipient leaves in the expected phyllotactic sequence, the more developed bundles being related to the first incipient leaves. Some secondary shoots displayed variable atypical patterns of meristem differentiation such as abrupt conversion of the apex resulting in sclerification with limited cell elongation and small, inhibited leaves. These observations raise questions concerning the nature of thorn induction and the commitment of meristems to thorns.     

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.     

Über den Verzweigungsvorgang beiPsilotum undSelaginella mit Anmerkungen zum Begriff der Dichotomie

After a critical evaluation of the concept of dichotomous branching in Cormophytes the shoot apical meristems ofPsilotum triquetrum andSelaginella speciosa are described. InPsilotum only the terminal meristems of the cryptophilic shoots have a three sided apical cell. Those of the photophilic shoots lack a typical apical cell.Selaginella has a two sided apical cell. The process of branching is independent from apical cells. It is due to an equal or unequal fractionation of the initial zone of the shoot apex which embraces all tissues above the leaf producing zone of the apical meristem.

Herrn Univ.-Prof. Dr.Walter Leinfellner zum 70. Geburtstag gewidmet.     

Formation and maintenance of the shoot apical meristem

Development in higher plants is characterized by the reiterative formation of lateral organs from the flanks of shoot apical meristems. Because organs are produced continuously throughout the life cycle, the shoot apical meristem must maintain a pluripotent stem cell population. These two tasks are accomplished within separate functional domains of the apical meristem. These functional domains develop gradually during embryogenesis. Subsequently, communication among cells within the shoot apical meristem and between the shoot apical meristem and the incipient lateral organs is needed to maintain the functional domains within the shoot apical meristem.     

Coordination of cell proliferation and cell fate decisions in the angiosperm shoot apical meristem.

A unique feature of flowering plants is their ability to produce organs continuously, for hundreds of years in some species, from actively growing tips called apical meristems. All plants possess at least one form of apical meristem, whose cells are functionally analogous to animal stem cells because they can generate specialized organs and tissues. The shoot apical meristem of angiosperm plants acts as a continuous source of pluripotent stem cells, whose descendents become incorporated into organ primordia and acquire different fates. Recent studies are unveiling some of the molecular pathways that specify stem cell fate in the center of the shoot apical meristem, that confer organ founder cell fate on the periphery, and that connect meristem patterning elements with events at the cellular level. The results are providing important insights into the mechanisms through which shoot apical meristems integrate cell fate decisions with cellular proliferation and global regulation of growth and development.     

Developmental anatomy of the shoot apical cell,rhizophore and root ofSelaginella uncinata

The developmental anatomy of the shoot apex, rhizophore and root ofSelaginella uncinata was examined by the semi-thin section method. The shoot apex has a single, lens-shaped apical cell with two cutting faces. Rhizophore primordia are initiated exogenously at the branching point of the second youngest lateral shoot. The rhizophore apex has a tetrahedral apical cell with three cutting faces. A pair of root primordia is initiated endogenously from inner cells of the rhizophore apex, after the rhizophore apical cell becomes unidentifiable losing its activity, and subsequently a root cap is formed from the distal face of the root apical cell. During the course of successive root branching the apical cell in an original root apical meristem becomes unidentifiable and then a new apical cell is initiated in each of the bifurcated root apical meristems. The root branching mode seems to be equivalent to the described dichotomous branching mode of fern shoots. Our results demonstrate a distinct morphogenetical difference between the rhizophore and the root, and confirm the exogenous origin of the rhizophore, as described for other species ofSelaginella. This evidence indicates that the rhizophore is not an aerial root but a leafless, root-producing axial organ.     

Aspects of Morphogenesis in a Dorsiventral Fern, Pteridium aquilinum (L.) Kuhn: With one Plate and ten Figures in the Text

The formation of the horizontal dorsiventral rhizome of Pteridiumaquilinum from the erect radial axis of the young sporelingis described. The shoot apex and the inception of leaves andbuds at the apical meristem have been investigated, and theirinception is shown to be essentially similar in long and shortshoots, later differences being due to differing rates of growthand internodal elongation in the two types of shoot.     

Comparative development of heavily asymmetric-cordate gametophytes of <Emphasis Type="Italic">Anemia phyllitidis</Emphasis> (Anemiaceae) focusing on meristem behavior

Development of heavily asymmetric cordate gametophytes of Anemia phyllitidis (Anemiaceae), one of the schizaeoid ferns, was examined using a sequential observation technique; epi-illuminated light micrographs of the same growing gametophytes were taken approximately every 24 h. The apical cell-like wedge-shaped cell was produced once from the terminal cell of a germ filament, but it stopped dividing soon after production of one or two derivative cells. Without a functional apical cell, the gametophyte developed by intercalary growth until the early stage of wing formation, and then the multicellular (pluricellular) meristem arose from the lower lateral side of the gametophyte. This was in sharp contrast to the observation that the multicellular meristem forms in place of the apical cell in typical cordate gametophytes. Loss of the functional apical cell probably caused a site-shift in the multicellular meristem of the Anemia phyllitidis gametophyte during evolution from apical to lateral. The results suggest that apical cell-based and multicellular meristems are primarily independent of each other. The multicellular meristem produced cells equally in the distal and proximal directions to form wings in both directions but proximally produced cells divided much less frequently. As a result, a heavily asymmetric gametophyte was formed.     

Anatomical analysis of growth and developmental patterns in the internode of deepwater rice

Submergence of the stem induces rapid internodal elongation in deepwater rice (Oryza sativa L. cv. Habiganj Aman II). A comparative anatomical study of internodes isolated from airgrown and partially submerged rice plants was undertaken to localize and characterize regions of growth and differentiation in rice stems. Longitudinal sections were examined by light and scanning-electron microscopy. Based on cell-size analysis, three zones of internodal development were recognized: a zone of cell division and elongation at the base of the internode, designated the intercalary meristem (IM); a zone of cell elongation without concomitant cell division; and a zone of cell differentiation where neither cell division nor elongation occur. The primary effects of submergence on internodal development were a threefold increase in the number of cells per cell file resulting from a decrease in the cell-cycle time from 24 to 7 h within the IM; an expansion of the cell-elongation zone from 5 to 15 mm leading to a threefold greater final cell length; and a suppression of tissue differentiation as indicated by reduced chlorophyll content and a lack of secondary wall formation in xylem and cortical sclerenchyma. These data indicate that growth of deepwater-rice internoes involves a balance between elongation and differentiation of the stem. Submergence shifts this balance in favor of growth.Abbreviations GA gibberellin - IM intercalary meristem     

Morphogenesis in Selaginella. III. Meristem determination and cell differentiation.

Selaginella willdenovii Baker is a prostrate vascular cryptogam with a dorsiventral stem. At each major branching of the stem tip a dorsal and a ventral angle meristem are formed. The ventral meristem becomes determined as a root and the dorsal meristem as a shoot. Indoleacetic acid (IAA) is transported basipetally in the stem and has been found to be the regulatory agent for meristem determination both in vitro and in vivo.Growth measurements of intact plants indicated that the sequence of development for each stem unit is frond expansion, internodal elongation, ventral meristem growth as a root, and dorsal meristem growth as a shoot. The principal experimental findings of this study are as follows. Triiodobenzoic acid (TIBA), an inhibitor of auxin transport alters the normal pattern of development in intact plants, causing ventral meristems to develop as shoots and dorsal meristems to develop precociously. Dorsal meristems grown in sterile culture on an auxin-free medium develop as shoots, but in the presence of IAA develop as roots. Meristems transferred after excision from auxin-free to plus-auxin medium on successive days showed an increasing tendency to develop as shoots, with more than 50% doing so by day 5. The mitotic index is low at the time of excision of the meristem, rises to a peak on day 5 and then declines.     

ORGANOGRAPHY,BRANCHING, AND THE PROBLEM OF LEAF VERSUS BUD DIFFERENTIATION IN THE VINING EPIPHYTIC FERN GENUS MICROGRAMMA

Investigation of the development and organography of the shoot systems of Microgramma vacciniifolia and M. squamulosa was undertaken for the purpose of determining: (1) the features of shoot growth that are responsible for the distinctive vining character of these epiphytic ferns; and (2) the mode of origin of branches and their contrast with leaf initiation. Shoots of both species are dorsiventral and plagiotropic (i.e., parallel to the substrate) in habit. Since the shoot apical meristem is radial in transectional symmetry, shoot dorsiventrality in Microgramma is a postgenital or secondary developmental event, and its inception is related to the initiation of lateral appendages. Leaves and buds arise in a distichous phyllotaxis and occupy opposite and alternating positions on the dorsal surfaces and flanks of the rhizome. Endogenous roots are initiated in two rows from the ventral surface of the stem, in the vicinity of the rhizome meristem; however, they do not emerge from the rhizome until some distance behind the tip and do not elongate until the region of substrate contact. We conclude that the vining nature of this fern rhizome is a result of precocious internodal elongation and the concomitant delay of leaf and bud expansion in the region of stem elongation. In addition, observation of branch origin confirms previous suggestions that branching in Microgramma is strictly lateral and extra-axillary and not a dichotomous derivative as proposed by some workers. Leaf and bud primordia differ not only in the nature of their respective vascular supplies but also in their actual course of initiation. In the case of the leaf, the primordium is precociously emergent and exhibits a lenticular apical cell at its summit when it is only one plastochron removed from the flanks of the apical meristem. By contrast, initials of the bud primordium divide less actively and remain in a sunken position for at least 5–6 plastochrons; only when the bud apex becomes expanded and emergent does a tetrahedral apical cell become recognizable at the tip of the bud promeristem. Because of the distinctive pattern of branch and leaf origin, as well as the lack of adventitious and phyllogenous origin of branch primordia, we suggest that the shoot of Microgramma is a useful test organism for the re-examination of the problem of leaf and bud determination in the ferns.     

COMPARISON OF SHOOT APEX DYNAMICS AND EARLY LEAF ONTOGENY IN TWO SPECIES OF SARACA (LEGUMINOSAE)

Shoot apices of Saraca indica produce adult leaves that have 4 to 6 pairs of leaflets, whereas those of S. bijuga usually have only 2 pairs. In both species one leaflet pair is found during the juvenile phase. Juvenility lasts many plastochrons in S. bijuga but is restricted to a few in S. indica. The shoot apical meristems of these two taxa are similar in structure, cell number, and cell size; however, the shoot apex of Saraca bijuga is slightly more stratified, having 2–3 tunica layers as opposed to 1–2 in S. indica. For most of the plastochron the apical meristem in both species is situated laterally at the base of the most recently formed leaf. A newly forming primordium and its internode shift the apical meristem upward unilaterally; the meristem passes through a brief apical dome stage and becomes positioned 180° from its origin at the beginning of the plastochron. Hence, there is a true pendulum meristem in both species. In S. bijuga the maximum length of the youngest leaf primordium, just prior to the formation of its successor, is twice that of S. indica. The internodes immediately below the shoot apex and the axillary buds develop more rapidly in S. bijuga than in S. indica. It is suggested that the bijugate leaf of S. bijuga represents a case of neoteny in plants.     

EARLY HISTOGENESIS AND SEMIQUANTITATIVE HISTOCHEMISTRY OF LEAF INITIATION IN TRITICUM AESTIVUM

The shoot apex of Triticum aestivum cv. Ramona 50 was investigated histologically to describe cell lineages and events during leaf initiation. During histogenesis three periclinal divisions occurred in the first apical layer, with one or two divisions in the second apical layer. This sequence of cell divisions initially occurred in one region and spread laterally in both directions to encircle the meristem. Cells of the third apical layer were not involved in leaf histogenesis. Initially, young leaf primordia were produced from daughter cells of periclinal divisions in the two outer apical layers. Nuclear contents of protein, histone, and RNA in the shoot apex were evaluated as ratios to DNA by means of semiquantitative histochemistry. Daughter cells of periclinal divisions in the outer apical layer which produced the leaf primordia had higher histone/DNA ratios than cells of the remaining meristem. However, protein/DNA and RNA/DNA ratios were similar in both regions. Leaf initial cells had a higher ³H-thymidine labeling index, a higher RNA synthesis rate, and smaller nuclear volumes than cells of the residual apical meristem.     

Internodal elongation and orientation of cellulose microfibrils and microtubules in deepwater rice

Excised stem sections of deepwater rice (Oryza sativa L.) containing the highest internode were used to study the induction of rapid internodal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and control stem sections, the basal 5-mm region of the highest internode grows at the fastest rate. During 24 h of GA treatment, the internodal elongation zone expands from 15 to 35 mm. Gibberellin does not promote elongation of internodes from which the intercalary meristem has been excised. The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The orientation of CMFs in parenchymal cells of GA-treated and control internodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation were observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epidermal cell walls are transversely oriented in both GA-treated and control stem sections. In slowly growing control internodes, CMF orientation changes to the oblique as cells are displaced from this basal 5-mm zone to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more gradual and extends over the 35-mm length of the elongation zone. The CMFs of older epidermal cells are obliquely oriented in control and GA-treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cortical microtubules determine the direction of CMF deposition. We conclude that GA acts on cells that have transversely oriented CMFs but does not promote growth of cells whose CMFs are already obliquely oriented at the start of GA treatment.     

THE SHELL ZONE: ITS DIFFERENTIATION AND PROBABLE FUNCTION IN SOME DICOTYLEDONS

Thirty-five species belonging to various dicotyledonous families were investigated to study the origin, development, and probable function of the shell zone, which is defined as an arcuate zone of cambiform cells delimiting the early axillary bud meristem. It is present in the majority of the investigated plants and five intergrading patterns of origin are described: (i) from the parenchymatized derivatives of the cells of the peripheral meristem of the shoot apex, adaxial to the bud meristem, (ii) from the peripheral meristem of the shoot apex along with the initiation of the early bud meristem, (iii) from the adaxial cells of the bud meristem, (iv) from the derivatives of the cells of the bud meristem at its base, and (v) partly from the parenchymatized cells of the peripheral meristem adaxial to the bud and partly from the adaxial derivatives of the bud meristem. The shell zone loses its identity at different stages of bud development in various species. Its cells ultimately contribute to the ground meristem, procambium, and pith cells of the axis. In Cuminum cyminum and lpomoea cairica the shell zone contributes in bringing about the axillary position of the bud from its early lateral position. In Solarium melongena, derivatives of the shell zone initiate the internodal elongation between the flower or inflorescence and the shoot apex, ultimately shifting the bud to an extra-axillary position on the internode.     

Effect of mutations in the PINHEAD gene of Arabidopsis on the formation of shoot apical meristems

The primary shoot apical meristem of angiosperm plants is formed during embryogenesis. Lateral shoot apical meristems arise postembryonically in the axils of leaves. Recessive mutations at the PINHEAD locus of Arabidopsis interfere with the ability of both the primary shoot apical meristem as well as lateral shoot apical meristems to form. However, adventitious shoot apical meristems can form in pinhead mutant seedlings from the axils of the cotyledons and also from cultred root explants. In this report, the phenotype of pinhead mutants is described, and a hypothesis for the role of the wild-type PINHEAD gene product in shoot meristem initiation is presented. © 1995 Wiley-Liss, Inc.     

REGIONS OF CELL DIVISION AND ELONGATION DURING STEM GROWTH OF XANTHIUM

Regions of cell division and cell elongation were established during stem growth of Xanthium pensylvanicum (cocklebur). From percent mitosis it was determined that the region of cell division in a Xanthium stem of Plastochron Index (PI) 13.89 was 20 mm long, starting from the stem apex and proceeding in a basipetal direction. Measurements of cortical cell lengths demonstrated that mature cell length in the stem of the same plant was reached at about 55 mm distance from the stem apex. Between 20 and 55 mm distance from the apex, shoot growth occurred by cell elongation alone. The maximum rate of change in cell length (dC/dX) was at about 15 mm distance from the stem apex. The length of the apical growth region was a function of the age of the plants. The older the plant, the longer its apical region of elongation. Growth of Xanthium stems was due primarily to elongation of internodes; the nodal regions did not seem to elongate.     

ONTOGENY OF THE INFLORESCENCE OF SAURURUS CERNUUS (SAURURACEAE)

The spicate inflorescence of Saururus cernuus L. (Saururaceae) results from the activity of an inflorescence apical meristem which produces 200–300 primordia in acropetal succession. The inflorescence apex arises by conversion of the terminal vegetative apex. During transition the apical meristem increases greatly in height and width and changes its cellular configuration from one of tunica-corpus to one of mantle (with two tunica layers) and core. Primordia are initiated by periclinal divisions in the subsurface layer. These are “common” primordia, each of which subsequently divides to produce a floral apex above and a bract primordium below. The bract later elongates so that the flower appears borne on the bract. All common primordia are formed by the time the inflorescence is about 4.4 mm long; the apical meristem ceases activity at this stage. As cessation approaches, cell divisions become rare in the apical meristem, and height and width of the meristem above the primordia diminish, as primordia continue to be initiated on the flanks. Cell differentiation proceeds acropetally into the apical meristem and reaches the summital tunica layers last of all. Solitary bracts are initiated just before apical cessation, but no imperfect or ebracteate flowers are produced in Saururus. The final event of meristem activity is hair formation by individual cells of the tunica at the summit, a feature not previously reported for apical meristems.     

APPLICATION OF THE 22.5 MEV DEUTERON MICROBEAM TO THE STUDY OF MORPHOGENETIC PROBLEMS WITHIN THE SHOOT APEX OF OSMUNDA CLAYTONIANA

Excised shoot apices of Osmunda claytoniana were grown under controlled sterile conditions. Histological examination of the normal shoot apex shows that it is comprised of: (1) a promeristem, which possesses 1 or more apical initiating cells at its center; (2) a prestelar tissue consisting of an incipient vascular tissue which flanks the pith-mother-cell zone; the pith-mother-cell zone gives rise to the pith rib meristem and subsequently to the fundamental parenchyma of the pith; (3) the fundamental parenchyma of the cortex and the fundamental parenchyma of the dermal system both arising from flank cells of the promeristem. Apical initial cells of meristems irradiated with a 127,000 rad acute exposure of a deuteron beam having a diameter of 25μ, histologically examined at 7-day intervals for a 12-week period, as early as 3 weeks’ postirradiation, showed the apical initiating cell(s) together with certain of the cells of the pith-mother-cell zone to be destroyed. A wound response develops peripherally to the destroyed initials. In addition, an isolated, organized growth center is observed to develop from normal promeristem cells. Incipient vascular tissue and a new pith-mother-cell zone are also observed to develop in association with the new center of growth. Implications of the role of the interrelationships between apical initiating cell(s) and other cells of the meristem and the role they may play in maintenance of meristematic integrity within the shoot meristem are discussed.