A Sequential Study of Cell Divisions and Expansion Patterns on a Single Developing Shoot Apex of Vinca major

During the growth of a single developing vegetative apex ofVinca major, both the orientation and frequency of cell divisions,and the pattern of cell expansion, were observed using a non-destructivereplica technique. Micrographs taken at daily intervals illustratethat the central region of the apical dome remains relativelyinactive, except for a phase of cell division which occurs after2 d of growth. The majority of growth takes place at the proximalregions of the dome from which develop the successive pairsof leaves. The developing leaf primordia are initiated by aseries of divisions which occur at the periphery of the centraldome and are oriented parallel to the axis of the subsequentleaves. The cells which develop into the outer leaf surfaceof the new leaves undergo expansion and these cells divide allowingfor the formation of the new leaf. This paper describes thefirst high-resolution sequential study of cell patterns in asingle developing plant apex. Sequential development, cell division, expansion patterns, SEM, Vinca major, apical dome, leaf primordium, leaf initiation     

The Relationship Between the Distribution of Periclinal Cell Divisions in the Shoot Apex and Leaf Initiation

Periclinal cell divisions in vegetative shoot apices of Pisumand Silene were recorded from serial thin sections by mappingall the periclinal cell walls formed less than one cell cyclepreviously. The distribution of periclinal divisions in theapical domes corresponded to the distributions subsequentlyoccurring in the apices when the young leaf primordia were forming.In Pisum, periclinal divisions were almost entirely absent fromthe I₁ region of the apical dome for half a plastochron justafter the formation of a leaf primordium and appeared, simultaneouslyover the whole of the next potential leaf site, about half aplastochron before the primordium formed. In Silene periclinaldivisions seemed to always present in the apical dome at thepotential leaf sites and also round the sides of the dome wherethe ensheathing leaf bases were to form. Periclinal divisionstherefore anticipated the formation of leaf primordia by occuring,in Pisum about one cell cycle and in Silene two or more cellcycles, before the change in the direction of growth or deformationof the surface associated with primordial initiation. Pisum, Silene, planes of cell division, orientation of cell walls, leaf primordia, shoot apical meristem, plastochron     

Flower primordium formation at the Arabidopsis shoot apex: quantitative analysis of surface geometry and growth

Geometry changes, especially surface expansion, accompanying flower primordium formation are investigated at the reproductive shoot apex of Arabidopsis with the aid of a non-invasive replica method and a 3-D reconstruction algorithm. The observed changes are characteristic enough to differentiate the early development of flower primordium in Arabidopsis into distinct stages. Primordium formation starts from the fast and anisotropic growth at the periphery of the shoot apical meristem, with the maximum extension in the meridional direction. Surprisingly, the primordium first becomes a shallow crease, and it is only later that this shape changes into a bulge. The bulge is formed from the shallow crease due to slower and less anisotropic growth than at the onset of primordium formation. It is proposed that the shallow crease is the first axil, i.e. the axil of a putative rudimentary bract subtending the flower primordium proper, while the flower primordium proper is the bulge formed at the bottom of this axil. At the adaxial side of the bulge, the second axil (a narrow and deep crease) is formed setting the boundary between the flower primordium proper and the shoot apical meristem. Surface growth, leading to the formation of the second axil, is slow and anisotropic. This is similar to the previously described growth pattern at the boundary of the leaf primordium in Anagallis.     

Planes of Cell Division and Growth in the Shoot Apex of Pisum

The planes of cell division and growth were examined in thecourse of a single plastochron in the shoot apical meristemby observing the orientations of mitotic spindles. In the I₁region of the apical dome, cell divisions were at first anticlinalbut 30 h before a leaf primordium emerged at this site 20 percent of the cell divisions became periclinal. These periclinaldivisions were found only in the corpus. Periclinal divisionsin the tunica were coincident with the appearance of the primordiumas a bulge. The change in the direction of growth in I₁ at thesite of the incipient leaf primordium occurred without any changein the rate of growth in this region of the meristem.     

Flowering and apical meristem growth dynamics

The shoot apical meristem generates stem, leaves, and lateralshoot meristems during the entire shoot ontogeny. Vegetativeleaves are generated by the meristem in the vegetative developmentalphase, while in the reproductive phase either bracts subtendinglateral flower primordia (or paraclades), or perianth and strictlyreproductive organs are formed. Meristem growth is fully characterizedby the principal growth rates, directions, volumetric, and arealgrowth rates. Growth modelling or sequential in vivo methodsof meristem observation complemented by growth quantificationallow the above growth variables to be estimated. Indirectly,growth is assessed by cell division rates and other cell cycleparameters. Temporal and spatial changes of growth and geometrytake place at the meristem during the transition from the vegetativeto the reproductive phase. During the vegetative phase, meristemgrowth is generally indeterminate. In the reproductive phaseit is almost always determinate, but the extent of determinacydepends on the inflorescence architecture. In the vegetativephase the central meristem zone is the slowest growing region.The transition from the vegetative to the reproductive phaseis accompanied by an increase in mitotic activity in this zone.The more determinate is the meristem growth, the stronger isthis mitotic activation. However, regardless of the extent ofthe activation, in angiosperms the tunica/corpus structure ofthe meristem is preserved and therefore the mitotic activityof germ line cells remains relatively low. In the case of thethoroughly studied model angiosperm plant Arabidopsis thaliana,it is important to recognize that the flower primordium developsin the axil of a rudimentary bract. Another important featureof growth of the inflorescence shoot apical meristem is theheterogeneity of the peripheral zone. Finally, the role of mechanicalfactors in growth and functioning of the meristem needs furtherinvestigation. Key words: Flower primordium, geometry, growth, inflorescence, shoot apical meristem, transition from vegetative to reproductive phase Received 4 October 2007; Revised 5 November 2007 Accepted 6 November 2007     

Growth of the Surface and Inner Parts of the Pea Shoot Apical Meristem during Leaf Initiation

The rate of cell division and the rate of increase in cell numberwere compared in the epidermis and in the underlying cells ofthe apical dome, the incipient primordium, and the axis of thepea shoot apex. These rates did not coincide in any part ofthe apex, but in the primordium and the apical dome there wasa closer correspondence in the epidermis than in the underlyingcells. This is interpreted as showing that the changing shapeof the apex, during growth of the primordium and the apicaldome, is associated with a tendency to local changes in therate of growth in the epidermis but to a tendency to changesin the direction of growth in the underlying cells.     

Mechanical Stress in the Shoot Apices of Euphorbia, Lycopersicon, and Pisum under controlled Turgor

Cuts were made in the surface of the shoot apices of Euphorbialathyris, tomato (Lycopersicon esculentum), and Pea (Pisum sativum)while they were completely immersed in water or aqueous mannitolat various concentrations, or in near-saturated air. Gapingoccurred all over the apical dome of Euphorbia and on the tomatoapex at the site of emergence of the primordial bulge. Maximumgaping occurred in near-saturated air and under water, and wasprogressively reduced with increasing osmotica. It is concludedthat the gaping results from tension in the surface cells andis not caused by superficial drying out. No gaping occurred in the axil of the newly formed primordiumof the tomato nor anywhere in the apex of the pea. Histologicalevidence suggests that these tissues are under lateral compression. The mechanical stresses involved are discussed in relation tothe morphology of the apices together with existing data onthe distribution of cell division during primordia formation.     

Rates of Cell Division in the Shoot Apical Meristem of Pisum

The relative rates of cell division in different regions ofthe pea shoot apical meristem were obtained by measuring theincrease in the numbers of metaphases following applicationof colchicine to the plants. Absolute values for the rates ofcell division could be calculated since the average rate ofcell division for the whole apex was known. Measurements ofthe rates of cell division were obtained at defined intervalsduring the course of a single plastochron. Within each regionof the apex the rate of cell division did not change more thanabout two-fold throughout the plastochron. There was very littleor no increase in the rate of cell division associated withleaf initiation. The formation of a leaf primordium and thesubsequent growth of the apical dome apparently result fromchanges in the direction of growth rather than changes in therates of growth. Three main regions were discernible withinthe apical meristem: a region with a slow rate of cell divisionin the apical dome, a region of a faster rate of cell divisionat the base of the apical dome and at the site of initiationof procambial strands, and a region of an intermediate rateof cell division in the newly initiated leaf primordium andthe adjacent part of the shoot axis.     

Changes in Poparity of Growth During Leaf Initiation in the Pea, Pisum sativum L.

In the apical dome of the pea shoot apex the axis of growthof the epidermal cells becomes predominantly longitudinal inthe I₁ region one plastochron before a leaf is initiated, andthis orientation persists into the young primordium. In contrast,in the underlying, non-epidermal cells the growth axis in theI₁ region becomes randomized half a plastochron before leafinitiation, but in the young primordium it becomes the sameas in the epidermis. The initiation of a leaf primordium thereforetakes place without any major change in the orientation of growthaxes in the epidermis. A controlling role for the epidermisis therefore suggested. No marked reorientation of the growthaxis occurs on the sides of the newly initiated primordium.The shape of the young primordium can be related to the differentialrates of growth and division within it rather than to changesin growth orientation. Pisum sativum, pea, shoot apex, meristem, growth, epidermis, polarity     

A Model of Flowering in Chrysanthemum

A mathematical model of flowering in Chrysanthemum morifoliumRamat. is described which may be used to predict quantitiessuch as the number of primordia initiated by the apex, plastochronduration and apical dome mass before, during and after the transformationof the apical meristem from vegetative to reproductive development.The model assumes that primordial initiation is regulated byan inhibitor present in the apical dome. Within each plastochronthe apical dome grows exponentially, and the inhibitor concentrationdeclines through chemical decay and dilution. When the inhibitorconcentration falls to a critical level a new primordium isinitiated. There is instantaneous production of inhibitor, anda decrease in dome mass corresponding to the mass of the newprimordium. The process continues until the apical dome attainsa particular mass when the first bract primordium is produced.Subsequent primordia compete with the apical dome for substrates,and the specific growth rate of the dome declines with successiveplastochrons. Eventually, the net mass of the dome starts todecline until it is entirely consumed in the production of floralprimordia. Chrysanthemum morifoliumRamat, flowering, primordial initiation     

The Mode of Origin of a Leaf Primordium in the Shoot Apex of the Pea (Pisum sativum)

The movement of carbon-particle markers on the surface of acultured pea apex resembled that previously found for the tomatoapex. In the pea the primordium originated lower down on theside of the apical dome than in the tomato, and its generaldirection of growth was more upright. The results accord wellwith existing data on the rates and directions of cell divisionin the pea apex, and show that the primordium is formed by increasedcell division on the flank of the apex in a growth centre (orregion) analagous to that found in the tomato apex. Becauseof the distichous phyllotaxis of the pea it appears that inlongitudinal section two such growth centres at different stagesare visible, whereas in the tomato, which has spiral leaf arrangement,only one is apparent. It is concluded that, while a change indirection of division inevitably occurs in the primordium asit begins to bulge outwards away from the centre of the apex,its initiation can be traced to a local increase in the rateof division some 2 plastochrons before the bulge is well formed.     

Growth and morphogenesis at the vegetative shoot apex of Anagallis arvensis L

A non-destructive replica method and a 3-D reconstruction algorithm are used to analyse the geometry and expansion of the shoot apex surface. Surface expansion in the central zone of the apex is slow and nearly isotropic while surface expansion in the peripheral zone is more intense and more anisotropic. Within the peripheral zone, the expansion rate, expansion anisotropy, and the direction of maximal expansion vary according to the age of adjacent leaf primordia. For each plastochron, this pattern of expansion is rotated around the apex by the Fibonacci angle. Early leaf primordium development is divided into four stages: bulging, lateral expansion, separation, and bending. These stages differ in their geometry and expansion pattern. At the bulging stage, the site of primordium initiation shows an intensified expansion that is nearly isotropic. The following stages develop sharp meridional gradients of expansion rates and anisotropy. The adaxial primordium boundary inferred from the surface curvature is shifting until the separation stage, when a crease develops between the primordium and the apex dome. The cells forming the crease, i.e. the future leaf axil, expand along the axil and contract across it. Thus they are arrested in this unique position.     

Growth of the Shoot Apex of Agropyron repens (L.) Beauv. During Successive Plastochrons

Two kinds of size change occur in the apical dome of Agropyronrepens during development of the shoot. A cyclic increase anddecrease in size results from the production of a new stem segmentand associated leaf primordium during each plastochron. A progressiveincrease and then decrease in size, which occur over a periodof several plastochrons, is attributable to discrepancies betweenthe size increment during each plastochron and the size of thestem segment formed at the end of the plastochron. The volumedoubling time of the dome remains constant at approximatelyone plastochron. Fluctuations in mean cell generation time correlatewith changes in mean cell volume and do not contribute to thesize changes of the dome. Agropyron repens (L.), Beauv, couch grass, shoot apex, cell growth, cell divisions     

The Effects of Photoperiod and Mineral Nutrient Supply on Growth and Primordia Production at the Stem Apex of Barley Seedlings

Growth and development of the shoot apex in seedlings of threebarley cultivars was examined in two daylengths (8, 16 h) andat two mineral nutrient levels (x 1, x 0.1). Production of primordiawas greater at the higher nutrient level and in the longer days.The rate of production varied with cultivar but in all casesthe plastochron shortened with transition to spike formation.Early flowering (cv. Clipper) was associated with a high rateof primordial production and early transition to spike formation,late flowering (cv. Proctor) with a low rate of production anda longer vegetative phase. The cultivar Akka showed intermediatecharacteristics. The volume of the apical dome increased linearlywith increasing numbers of primordia, the rate of increase varyingwith cultivar and treatment. Enlargement of the dome was duemainly to increase in cell number. The transition of the apexto produce spikelet primordia occurred with widely differingvolumes of the apical dome, thus invalidating the hypothesisthat transition is dependent upon attainment of a critical domesize. Although both the rate of production of primordia andenlargement of the dome were markedly affected by photoperiod,both were unaffected when the photoperiodic treatment was givendirectly to the shoot apex. It is considered that the fate of a primordium once initiatedis determined by competition for available metabolites betweenit, other primordia and the apical dome. Hordeum vulgare L, barley, apical dome, primordia, plastochron, cell division     

Cell Cycling and the Fate of Potato Buds

The cell cycling characteristics of the regions of the apicalmeristems of underground shoots and buds of Solanum tuberosumL. were investigated by stathmokinesis and labelling. The apicaldomes of orthotropic shoots produce cells at twice the elementalrate of those of stolons, and their youngest leaf primordiaat twelve times the rate. Changing the fate of stolons so thatthey will become orthotropic by decapitating the tuber sproutsthat bear them results within 24 h in a general increase incell production especially in the leaf primordia. Axillary buds on tuber sprouts induced to become precursorsof orthotropic shoots instead of stolons undergo a spectacularincrease in cell division within 24 h in all regions, especiallyin the primordia and bud anlagen where the rate increases 20-foldor more. The summit is slower to react than other regions, but,by 24 h, its rate of cell division increases 11-fold and itis contributing 14 cells per day to the flanks from its 80 cells. In all the axillary buds the rate of mitosis in the summit ishalf that of the flanks of the apical dome, but, in both stolonsand orthotropic underground shoots, the rate is higher in thesummit than in the flanks or rib meristem. The results are discussed in relation to what is known of cellcycling changes after floral evocation. Solanum tuberosum L., potato, fate, cell division, apical meristem, stathmokinesis     

Development During Vegetative Growth in the Apical Region of the Shoot of Lupinus albus

This investigation was designed to show the nature of the developmentin terms of cell numbers and of rates of division that proceedsin the terminal region of the shoot during the vegetative phasein Lupinus albus. Data are assembled that show that with eachsuccessive plastochron in the dome the number of cells increasesand the rate of division decreases; in the first internode againthe number of cells increases and the rate of division decreases,and in the first primordium both the number of cells and therate of division decrease. The data also show that with thetransition from one plastochron to the next the rate of divisionin any particular intemnode or primordium decreases. It is asignificant feature that all the changes in the system can becharacterized through constant numerical factors. The significanceof the results is discussed.     

Initiation of axillary and floral meristems in Arabidopsis

Shoot development is reiterative: shoot apical meristems (SAMs) give rise to branches made of repeating leaf and stem units with new SAMs in turn formed in the axils of the leaves. Thus, new axes of growth are established on preexisting axes. Here we describe the formation of axillary meristems and floral meristems in Arabidopsis by monitoring the expression of the SHOOT MERISTEMLESS and AINTEGUMENTA genes. Expression of these genes is associated with SAMs and organ primordia, respectively. Four stages of axillary meristem development and previously undefined substages of floral meristem development are described. We find parallels between the development of axillary meristems and the development of floral meristems. Although Arabidopsis flowers develop in the apparent absence of a subtending leaf, the expression patterns of AINTEGUMENTA and SHOOT MERISTEMLESS RNAs during flower development suggest the presence of a highly reduced, "cryptic" leaf subtending the flower in Arabidopsis. We hypothesize that the STM-negative region that develops on the flanks of the inflorescence meristem is a bract primordium and that the floral meristem proper develops in the "axil" of this bract primordium. The bract primordium, although initially specified, becomes repressed in its growth.     

Development of Axillary and Leaf-opposed Buds in Rattan Palms

Axillary vegetative buds are present in Calamus, Ceratolobus,and Plectocomiopsis. Two species of Daemonorops Sect. Piptospathaalso have axillary vegetative buds. All species of Daemonoropshave only displaced adnate axillary inflorescence buds. A singlebud is initiated in the axil of the first or second leaf primordiumin a way similar to that for axillary inflorescence buds. Themeristem is displaced during development on to the internodeabove and sometimes on to the base of the leaf above. Leaf-opposedvegetative buds occur in five species of Daemonorops Sect. Cymbospathaand in one species of Daemonorops Sect. Piptospatha. This typeof bud is initiated 180° away from the axil of the firstor second leaf primordium. It is not a displaced axillary bud,but does become adnate to the internode above like the axillarybuds. One or more leaves, transitional between juvenile andadult, on a shoot often subtend both types of buds. Myrialepishas leaf-opposed vegetative buds, but their development wasnot observed. Korthalsia has buds that are displaced about 130°from the leaf axil and are intermediate between the axillaryand the leaf-opposed condition. Other forms of vegetative budsare described: multiple buds in Plectocomia, aerial forkingin Korthalsia, and suckering from inflorescences and from aerialstems in Calamus. bud development, rattan palms, palm taxonomy, branching     

ANATOMY AND ASPECTS OF DEVELOPMENT OF THE STAMINATE INFLORESCENCES AND FLORETS OF SEVEN SPECIES OF ALLOCASUARINA (CASUARINACEAE)

The morphology, ontogeny, and vascular anatomy of the staminate inflorescences and florets of seven species of Allocasuarina are described. The generally terminal but open-ended inflorescences occur on monoecious or staminate dioecious trees and consist of whorls of bracts, each subtending a sessile axillary floret. Each floret consists of one terminal stamen with a bilobed, tetrasporangiate anther enclosed typically by cuculliform appendages, commonly considered bracteoles, an inner median pair and an outer lateral pair. The mature stamen is exerted, the anther is basifixed and is extrorsely dehiscent. In early development of a male inflorescence very little internodal elongation occurs and enclosing cataphylls appear. The inflorescence apex is a low dome with a uniseriate tunica and a small group of central corpus cells. Bract primordia are initiated by periclinal divisions of C₁ followed by further divisions of the corpus and anticlinal divisions in the tunica. The bracts are epinastic and become gamophyllous except apically by cell divisions in both sides of each primordium. Stomata are restricted to the axis furrows and the abaxial tips of the bracts. The axillary florets arise in acropetal succession initiated by periclinal divisions in C₁ accompanied by anticlinal divisions in the tunica. The lateral floral appendages are also initiated by C₁ followed by anticlinal divisions in the tunica. They become adnate basally later with the subtending bract. The median sterile appendages are initiated in a manner similar to the initiation of the outer appendages. The stamen is initiated by divisions in the outer layers of the corpus and in the tunica, and then develops first by apical growth followed by intercalary growth. The vascular system of the inflorescence is identical to that of the vegetative stem. Each floret is supplied by a single bundle that has its source in a branch from each of the two traces supplying a bract. Six bundles arise from the floral bundle; four of these terminate in the base of the stamen and two form an amphicribal bundle that supplies the anther. Pollen is binucleate, 3- to 7-porate. The exine is tegillate.     

FLORAL ORGANOGENESIS IN FIVE GENERA OF THE MARANTACEAE AND IN CANNA (CANNACEAE)

The paired flowers of all species of the Marantaceae studied, except Monotagma plurispicatum, are produced through the division of an apical meristem with a tunica-corpus structure. The solitary flowers of M. plurispicatum develop from a similar meristem which does not bifurcate. The paired flowers of Canna indica are produced in the axil of a florescence bract through the formation of a bract and an axillary flower on the side of the primordium which gives rise to the largest flower of the pair. The sequence of organ initiation for both families is: calyx, corolla and inner androecial whorl, outer androecial whorl, gynoecium. The sequence of sepal formation is opposite in the two families. In the Cannaceae it leads directly into the spiral created by the formation of the other organs, while in the Marantaceae the sequence of sepal formation follows a spiral opposite to that of the other floral organs. The members of the corolla and inner androecial whorl separate from common primordia. In general these common primordia separate into a petal and an inner androecial member through the initiation of two growth centers, at the same level, in the dorsal and ventral flanks of the primordium. In Ischnosiphon elegans and Pleiostachya pruinosa the stamen is initiated at a lower position than the petal in the ventral flank of the common primordium. A similar pattern of initiation is described for the callose staminode in Marantochloa purpurea and Canna indica. This pattern is interpreted as a variation on the more generalized pattern of inner androecial formation found in the other genera.