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.     

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     

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     

Leaf Ontogeny in Digitaria eriantha

The shoot apex consists of two layers, the dermatogen and thehypodermis. The leaf primordia arise through periclinal divisionswithin these two layers on the side of the apex. Further divisionsof the dermatogen push the little protuberance upward and togetherwith divisions the hypodermis add internal tissues of the youngleaf. When the median and lateral bundles of the primordia arisein Digitaria eriantha they are isolated from the vascular supplyof the rest of the plant. The median strand, the first to form,and the first order laterals form at the disc of insertion ofthe primordium. The other laterals form higher up in the primordium.These strands extend both acropetally and basipetally to linkwith the vascular supply of the rest of the plant. Digitaria eriantha, apical meristem, leaf primordium, vascular bundle, orange G, tannic acid, iron alum     

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.     

Changes in Volume and Cell Number in the Different Regions of the Shoot Apex of Pisum during a Single Plastochron

The length of the ninth plastochron in shoot apices of Pisumsativum was measured and found to be almost 46 h. This singleplastochron was divided into 11 morphologically recognizablestages and the time taken to reach each stage was measured.The cell number and cell volume of five regions of the apexwas measured at each stage of the plastochron. Although theapex as a whole grew exponentially, growth during the first30 h of the plastochron was predominantly in the primordiumand the adjacent tissues, whereas in the last 16 h growth wasmainly in the apical dome. Since the mean cell volume remainedconstant, different rates of growth were due to different ratesof cell division. The data suggested that the apex probablygrows by the formation of growth centres on alternate sidesof the apex, the beginning of each new growth centre being apparentas an increased rate of growth in the apical dome 16 h beforethe beginning of the next plastochron. The inception of a newprimordium may therefore precede its appearance as a bump byabout 16 h, and precede the first periclinal division in thetunica by 26 h. A central zone of larger cells with lightly-stainingnuclei was found at the extreme apex. This central zone becamereduced in size or disappeared at the time at which a new primordiumwas about to become visible.     

Floral transition as a sequence of growth changes in different components of the shoot apical meristem ofChenopodium rubrum

The changes in cell division rate were studied in different components of the shoot apex ofChenopodium rubrum during short-day photoperiodic induction and after the inductive treatments. Induced and vegetative apices were compared. Accumulation of metaphases by colchicine treatment was used to compare the mean cell cycle duration in different components of the apex. A direct method of evaluating the increase in cell number obtained by anticlinal or periclinal divisions was applied if the corresponding components of induced and non-induced apices had to be compared. The short-day treatment prolonged the cell cycle more in the peripheral zone than in the central zone and still more in the leaf primordia. The importance of changing growth relations for floral transition was shown particularly if the induced plants were compared with the vegetative control with interrupted dark periods. Induced plants transferred to continuous light showed further changes in the rates of cell division. The cell cycle was shortened more in the central zone than in the peripheral zone,i.e. there was a further shift in growth relations within the apical dome. The cell cycle in the leaf and bud primordia was also shortened if compared with the vegetative control, the acceleration being stronger in the bud primordia. There was a subsequent retardation in cell division in the leaf primordia formed during and after the inductive treatment if the plants were fully induced. An inhibition of the oldest bud primordia was observed in fully induced apices, as well.     

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     

LEAF AND APICAL GROWTH CHARACTERISTICS IN TRITICUM

Leaf initiation rate, leaf primordium growth rates, and apical volume growth rates were determined for seedlings of Triticum aestivum cv. Ramona 50 under controlled environmental conditions. Three leaf primordia are present in the caryopsis, and three more leaves are initiated within the first two weeks after germination with a mean plastochron length of 95.5 hr. Volume growth rates of the apical region were determined on six apices which had six primordia each. The mean radial expansion rate was 0.467/plastochron, and the vertical expansion rate was 0.457/plastochron. The volume expansion rate was 1.393/plastochron. The mean volume doubling time was 0.498 plastochrons or 47.1 hr.     

In vitro Growth of Vegetative Tomato Shoot Apices

Explants from the shoot apex of the tomato, comprising the apicaldome and youngest primordium together with small amounts ofsub-apical tissue were cultured for periods of 1 to 4 plastochrons.By the use of a simple parameter, the axillary distance, thegrowth-rate could be accurately monitored throughout each plastochron. Gibberellic acid, coconut milk, and kinetin, in addition tosucrose and inorganic salts, all promoted growth of the apex;a combination of gibberellic acid and coconut milk gave thefastest growth. Temperature had a large effect on the growth-ratewith an in vitro Q₁₀ of 2.1 contrasted with an in vivo Q₁₀ of1.2 over the range of 15 to 25 ?C. On gibberellic acid and coconutmilk at 15 ?C two-thirds of the in vivo growth--rate was sustainedin culture for two plastochrons after which the growth-rategradually declined; at 20 and 25 ?C growth-rates slightly higherthan in vivo rates were sustained for 1 plastochron before amore rapid decline. The anatomy of these in vitro apices wasnormal for 1? plastochrons after which there were small increasesin cell volume in the developing primordium. Reducing the amount of sub-apical tissue drastically reducedthe growth rate but had little effect on the responses to gibberellicacid and coconut milk. Explants are considered to be useful material for studying thechanges that take place in the apex during the course of 1 or2 plastochrons, but inadequate on the media tested for experimentsinvolving longer periods of growth. Explants also provide asensitive assay system for the effects of growth factors onthe rate of shoot apical growth.     

Identification of Shoot Apical Meristem Cells Committed to Form Vascular Elements in Pisum sativum L. and Vicia faba L.

A cytochemical study of naphthol AS-D esterases in vegetativeshoot apices of Pisum sativum and Vicia faba L. has shown thepresence of carboxyl esterases (E.C. 3.1.1.1 [EC] .) in those meristemcells already committed to form vascular elements. These cellsform a sequence linking the morphologically identifiable procambiumto the cells of the tunica layers at a site either already identifiableas the next primordium or which will form the next primordium.The implications of this result are briefly discussed in relationto the control of primordia formation and procambial cell development. Pisum sativum, Vicia faba, determination, vascular tissue, shoot apex, cytochemistry     

Independence of Organogenesis and Cell Pattern in Developing Angle Shoots of Selaginella martensii

Angle meristems are mounds of meristematic tissue located atdorsal and/or ventral branch points of the dichotomising stemaxes of many species of Selaginella (Lycophyta). The presentstudy examined the development of ventral angle shoots of S.martensii in response to removal of distal shoot apices (decapitation).Scanning electron microscopy of sequential replicas of developingangle meristems and angle shoots revealed that for the firsttwo pseudowhorls of leaf primordia, particular leaves are notattributable to particular merophytes of the angle meristemapical cell. Individual leaf primordia of the first (outer)pseudowhorl often form from more than one merophyte. Neitherthe shape of the angle meristem apical cell nor the directionof segmentation has any effect on the development of the angleshoot. Additionally, the apical cell of the angle meristem doesnot necessarily contribute directly to either of the new shootapices of the developing angle shoot. The first bifurcationof the angle shoot shows a remarkably consistent relationshipto the branching pattern of the parent shoot. The strong branchof the first angle shoot bifurcation typically occurs towardthe weak side branch of the parent shoot. Anatomical studiesshowed that bifurcation of the young angle shoot involved theformation of two new growth centres some distance away fromthe original angle meristem apical cell; new apical cells subsequentlyformed within these. These results provide additional supportfor the view that cell lineage has little or no effect on finalform or structure in plants.Copyright 1994, 1999 Academic Press Selaginella martensii Spring, Lycophyta, angle meristem, apical cell, shoot apical meristem, leaf primordium, branching, dichotomy, morphogenesis, determination, competence, development, mould and cast technique, replica technique, scanning electron microscopy     

LEAF DEVELOPMENT IN DOXANTHA UNGUIS-CATI (BIGNONIACEAE)

Leaf structure in Doxantha unguis-cati is polymorphic. The usual mature compound leaf is composed of two lanceolate leaflets and a terminal tripartite spine-tendril. Leaf primordia are initiated simultaneously in pairs on opposite flanks of the shoot apical meristem by periclinal cell divisions in the third subsurface layer of the peripheral flank meristem. Two leaflet primordia are the first lateral appendages of the compound leaf. Initiation of these leaflet primordia occurs on the adaxial side of a compound leaf primordium 63–70 μm long. Lamina formation is initiated at the base of a leaflet primordium 70–90 μm long and continues acropetally. Mesophyll differentiation occurs in later stages of development of leaflets. The second pair of lateral appendages of the leaf primordium differentiate as prongs of the tendril. Initiation of the second pair of lateral appendages occurs on the adaxial side of a primordium approximately 168 μm long. Acropetal procambialization and vacuolation of cells extend to the apex of tendrils about 112 μm long, restricting the tendril meristem to the adaxial side of the primordium and resulting in curvature of the tendril. The tendril meristem is gradually limited to a more basipetal position as elongation of apical cells continues. Initiatory divisions and early ontogenetic stages of leaflets and tendrils are similar. Their ontogeny differs when the lateral primordia are approximately 70 μm long. Marginal and submarginal initials differentiate within leaflets but not in tendrils. Apical growth of tendrils ceases very early in ontogeny as compared with leaflets.     

Growth changes in the shoot apex of Sinapis alba during transition to flowering

The aim of the work was to report morphological changes whichoccur in the shoot apex during the morphogenetic switch to floweringin the model long day (LD) plant, Sinapis alba. During the floraltransition induced by 1 LD the growth rate of all componentsof the shoot apex is modified profoundly. The earliest changes,detected at 24 h after start of LD, include a decrease in plastochronduration and an increase of growth of leaf primordia. One daylater, the meristem dome starts to increase in volume, apicalinternodes have an increased height and there is a precociousoutgrowth of axillary meristems. All these changes precede initiationof flower primordia, which starts at about 60 h after the startof LD. Later changes include meristem doming, a decrease inthe plastochron ratio and a shift to a more complex phyllotaxis.All the changes, except the decreased plastochron ratio, arecharacteristics of an apex with an increased tempo of growth.The stimulation of longitudinal growth (height of apical intemodes)is more marked and occurs earlier than the reduction of radialgrowth (plastochron ratio). Key words: Axillary meristem, internode growth, leaf growth, plastochron ratio, plastochron duration     

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.     

Development of Lateral Root Primordia in Vicia faba, Pisum sativum, Zea mays andPhaseolus vulgaris: Rates of Primordium Formation and Cell Doubling Times

Lateral root primordium development has been examined in primaryroots of Vicia faba L., Pisum sativum L., Zea mays L. and Phaseolusvulgaris L. Following their initiation from an estimated minimumnumber of 77–162, 20–57, 17 and 12 cells respectivelyin Vicia, Phaseolus, Pisum and Zea, the primordia rapidly increasedin cell number to emerge as secondary roots about 2.8–3.6days later depending on the species being examined. Cell doublingtimes were estimated directly from cell numbers at differenttimes following primordium inception and were found to increasewith increase in primordium size in each of the species investigated. The number of primordia formed per cm of root growth per daywas greatest in Zea and least in Pisum. A comparison of thedata obtained for Vicia with that in the literature led to theconclusion that although the number of primordia produced percm of root growth was independent of the rate of primary elongation,the number produced per day increased in a linear fashion withincrease in the rate at which the primary lengthened. Vicia faba L, Pisum sativum L, Zea mays L, Phaseolus vulgaris L, broad bean, garden pea, maize, dwarf bean, root primordia, cell division, cell doubling time     

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.     

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.     

Origin of Nodes and Internodes in Plant Shoots. II. Models of Node and Internode Origin from One Layer of Cells

In the promeristem of some plant apices there are two layers'ofcells giving rise to the plant shoot. The first layer on thesurface forms the protoderm, and developing from it the epidermis;it is easily distinguished when there are phenolic compoundsexclusively in that layer, e.g. Crataegus, Vitis. The secondlayer may divide periclinally (a process here called differentiatingdivision) producing, successively, node and internode mothercell layers. Mitoses can start from the central part and continueradially or, more commonly, from leaf primordia and continuecentripetally. The number of periclinal divisions in that layerdetermines how many nodes and internodes the shoot will have.Differences in the number of cell divisions in these two motherlayers lead to the formation of long internodes and short nodes Nodes, internodes, idioblasts, phenolics, development, Crataegus, Vitis