Some Aspects of Ontogeny of the Shoot Apices of Solanum melongena L. and Capsicum annuum L.

The ontogeny of the vegetative shoot apices of two importantvegetable plants, Solanum melongena L. and Capsicum annuum L.is described. Each shoot apex is studied at different stagesof seed germination. The relationship between time of germinationand (i) area of vacuoles in the cell, (ii) total area of thecell, (iii) area of the nucleus in the cell, and (iv) ratioof area of vacuoles in the cell to the total area of the cellin each apex is examined. The differentiation of cytohistologicalzonation in both apices is distinct only after one or two leavesare initiated and developed. At a certain stage heterogeneityin staining in the peripheral region of the shoot apex of S.melongena is found. The zonation in both plants differentiatesgradually in histological and cytological features. Vacuolationincreases or decreases in all the cells of the shoot apex orin the cells of a particular region of the shoot apex at differentstages of its ontogeny.     

Characterization of Stelar Initiation in Shoot Apices of Ferns

Procambium is commonly recognized as a vascular meristem inshoot apices of vascular plants. Prestelar tissue comprisingprovascular tissue (PVT) and pith mother cells (PMCs) immediatelysubjacent to the single cell layer of promeristem has been consideredto represent the initial stage of stelar differentiation precedingprocambium and rib meristem in ferns. In addition to characterizationof PVT and PMCs on the basis of cell morphology, cytologicalfeatures and developmental continuity with procambium and ribmeristem, four lines of evidence from studies of shoot apicesof Matteuccia struthiopteris and Osmunda cinnamomea supportthis interpretation of initial differentiation. (1) Differentialstaining by safranin-fast green and crystal violet-erythrosinshows that PVT and PMCs differ in colour reactions from promeristemand resemble procambium and pith meristem, respectively. (2)Comparative ultrastructural study reveals qualitative differencesin the cell membrane system, nuclei, cytoplasm, vacuoles andplastids between promeristem and PVT but similarity of PVT toprocambium. (3) Large droplets of tannins occur in promeristembut not in PVT, PMCs and procambium. (4) Cytochemical studyof the shoot apex of Osmunda shows that carboxylesterase activityis strongly demonstrated in PVT and procambial cells but notin promeristem cells and PMCs. These observations further substantiatethe interpretation that PVT represents initial vascular differentiationand PMCs reflect a commitment to pith development.Copyright1995, 1999 Academic Press Initial vascular differentiation, provascular tissue, differential staining, ultrastructure, tannins, carboxylesterase, shoot apex, Matteuccia struthiopteris, Osmunda cinnamomea     

The Natural Philosophy of Plant Form: Cellular Autoreproduction as a Component of a Structural Explanation of Plant Form

A map-L-system is described which simulates the developmentof the two-dimensional patterns of cell walls displayed at thesurfaces of shoot apices of Psilotum nudum. The simulation ofthese cellular patterns commences with the division of a triangularcell and continues until a complete set of ten different cells,including new triangular cells, is formed amongst the descendantsof each merophyte. The triangular cells generated by means ofthis division pathway, P1, are, in their three-dimensional aspect,four-sided apical cells. In the plant, they have the potentialityto support the development of a shoot apex. The generation ofnew triangular cells by pathway P1 therefore seems to be a preconditionfor the branching of the shoot. Observed variations upon thecellular pattern developed by pathway P1 have also been analysed.Two of these variant pathways, P2 and P3, suggest the typesof controls which are required to bring about all three (P1–P3)patterns of cells. These controls may involve the participationof the plant cytoskeleton and may also require an influencefrom the apical cell itself. The triangular shoot apical cellsof Psilotum are autoreproductive cells: that is, at each division,one of the daughters is a new triangular cell, the other daughterhas some other shape. This example of triangular cell autoreproductionand self-maintenance and its relation to organogenesis is discussedin light of the views on reproduction and self-maintenance expressedby Agnes Arber (1950) in her book The natural philosophy ofplant form(Cambridge: Cambridge University Press). Copyright2001 Annals of Botany Company Agnes Arber, apical cell, cell division patterns, computer simulation, cytoskeleton, L-systems, Living Systems Theory, meristems, Psilotum, shoot apex, stem cell     

Surface Changes on Fresh Carnation Shoot Apices During Scanning Electron Microscopy

The general morphology of fresh intact shoot apices of carnation,Dianthus caryophyllus L. cv. Improved White Sim was studiedwith a scanning microscope. The changes in the surface withtime under the electron beam were: (1) individual cells beingvisible, (2) a smooth apex and (3) wrinkling until the specimenbecame unsatisfactory for study. As indicated by time coursestudies, the initial individual cell observations representthe true features of the intact apex. The observation of a smoothapex was related to a ballooning of the surface layer resultingfrom water vaporization. Incisions in the apex to prevent pressurebuild-up under the surface layer revealed the smooth characteristicof the apex surface was due to lifting of the cuticle from underlyingcells. Areas adjacent to a micropuncture in the apical domedid not exhibit ballooning.     

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     

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.     

Chalcone Synthase Localization in Shoot Apices of Fagopyrum, Brassica and Pisum

Chalcone synthase (CHS), a key enzyme of flavonoid synthesis,was localized in shoot apices of Fagopyrum, Brassica and Pisum.The enzyme was detected in initial cells of the shoot apex,which gives rise to the whole plant body. In Fagopyrum and BrassicaCHS was located in the rib and flank meristems, whereas in theArgenteum mutant of Pisum this enzyme was localized at an earlierstage in the ontogenesis of the shoot. It occurs in a few cellsof the tunica, which gives rise to the protoderm, and then tothe epidermis which contains anthocyanins in these plants. Chalcone synthase, immunogold labelling, promeristem, shoot apex, Brassica, Fagopyrum, Pisum     

RNA Synthesis in the Shoot Apex of Polytrichum formosum Hedw.

RNA metabolism in the cells of the gametophyte shoot apex ofPolytrichum formosum was investigated using both microspectrophotometricand autoradiographic methods along with an accurate measurementof surfaces and volumes of nuclei, nucleoli, free cytoplasmand vacuolar systems. On a per cell basis, the amount of RNAand the rate of RNA synthesis were shown to be highest in theapical cell. On the other hand, both RNA concentration and rateof synthesis for a unit quantity of cytoplasm were found tobe higher in leaf initials and in the cells of young leavesthan in theapical cell, the segments and the segmental derivatives.For the various types of cells in the shoot apex it was establishedthat the more voluminous the nucleolus, the higher the RNA syntheticrate per cell. These results were correlated with the data previouslyobtained on the mitotic cycles in Polytrichum. It is concludedthat in the apical cell, notwithstanding its differentiatedfeatures, RNA metabolism must be considered on the whole tobe very active. These various data are compared with those obtainedon angiosperm shoot apices.     

The Difference Between Open and Closed Meristems

An open and a closed root meristem have been compared by investigatingthe cell kinetics of small regions of the apices of Helianthusand Zea. The cells of the stelar pole are quiescent in both and thereis no exchange of cells between stele and cortex or stele andcap. The immediately distal cells in the closed meristem (Zea)are also quiescent and the few divisions that do occur can betransverse or longitudinal. In the open meristem (Helianthus)these cells are not quiescent, but they go out of cycle transiently,prolonging the potential cell-doubling time. Their divisionsare transverse. It is a consequence of these differences thatclosed meristems form root caps discrete from the cortex whereasopen meristems force instability in the boundary between theperipheral part of the cap and the cortex. Another consequencein roots with open meristems is a succession of columella complexestransversely displaced from each other by the state of fluxin the meristem during the non-cycling phase of the proximaltier of cells, those immediately distal to the stelar pole. The results are discussed in relation to the ontogenetic onsetof quiescence and the evidence for switches between open andclosed operation of meristems. meristem, root apex, Helianthus annuus, Zea mays L.     

The Development of the Shoot Apex of Agropyron repens (L.)Beauv.

During the phase of growth between the production of one and10 mature leaves the primary shoot apex of Agropyron repensundergoes first an increase and then a decrease in size. Theapical dome was found to attain a maximum size at the six leafstage. The changes are attributable to changes in cell number,mean cell size remaining constant after an initial decrease.The zonation pattern, and particularly the number of tunicalayers, varies with the size of the dome. The apex as a wholeundergoes a greater increase in height than in diameter, withcommensurate changes in the number of leaf primordia presentupon it. Similar changes occur in the tiller but no such obvioustrends were found in the rhizome. In an experiment in whichall axillary buds were excised from the primary shoot it wasfound that the apex continued to increase in size well beyondthe maximum attained by that of an intact plant. On this evidenceit is suggested that the eventual decline of the primary shootand tiller apices is due to an inhibitory effect on the mainshoot by the developing axillary shoots.     

Experimental and Analytical Studies of Pteridophytes: XXXVIII. Some Observations on Spiral and Bijugate Phyllotaxis in Dryopteris aristata Druce

In an attempt to assess some of the factors which determineleaf arrangement at the shoot apex, a study has been made ofphyllotaxis in the fern Dryopteris aristata. It is shown thatclockwise and counter-clockwise spirals occur with equal frequencyboth in field collections and in adventitious and experimentallyinduced buds which arise on the rhizomes and apices of plantsgrown in the laboratory. In addition to the two common spiralarrangements, a form with bijugate phyllotaxis has been noted.It is concluded that the three types of leaf arrangement arenot under direct genetic control, but rather are dependent oncircumstances of growth at the apex.     

Changes in Apical Growth and Phyllotaxis on Flowering and Reversion in Impatiens balsamina L.

When plants of Impatiens balsamina L were subjected to 5 shortdays and then re-placed in long days, they began to form a terminalflower and then reverted to vegetative growth at this terminalshoot apex The onset of flowering was accompanied by an increasein the rate of initiation of primordia, an increase in the growthrate of the apex, a change in primordium arrangement from spiralto whorled or pseudo-whorled, a lack of internodes, and a reductionm the size at initiation of the primordia and also of the stemfrusta which give rise to nodal and internodal tissues On reversion,parts intermediate between petals and leaves were formed, followedby leaves, although in reverted apices the size at initiationand the arrangement of primordia remained the same as in thefloweing apex The apical growth rate and the rate of primordiuminitiation were less in the reverted apices than in floral apicesbut remained higher than in the original vegetative apex Sincethe changes in apical growth which occur on the transition toflowering are not reversed on reversion, the development oforgans as leaves or petals is not directly related to the growthrate of the apex, or the arrangement, rate of initiation orsize at initiation of primordia Impatiens balsamina L, flower reversion, evocation, phyllotaxis, shoot meristem     

Exit from the Mitotic Cycle in Root Meristems of Zea mays L.

The choice between two modes of exit from the mitotic cycleat the margins of meristems has been made easier by surveyingthe range of the numbers of cell contacts between contiguousfiles in root apices of Zea mays L. The range shows that somecells must go out of cycle while others remain in cycle forat least three further generations. The view that cycling endsby a fall in the proliferative fraction is supported by theexistence of pulse-labelled telophases in the proximal regionof the menstem. These are most likely due to acceleration ofthe mitotic cycle which has to be contrasted with decelerationof the overall rate of cell proliferation. The work is discussedin relation to patterns of cycling in the different tissuesof the apex. mitotic cycle, cell size, meristem, proliferative fraction, Zea mays L, maize     

Surface-viewed shoot apex ofAngiopteris lygodiifolia Ros. (Marattiaceae)

Marattian ferns are thought to be an exception to the rule that a single apical cell is always present in the shoot apex of ferns; the occurrence of plural apical initials has been generally accepted for these ferns. However, a contradicting conclusion was reached in this study which examined the apical organization of the shoot ofAngiopteris lygodiifolia Ros., using fresh materials which had not been fixed. Shoot apices were hand-sectioned transversely into thin sections, including the surface layer of the shoot apex, which were observed by differential interference contrast microscopy without staining. In contrast with the generally accepted view, the shoot apex ofA. lygodiifolia was found to usually possess a single apical cell with three cutting faces. The segments cut off from the apical cell are regularly arranged in a helical sequence. The apical cell seems to actually function as an initial cell of the whole shoot apex. The shoot apices, particularly those of plants cultivated in a greenhouse, sometimes show somewhat irregular organization. In extreme cases, no apical cell is recognizable. However, even in these exceptional cases of such apparently irregular shoot apices, plural apical initials are not found.     

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.     

Observations of the Effect of Water on the Hyphal Apices of Fusarium oxysporum

When colonies of Fusarium oxysporum, growing on plates of mineral-sucroseagar, are flooded with the mineral-sucrose solution, withoutadded agar, or with solutions of any of the constituents ofthe mineral-sucrose mixture at a concentration of 0.076 M theleading hyphal apices at the agar surface continue to grow onunchecked. If, however, the colonies are flooded with solutionsof decreasing and increasing molarity from 0.076 M an increasingproportion of the leading hyphal apices at the agar surfacestop growing, and branch subterminally. In distilled water about50 per cent. of the apices branch and this branching is precededby swelling, whereas in 0.5 M sucrose more than 90 per cent.of the apices branch and the branching is not accompanied byswelling. In the distilled water those hyphae which do not branchswell a little and grow on from the apex within 40 seconds. When hyphal apices are flooded with distilled water for from10 to 40 seconds and then transferred to mineral-sucrose solutionmore than 90 per cent, of the hyphal apices branch, whereasflooding with distilled water for 60 seconds or longer givesthe same percentage of branched apices as does flooding withdistilled water alone. It is shown that swelling and branching of the hyphal apex arenot causally related but that branching always occurs followingarrestment of the hyphal apex for more than 60 seconds. It issuggested that the phenomena reported can be explained in termsof an irreversible change in the apical cap of the arrestedhypha such that continued extension can no longer take placein this region and fresh outlets for growth must then be foundsubterminally. Such a mechanism, however triggered, could accountfor a wide variety of morphogenetic forms in the fungi.     

Pre-flowering Growth and Development of the Inflorescences of Maize: I. PRIMORDIA PRODUCTION AND APICAL DOME VOLUME

The growth and development of the main stem and the two uppermostaxillary apices of maize were studied during the period fromemergence until flowering. Plants were grown in the field undervarying levels of applied nitrogen fertilizer at two times ofsowing. The effects of daylength were isolated from those oftemperature by making comparisons of growth and developmenton a thermal time basis. The growth and development of the terminal (male) apex and thetwo uppermost axillary (female) apices followed the same patterns,with apex volumes increasing curvilinearly with increase innumber of leaf or husk primordia. The RGR(relative growth rateof volume) of the terminal apex was, however, only approximatelyone-tenth of the axillary apices. There was no difference ingrowth and development between the first and second axillaryapices before flowering: other factors, such as accumulationof dry weight, rather than primordia production, must be responsiblefor an axillary apex's potential to bear grain. Applied N, andto a lesser extent short days, increased the rates of growthand development of all the apices. For example, applied N increasedthe RGR (volume) of the apical domes, and the rate of productionof spikelet primordia, by about 25%. All axillary apices and treatments showed a single relationshipbetween number of spikelets and surface area of the ear: a favourableenvironment (e.g. high N) simply accelerated the progressionof spikelet production and area expansion along this singlepath. We conclude that this path is probably determined geneticallyand that N and time of sowing influence potential yield of maizethrough effects on the surface area of the ear but not on thedensity of spikelets formed. Key words: Maize, inflorescence, N application, daylength, temperature, apex volume     

Experimental and Analytical Studies of Pteridophytes: XXXV. The Effects of Direct Aplications of Varous Substances to the Shoot Apex of Dryopteris austriaca (D. aristata)

The organization and morphogenetic activity of a fern apex,that of Dryopteris austriaca (D. aristata), have been furtherinvestigated by observing how it responds to direct applicationsof solutions of various physiologically active organic substances.Large apices were laid bare and treated for periods of up toabout 20 days with solutions of 3-indoleacetonitrile, dinitrophenol,maleic hydrazide, yeast extract, glutamine, &c., with directexamination at frequent intervals using a binocular microscope. In most cases some reaction was obtained. Developments of morphogeneticinterest include: an increase in the rate of growth in the subapicalregion; an arrest or slowing down of growth in the apical meristem;the formation of flattened and of sunken, cup-shaped apices;the formation of double, treble, and quadruple leaf primordia,of two primordia in a single leaf site, and of centric (or radial)leaves; the occasional suppression of primordia; the reversalof the phyllotactic spiral; the induction of buds on meristemswith an uninjured apical cell; the promotion of root formationand growth; the formation of scales on the apices of young leafprimordia and leaf sites, and the precocious formation of scalesover the whole of the apical meristem, including the apicalcell; and the precocious parenchymatization of the meristemincluding the apical cell. The significance of these resultsis discussed.     

Regeneration of the Root Cap of Zea mays L. and Pisum sativum L.: A Study with the Scanning Electron Microscope

Removal of the root cap from a root apex initiates regenerationof a new cap. The process has been followed using scanning electronmicroscopy. Quantitative data have been obtained for the growthin area of the exposed acroscopic surface of the quiescent centre(QC) and the increase in volume of the regenerating cap tissue.In Zea the surface of the QC shows an initial rapid increasein area followed by a slower increase. In Pisum the surfacearea increases uniformly, a rapid initial phase being absent.Together with observations on the behaviour of an incision atthe exposed surface, the results indicate that in Zea the capnormally imposes a constraint upon radial growth at the acroscopicsurface of the QC; in Pisum the QC appears not to be so constrained.The different responses may be related to the different arrangementsof cells at the apex of the meristem of these two species. Zea mays, Pisum sativum, maize, peao, scanning electron microscopy, root apex, regeneration     

Histochemical Study of Photoperiodic and Low Temperature Effects on Floral Induction of Spinacia oleracea

Shoot apices of Spinacia oleracea plants have been induced toflower either by: (a) subjecting leaves to 24 h long day, or(b) exposure to a short photoperiod but displaced by 8 h (displacedshort day) in the usual 24 h short-day cycle, or (c) exposureto low temperature (5 °C) during the dark period of thenormal short day. A quantitative cytochemical assay of pentosephosphate pathway activity during floral induction indicatesan approximate doubling of the rate of activity when comparedto that of vegetative apices (short day) (21 °C). Exposure to either low temperature, or a displaced short photoperiodstimulates pentose phosphate pathway activity in the shoot apexin a manner similar to that seen by long-day induction. Thischange in metabolic activity is accompanied by changes in theshape of the shoot apex which resembles that seen at an earlystage during floral induction. Spinacia oleracea, pentose phosphate pathway, shoot apex, glucose-6-phosphate dehydrogenase, floral induction, chilling, displaced short day