On the Mechanics of Growth and Morphogenesis in Hydroid Polyps

The growth and morphogenesis of Obelia loveni and Dynamena pumilaasexual generation were studied. Their cell layers consist oftightly packed cells which are able to contract, to feorlentand slide upon one another while remaining at the same timeanchored to elastic enveloping membranes. Both growth and morphogenesisare of pulsatory character due to coordinated periodic contractionsand relaxations of cells. Cell contraction results in a normalizationof cell orientations, an upward shift of the entodermal column,and an expansion of the rudiment's tip. Cell relaxation is accompaniedby reverse deformations and an upwards shift of the ectodermalcolumn. The spatial pattern of active and passive forces andsupporting structures is ascertained.     

Radiomimetic Effect of Cisplatin on Cucumber Root Development: the Relationship between Cell Division and Cell Growth

Cisplatin [DDP, cis-dichlorodiammine platinum (II)], a strongcytostatic and antineoplastic agent, was tested on seedlingsof cucumber Cucumis sativus L. for its general effect on rootdevelopment and its particular effects on root cell divisionand cell growth. DDP was characterized as a radiomimetic compoundsince both DDP (1·3 x 10^-5 M) and -irradiation (2·5-10kGy) drastically and irreversibly stopped development of embryoniclateral root primordia (LRPs) in the radicle by inhibiting bothmitotic activity and cell growth. In 20% of the LRPs of DDP-treatedroots, cells did not divide at all. Dividing cells completedno more than two cell cycles. These effects were specific becausewhen DDP was available to the roots only at the onset of celldivision, cell proliferation and cell growth were similar tothat produced by constant incubation. Neither DDP nor -irradiationaffected non-meristematic cell elongation. It was concludedthat cell growth of meristematic cells is closely related tocell division. However, non-meristematic cell growth is independentof DNA damage. This suggests DDP as a tool to reveal these autonomousprocesses in plants development and to detect tissue compartmentsin mature plant embryos which contain potentially non-meristematiccells.Copyright 1993, 1999 Academic Press Cucumis sativus, cucumber, cisplatin, cell growth, cell elongation, cell division, lateral root, root development     

The helix clock: a potential biomechanical cell cycle timer

A model based upon helical geometry that provides cylindrically shaped cells with a means to measure their length during growth and to time cell cycle events is presented. The helix clock arises from the change in pitch angle that accompanies the parallel packing of strands on a cylinder surface. A single strand inserted into the cylinder surface nearly perpendicular to the long axis of the cylinder starts the clock running. As additional strands are inserted parallel to those in place, the pitch angle of all strands must reorient. A limit is reached when all strands lie parallel to the long axis of the cylinder. By sensing either the pitch angle or a physical ramification thereof, cells can measure their length during growth and time events of the cell cycle. The helix clock model is discussed in relationship to the bacterial cell cycle. The idea that bacterial cells use one helix hand for cylinder elongation, the other for septation is presented. The negative twist so generated apparently drives folding in the helical bacterial macrofiber system of Bacillus subtilis.     

Morphogenetic Studies on the Distribution and Activities of Leaf Meristems in Ferns

In very young sporophytes of Dryopteris and Osmunda, the leafprimordia originate very close to the shoot apical cell andshow early differentiation of an apical cell, rapid growth,and an early transition from distal to marginal growth. In successively older primordia of adult Dryopteris, a gradualelaboration in the size of the leaf apical cell takes placeand the greatest size is attained before lateral pinnae beginto be formed. With the formation of lamina, the apical cellgradually decreases in size and is transformed into the marginaltype of meri-stematic cell, when the leaf unrolls. In ferns with a homogeneous marginal meristem, which consistsof a uniform layer of cells with an equal capacity for growth,a simple, entire leaf is formed, e.g. Phyllitis and Platyceriumand where an initially homogeneous marginal meristem becomesheterogeneous, with a consequential differentiation of areasof unequal growth, a lobed or pinnate configuration, as in Blechnumand Lomnaria, or a compound leaf, as in Dryopteris, results. There are some indications of the inception of vascular elementsbeing due to the activity of functioning meristems, the processbeing a basipetal one.     

Contributions to the Morphology of Some Species of Microsorium

Morphology of eleven epiphytic and rupicaulous species of Microsoriumis described. The paleae in the genus are either peltate withthe basal region developing secondarily as a hood over the stalk,or basally attached with auricles on either side of the stalk.The auricles in M. hancockii and M. pteropus develop from singleinitial cells adjacent to the stalk, while in the others nospecialized initial cells occur. Marginal and terminal glandularhairs occur on the paleae, except in M. hancockii and M. pteropusin which marginal hairs are absent. Slender sclerenchyma strands are scattered profusely in theground tissue of the rhizome. The stelar cylinder is dictyostelicand is dissected by lacunae into cylindrical vascular bundles.Leaf traces are multiple strands originating as branches fromthe dorsal median vascular bundle of the stelar cylinder andthe one next to it on each side. Two or three closely placedvascular bundles of the rhizome constitute the vascular connexionto each branch of the rhizome. Venation of the leaf lamina is reticulate with most of the free-endingveinlets entering foliar hydathodes. The juvenile leaves bearhairs similar to the pro-thallial hairs and are spatulate witha medianly placed forked vein. Sori are generally punctiform,but in M. hancockii and M. pteropus spread slightly over theveins, often forming elongated coenosori. Uniseriate (multiseriatein M. rubidum), multicellular paraphyses occur in all speciesexcept M. scolopendria. The spores are monolete and either psilateor granulate. The prothalli develop from 3-5 cells long germ filaments inwhich the anterior cells divide longitudinally and form an ameristicprothallial plate. An apical meristematic cell is formed later,and a cordate prothallus is developed, except in M. hancockiiand M. pteropus in which a definite meristem is never formedand the prothalli are ribbon-shaped and branched. The cordateprothalli possess polypodiaceous hairs: the ribbon-shaped onesare more or less naked and devoid of any midrib. The chromosome number in the species is n = 36 (zn = 72).     

Hydra and the evolution of apoptosis

Programmed cell death occurs in most, if not all life forms.It is used to sculpt tissue during embryogenesis, to removedamaged cells, to protect against pathogen infection and toregulate cell numbers and tissue homeostasis. In animals celldeath often occurs by a morphologically and biochemically conservedprocess called apoptosis. A novel group of cysteine proteases,referred to as caspases, constitute the central component ofthis process. Caspases are activated following the inductionof apoptosis and cleave a variety of cellular substrates, thusgiving rise to the characteristic morphological events of apoptosis.Apoptosis is rapid and cell corpses are removed by phagocytosis.Recent work has shown that apoptosis also occurs in Cnidariaand Porifera, thus extending the origin of this evolutionaryinnovation down to the first metazoan animal phyla. Here, wereview several examples of the role of apoptosis in cnidariansand then summarize new results on the subcellular localizationof caspases and the control of apoptosis in Hydra. We show byimmuncytochemistry that caspases in Hydra are localized in mitochondria.Following induction of apoptosis caspases are released frommitochondria as proenzymes and then activated by proteolyticcleavage in the cytoplasm. We also present evidence that apoptosisin Hydra is dramatically stimulated by inhibitors of PI3-kinase.Since PI3-kinase is a central component of growth factor signalingcascades in higher metazoans, this result suggests that controlof apoptosis by growth factors is also evolutionarily conserved.We speculate on the role of growth factors in the evolutionof apoptosis.     

An Automata-theoretical Model of Meristem Development as Applied to the Primary Root of Zea mays L.

Observations were made of the sequence of division within thecellular packets (groups of cells of common descent) which comprisethe cell files that run the length of the central cortex ofthe primary root meristem ofZea mays. These sequences, and alsothe relative lengths of the cells within the packets recordedat various times during root growth, indicate that cell-filedevelopment can be expressed using one, or a limited number,of deterministic ‘bootstrap’ L-systems which assigndifferent lifespans to sister cells of successive cell generations.The outcome is a regular pattern of divisions from which daughtercells emerge usually with unequal, but definite, lengths. Inthe immediately post-germination stage of root growth, one divisionpathway is especially common in the cortex and generates sequencesof unequal daughters having a particular basi-apical orientation.Later in root growth, the cellular pattern in the cortex indicatesthat this pathway is replaced by another where unequal divisionsare not so marked, but which nevertheless continues to maintaina regular arrangement of differently sized cells. This latterpathway is characteristic of a zone close to the initial cellsof the cortex. It is present at all stages of root growth andspreads along the length of the cortex as the descendants ofthese initials proliferate. The development of the whole corticalcell file can be simulated from knowledge of the growth functionsof the bootstrap systems. The files so generated contain allthe observed cell patterns. The growth functions also predictthe sequence in which cells cease dividing near the proximalmargin of the meristem, but for this it is necessary to incorporatea counter for the number of divisions that will be accomplishedin the cell file. Cytological requirements for the propagationof unequal divisions, together with a consideration of the natureof the division counter, as well as the significance of theswitch in division pathways encountered during early root growth,are discussed in the context of this deterministic model ofcell division. Cell division; root meristem; L-systems; Zea mays     

Hypertonic shock inhibits growth factor receptor signaling, induces caspase-3 activation, and causes reversible fragmentation of the mitochondrial network

Hyperosmotic stress can be encountered by the kidney and the skin, as well as during treatment of acute brain damage. It can lead to cell cycle arrest or apoptosis. Exactly how mammalian cells detect hyperosmolarity and how the cell chooses between cell cycle arrest or death remains to be established. It has been proposed that hyperosmolarity is detected directly by growth factor receptor protein tyrosine kinases. To investigate this, we tested whether growth factors and osmotic stress cooperate in the activation of signaling pathways. Receptors responded normally to the presence of growth factors, and we observed normal levels of GTP-bound Ras under hyperosmotic conditions. In contrast, activation of Raf, Akt, ERK1, ERK2, and c-Jun NH₂-terminal kinase was strongly reduced. These observations suggest that hyperosmotic conditions block signaling directly downstream of active Ras. It is thought that apoptotic cell death due to environmental stress is initiated by cytochrome c release from the mitochondria. Visualization of cytochrome c using immunofluorescence showed that hypertonic conditions result in a breakup of the mitochondrial network, which is reestablished within 1 h after hypertonic medium is replaced with isotonic medium. When we carried out live imaging, we observed that the mitochondrial membrane potential disappeared immediately after the onset of hyperosmotic shock. Our observations provide new insights into the hypertonic stress response pathway. In addition, they show that signaling downstream of Ras and mitochondrial dynamics can easily be manipulated by the exposure of cells to hyperosmotic conditions. protein tyrosine kinases; Ras; mitogen-activated protein kinase; hyperosmotic shock     

Cessation of cell elongation in rye coleoptiles is accompanied by a loss of cell-wall plasticity

The mechanism by which endogenous cessation of coleoptile elongationafter emergence of the primary leaf is brought about was investigatedin rye seedlings (Secale cereale L.) that were either grownin darkness or irradiated with continuous white light. In 3-d-oldetiolated (growing) coleoptiles a turgor pressure of 0.59 MPawas measured. In 6-d-old coleoptiles, which had ceased to elongate,cell turgor was 0.51 MPa and thus only 13% lower than in therapidly growing organ. Hence, the driving force for growth (turgor)is largely maintained. Cell-wall plasticity (E_pl) and elasticity(E_Ql were determined with a constant load extensiometer bothin vivo (turgid coleoptile segments) and in vitro (frozen-thawedsamples). Cessation of coleoptile elongation was correlatedwith a 95% reduction in E_pl9 whereas E_Ql was only slightly affected.Extension kinetics were measured with living and frozen-thawedsegments cut from growing and non-growing coleoptiles. The correspondingstress-strain (load-extension) curves indicate that the cellwall of the growing coleoptile behaves like an elastic-plasticmaterial whereas that of the non-growing organ shows the behaviourof an elastic solid. These data demonstate that E_pl representsa true plastic (irreversible) deformation of the cell wall.It is concluded that cessation of coleoptile growth after emergenceof the primary leaf is attributable to a loss of cell-wall plasticity.Hence, a mechanical stiffening of the cell wall and not a lossof turgor pressure may be responsible for the deceleration ofcell elongation in the rye coleoptile. Key words: Extension growth, rye coleoptile, cell-wall extensibility, turgor pressure     

Dunaliella salina: A model System for Studying the Response of Plant Cells to Stress

This review focuses on the biochemical and physiological responseof the halotolerant green alga, Dunaliella salina, to conditionsof stress. It is now well established that in response to stress,cells of Dunaliella salina var. bardawil show increased glycerolproduction, massive ßcarotene accumulation and enhancedabscisic acid metabolism. In this respect, cellular responsesare regulatory and seem to depend on a diversity of mechanismswhich may be linked to a modification of the abscisic acid balance.Dunaliella lacks a rigid cell wall and the cellular contentsare enclosed by an elastic plasma membrane that permits rapidcell volume changes in response to extracellular changes inosmolarity. Based on the ‘stretch activated ion channelsmodel’ reviewed recently by Kirst (1990) we propose thefollowing cascade of responses: volume change/distortion ofplasmalemma     

Studies on the Growth in Culture of Plant Cells: XI. THE INFLUENCE OF SHAKING RATE ON THE GROWTH OF SUSPENSION CULTUEES

The influence of shaking rates (expressed as revolutions permin) on orbital shaking platforms (1 in (2.54 cm) diam. rotarymotion) on the growth of cell suspension cultures of Acer pseudoplatanusL. and Atropa belladonna cultivar lutea Döll are described.By following cell growth and respiration and the levels of oxygenand carbon dioxide in the media during the progress of incubationit is concluded that the reduction of growth at sub-optimalshaking rates is not due to oxygen deficiency or toxic accumulationof carbon dioxide. The growth of the Atropa cell suspensionin ‘closed systems’ has been studied by the developmentof modified culture vessels and evidence obtained that the reducedgrowth in the systems is due to the formation by the culturesof an unidentified volatile growth inhibitor and not to eitheroxygen depletion or toxic accumulation of either carbon dioxideor ethylene. It is suggested that the reduced growth in ‘opensystems’ cultures at sub-optimal shaking speeds is eitherdue to retention of this volatile inhibitor or to restrictionof nutrient uptake by the existence of a stationary liquid-phaseboundary to the cells.     

The Structure and Development of the Reproductive Organs and Carposporophyte in two British Species of Gelidium

The structure and development of the reproductive organs andcarposporophyte are described for two British species of Gelidium,G. latifolium (Grev.) Born. & Thur. and G. pulchellum (Turn.)Kütz. The gonimoblast does not develop directly from theunchanged carpogonium, as was thought by Kylin, but a swollenmultinucleate cell of irregular outline is formed first, eitherfrom the carpogonium alone, or by the fusion of the carpogoniumand certain neighbouring cells; the gonimoblast develops fromlong, non-septate processes of this multi-nucleate cell. Thesignificance of the fusion of the carpogonium and certain vegetativecells is discussed in relation to the various definitions ofthe auxiliary cell which have been proposed. It is concludedthat an auxiliary cell does not occur in Gelidium.     

Structure and Organization of the Vascular System in the Rhizome of Drynarioid Ferns

Form and structure of the vascular cylinder of the rhizome of14 species belonging to six genera of drynarioid ferns are described.The study reveals that drynarioid ferns fall into two categories:the Drynaria group (Drynaria, Photinopteris), which is probablythe more primitive, exhibits the alternate two-ranked leaf arrangementand this could have led through suppression to the one-rankedcondition occurring in the Aglaomorpha group (Aglaomorpha, Drynariopsis,Pseudodrynaria, Merinthosorus, Thayeria). It seems that rhizomemorphology provides characters which help to separate differentspecies into the two distinct groups and indicates some phylogenetictrends. The most characteristic feature of the rhizome of drynarioidfern is seen in Thayeria, where some of the leaves on the dorsalsurface of the rhizome are dormant and some of them developon a prominent short, stout phyllopodium. Pteridophyta, Polypodiaceae, rhizome, vascular system, drynarioid ferns     

Development of the Sieve Plate in Saxifraga sarmentosa L.

The differentiating sieve plate in the phloem of the stolonof Saxifraga sarmentosa L. was studied with the electron microscope.Development of the pore site begins with differentiation ofa pair of collar-like areas around the plasmodesma which canbe seen in the youngest identifiable sieve plates. Further growthof the collars occurs by deposition of an amorphous substance,presumably caflose. Although the growth of the collars is simultaneouswith the growth of the surrounding cell wall it is rapid atfirst and the pore sites appear asdome-shaped protuberances.It also involves deposition of callose over an increasinglywider area of the cell wall and since the thickening of thenormal cell wall continues only where notcovered by callose,the collars assume a conical form. There seems to be no displacementor lysis of normal cell wall material during growth of the collars.Eventually the growth of the cell wall in thickness overtakesthe pore sites so that when the growth of the cell wall is completethe pore sites appear as depressions in the sieve plate. Theperforation of a pore site is accomplished by widening of theplasmodesmatal cylinder which begins at the middle lamella byremoval of callose. Endoplasmic reticulum is found in closeproximity to the plasmodesma andis believed to penetrate it.     

Weak 'top-down' control of dinoflagellate growth in the coastal Skagerrak

Effects of planktivorous fish (Sprattus sprartus), copepods(mainly Oithona sp.) and benthic filter feeders (Mytilus edulis)on the biomass and species composition of a natural marine planktoncommunity, dominated by dinoflagellates, was studied experimentallyin September 1988. Plastic 300 I cylinders were filled withSkagerrak surface water, filtered through a 90 µm nylonnet in order to remove mesozooplankton. Copepods were addedin three different concentrations (0, 1x and 6x the ambientdensity) with and without nutrient additions (nitrate, silicateand phosphate). In two nutrient-enriched cylinders with addedcopepods (1x and 6x), 5 cm sprats were also added (10 per cylinder).In two nutrient-enriched cylinders without added copepods. bluemussels (20 per cylinder. 3 cm length) were introduced. Chlorophylla. inorganic nutrients and densities of phytoplankton, ciliatesand copepods were measured throughout the experimental periodof 10 days. Gyrodinium aureolum increased in cell numbers inall but the mussel cylinders, while the other dominating dinoflagellate,Ceratium furca, decreased. This decrease was not due to grazing,but probably a container artefact. Diatoms, Rhizosolenia fragilissinia,Leptocylindrus danicus and Chaetoceros spp., increased slowlyin treatments without fish. In the fish treatments, diatomshad appreciably higher growth rates and cell numbers, probablydue to predation by fish on the copepods. Gyrodiniun aureolunalso had a somewhat higher growth rate in the presence of fish.although this was probably not due to reduction of copepod grazing.The results show that planktivorous fish can affect the biomassand species composition of natural phytoplankton communitiesin marine waters, but that the effect on the late summer-autumndinoflagellate dominated community is weak. Blue mussels areefficient grazers of the whole phytoplankton community and arenot particularly selective with respect to either size or species.     

SYNCHRONIZATION OF BACTERIAL CULTURE BY PREINCUBATION OF CELLS AT HIGH CELL DENSITY

The growth of Escherichia coli strain B in a liquid medium wasfound to cease at a cell density of 5x10⁹ cells per ml. (Thiscritical concentration is designated as the maximum or M-concentration.)Even cells harvested from the logarithmic growth phase couldnot divide at this or higher cell densities. Investigationson the metabolic activities of such cultures, however, showedthat the synthesis of cellular protein and nucleic acid wastaking place under such circumstances, showing that only someprocess (or processes) particularly related to cell divisionwas suppressed at the critical cell concentration in question. This finding led us to devise a new method of synchronizationof E. coli: cells harvested from a logarithmic phase were preincubatedat the critical concentration of 5x10⁹ cells per ml for 45 minutes,and then diluted 100 times with fresh medium. This led to synchronizationof cell division, as shown by a stepwise multiplication in cellnumber. (Received June 20, 1961; )     

Studies on the Development of the Air Pores and Air Chambers of Marchantia paleacea: 1. Light Microscopy

The ontogeny of the air pores and air chambers of Marchantiapaleacea begins with the schizogenous development of protodermalintercellular spaces of the initial apertures, and is completedwith the formation of the air pores and giant sub-epidermalair chambers bearing numerous photosynthetic filaments. Intercellularspace formation commences from the thallus surface and proceedsinwards to the first internal layer of cells. The cells amongwhich spaces develop do not originate from one mother cell.Spaces are formed only in the regions of the intersection ofthe anticlinal walls of three, four, or sometimes more successivederivatives of S₁, S₃ and S₄ segments of the apical cell, oneor two of which have been divided periclinally and the restanticlinally. Protodermal intercellular spaces appear in mostor all the corners of these cells, the anticlinal walls of whichexhibit an opposite disposition. The S₁, S₂, S₃ and S₄ segmentsare produced by definite divisions of a five-sided apical celland by a series of divisions give rise to initial cells of theinternal layers of the thallus and initial cells of the protodermaland sub-protodermal layers. The concept of a quiescent apicalcell cannot be accepted, since dividing apical cells have beenobserved, and the pattern of wall disposition of the thallusapex cannot be explained without the active participation ofthe apical cell. The air chambers are apparently of exogenous origin. They resultfrom the broadening of the bottom of the initial apertures bythe coordination of the rate of anticlinal divisions and growthof the protodermal and sub-protodermal cells surrounding theintercellular spaces of the initial apertures. The ontogenyof the pore rings starts at an advanced stage of air chamberformation not from a mother cell but from the cells which surroundthe closed entrance of the air chamber, by a shift of the planeof division from anticlinal to periclinal. Before the periclinaldivisions a new axis of growth perpendicular to the thallussurface is established in the mother cells of the pore. By a polarized growth into the air chamber followed by periclinaldivisions, the cells of the floor form initial cells of thephotosynthetic filaments. The latter divide again to form singleor branched photosynthetic filaments. Marchantia paleacea, air pore, air chamber     

Further Observations on Root Contraction

Further observations, by light and electron microscopy, on theanatomy of root contraction in Hyacinthus orientalis cvs. arereported, together with some comparisons with Narcissus andGladiolus. It is maintained that root contraction in these andsome other species is essentially a growth process, as previousinvestigators have shown, and that an understanding of the re-directedgrowth of the cortical cells which brings it about may be basedon the specialized structural features of their cell walls,here described. Physical, anatomical and comparative reasonsare given for our belief that the alternative explanation ofthe contractile process in Gladiolus which relates it to cellcollapse resulting from transpirational ‘pull’ isuntenable. anatomy, contraction, Hyacinthus, Narcissus, Gladiolus, root     

Cell Expansion and Wall pH in the Fern Regnellidium diphyllum, A Plant Lacking Acidinduced Growth

Petioles of the semi-aquatic fern Regnellidium diphyllum donot show acid growth but low wall pH is a necessary conditionfor maximum rates of IAA-induced cell expansion. Measurementsof wall pH by two indirect methods indicate an unusually lowvalue, in the range pH 4 to 5. This is one to two pH units belowthat estimated for petioles of the semi-aquatic dicotyledonNymphoides peltata, a species in which IAA aand ethylene causegrowth responses very similar to those in Regnellidium but whereacid growth occurs. Having shown previously that fusicoccinenhances proton secretion in both Regnellidium and Nymphoides,we now show that although it causes a reduction in the estimatedapoplast pH to below 4·0 in Regnellidium, cell expansionis not promoted. The FC-induced reduction in pH in Nymphoidesis less and occurs more slowly, but growth is promoted significantly;when IAA and fusicoccin are present together, growth promotionis approximately additive for Nymphoides A model is proposed for Regnellidium in which equilibrium wallpH is maintained at a low value that is optimal for acid growth,the availability of acid-labile sites in the wall being thechief limitation to cell extension. We suggest that this controlmechanism may be widespread for organs without a cuticle, includingroots and the gametophytes of lower plants growing in acidicconditions. Key words: Acid growth, wall pH, fusicoccin, Regnellidium diphyllum, Nymphoides peltata     

Effect of the Rht3 dwarfing gene on dynamics of cell extension in wheat leaves, and its modification by gibberellic acid and paclobutrazol

The second leaf of wheat was used as a model system to examinethe effects of the Rht3 dwarfing gene on leaf growth. Comparedto the rht3 wild type, the Rht3allele decreased final leaf length,surface area and dry mass by reducing the maximum growth rates,but without affecting growth duration. Gibberellic acid (GA₃)increased final leaf length and maximum growth rate in the rht3wild type, but was without effect on the Rht3 mutant, whichis generally regarded as being non-responsive to gibberellin(GA). Paclobutrazol, an inhibitor of GA biosynthesis, decreasedfinal leaf length and maximum growth rate in the rht3 wild typeto values similar to those in the untreated Rht3 mutant. NeitherGA₃ nor paclobutrazol affected the duration of leaf growth.The decrease in leaf length was produced by reduction of celllength rather than cell number. The maximum relative elementalgrowth rate (REGR) for cell extension was essentially the samein all treatments, as was the time between the cells leavingthe meristem and achieving maximum extension rate. The differencesbetween the genotypes and treatments were all almost entirelydue to differences in the time taken from the attainment ofmaximum REGR of cell extension to the cessation of extension.This was reflected in the length of the extension zone, whichwas approximately 6–8 per cent of final leaf length. Theeffects of the Rht3 allele, GA₃ and paclobutrazol all appearto be on the processes which promote the cessation of cell elongation. Key words: Cell extension, gibberellin, leaf growth, Rht3 gene, Triticum, wheat