The Kinetics of Extracellular Glycollate Production by Chlorella pyrenoidosa

Extracellular glycollate is liberated by Chlorella pyrenoidosaduring growth in medium bubbled with air or 3 per cent carbondioxide in air. With air the rate of release of glycollate percell decreases, with 3 per cent carbon dioxide it increases,with increase in cell number. Glycollate is released duringshort-term experiments when C. pyrenoidasa, grown under lowlight and high carbon dioxide, is transferred suddenly to highlight and low carbon dioxide. No other combination of thesefactors produces a comparable release of glycollate. The quantityof glycollate released in short-term experiments increases exponentiallywith the relative growth-rate of the culture from which thecells are derived. A crucial condition for maximum glycollaterelease is that growth of the culture prior to the experimentshould not be limited by carbon-dixoide concentration. The effectof pH is related to its effect on growth-rate; i.e. C. pyrenoidosahas a lower relative growth-rate at pH 8.3 and produces correspondinglyless glycollate than faster growing cultures at pH 6.4. Duringshort-term experiments under high light and low carbon dioxidethe rate of glycollate release drops after 50–100 minutessuggesting exhaustion of the glycollate precursor.     

Amitosis and Endocytogenesis in the Fruit of Malus sylvestris

Cyto-histological investigations on the initiation and developmentof a non-pathogenic physiological ‘cork spot’ necrosisin the outer cortex of the fruit of Malus sylvestris Mill. ‘YorkImperial’ revealed two distinct aberrancies, namely, amitoticnuclear division and intracellular or endogenous proliferationswithout hyperplasia per se The incipient necrotic conditionbecomes evident internally about 3 weeks after fruit set asminute isolated, discoloured, amorphous spots of disorganizedruptured cells within otherwise healthy cortical tissue Thedisorganization continues slowly with adjoining cells becomingsimilarly necrotic After the lysis of the initial cork spotcells, about 1 month later, sporadic cellular changes occurin various healthy vacuolated cortical cells contiguous to andencompassing the necrotic tissues. The nuclei increase in sizeand volume as they assume distinctive positions in preparationfor amitotic nuclear divisions. The enlarged nucleus or macronucleus,containing one or several nucleoli, divides by a distinct cleavagedeveloping from a constriction perpendicular to its longitudinalaxis The division results in the amitotic formation of two daughtermicronuclei that usually become separated by the formation ofa cell wall. No evidence of cell plate formation was observedand the method of cell wall formation could not be determined.Repeated amitotic divisions of the micronuclei result in anintracellular or endocytogenetic proliferation of parenchymatouscells that are invariably confined within the original mothercell until its wall ruptures The endogenous proliferations arereleased into lacunae or intercellular spaces, eventually becomedisorganized, and disintegrate, with the accumulated residualincrements resulting in an overall ‘cork spot’ appearance. Malus sylvestris Mill., apple, amitosis, endocytogenesis, multinucleate cells, macronuclei, micronuclei     

Mechanical Defences to Herbivory

The two major mechanical defences of plants are toughness andhardness. These have different material causes and ecologicalfunctions. In any non-metal, high toughness is achieved by compositeconstruction (i.e. by an organized mixture of components). Theprimary source of toughening in plants is the composite cellwall (cellulosic microfibrils set in a hemicellulose and, sometimes,lignin matrix), with a toughness of 3.45 kJ m^-2, which is ten-timesthe probable toughness of its individual components if theycould be isolated. The toughness of most plant tissues is roughlyproportional to the volume fraction of tissue occupied by cellwall (V_c) and, compared to animal tissues and non-biologicalcomposites, is very low. High toughness in plant cells is notproduced by the walls themselves, but by their plastic intracellularcollapse. This is a truly cellular toughening mechanism, oneof the most potent ever discovered by materials scientists,depending on an elongate cell shape with microfibrils directeduniformly at a small angle to the cellular axis. Only ‘woody’cells, tracheids and fibres, have this framework and only inthe S2 layer of their secondary wall. Despite this non-optimumconfiguration, toughness is elevated by this mechanism ten-timesabove that due to cell wall resistance alone. The effectivenessof toughness in preventing herbivory is indisputable, but largelyindirect due to confusion over a false equivalence between nutritional‘fibre content’ and toughness. In contrast, generalizedhardness requires high density. If hardness is due to high V_c, this conflicts with ‘woody’ toughness becausethere is then no lumen for cell walls to collapse into. Thus,dense seed shells may be brittle (i.e. low toughness) even ifbuilt from fibres. However, solid cell wall is not very hard.Instead, high hardness in plants is associated with amorphoussilica and is always localized. The efficacy of hardness ismore difficult to evaluate than toughness because some animalsspecialize in coping with it. Copyright 2000 Annals of BotanyCompany Review, hardness, toughness, mechanical defence, herbivory, cell wall, plastic buckling, silica     

The Effect of Drugs on the Growth of Lemna minor C.

Lemna minor fronds were grown for periods of between 90 and190 days in nutrient solutions to which non-lethal doses ofone of the following drugs were added: sodium nitrite, sodiumazide, brilliant green, proflavine, propamidine isethionate,and dinitrobutyl phenol. Over these periods there was no signof any increase in the growth-rate with time, such as mightbe expected if Lemna showed adaptation to drugs similar to thatfound in micro-organisms. ‘Changeover’ experimentsshowed that the growth-rate of fronds in a drug solution wasgenerally less if they had been previously exposed to that drugthan if they had not. The sensitivity of the fronds, inducedby growth in solutions containing sodium nitrite, persistedover five to six generations of growth in nitrite-free solutions.     

Pollen--stigma Interaction in the Leguminosae: the Secretory System of the Style in Trifolium pratense L.

The canal that traverses the upper part of the style of Trifoliumpratense is derived lysigenously. The core tissue of the veryyoung style consists of elongated cells similar to those ofthe transmitting tissue of solid-style families such as theSolanaceae; as the style matures, these cells separate to formthe canal, which receives secretions both from the core tissueand the inner wall cells. The early secretion of proteins intothe intercellular spaces is associated with the presence ofparamural bodies (lomasomes) in the adjacent cells. In the cellsin the immediate vicinity of the canal, vesicles, probably derivedfrom the Golgi system enlarge during later development and accumulatea protein-carbohydrate content, which is later passed into thecytoplasm where it forms densely packed fibrillar nodules. Withthe dissolution of the cell membranes, this material is passedinto the canal, where it is progressively diluted by continuedingress of water until the cavity reaches its final volume. Leguminosae, Trifolium pratense L., pollen—stigma interaction, self-incompatibility, stylar secretion, protein secretion     

Studies on Mature Seeds of Cuscuta pedicellata and C. campestris by Electron Microscopy

The seeds of Cuscuta pedicellata have been investigated by transmissionand scanning electron microscopy. Additional observations havebeen made on seeds of C. campestris by SEM only. The seed coatconsists of an outer single epidermis, two different palisadelayers, and an inner multiparenchyma layer. The outer epidermalwall in C. pedicellata has a thick cuticle and zones rich inpectic substances. The thicker ‘U-shaped’ cell wallsin the outer palisade layer are strengthened by a wall layerof hemicellulose. The inner palisade layer has thick walledcells with a ‘light line’. The inner cell wall ofthe compressed multiparenchyma layer has a thin cuticle. A fairlythick cuticle is positioned directly on the endosperm surface.The aleurone cell walls are different from the remaining endospermwalls. The latter are thick and believed to be of galactomannans.There is a ‘clear’ zone between the plasmalemmaand the cell wall in the aleurone cells. The embryo cells arepacked with lipids and proteins. In Cuscuta campestris mostendosperm has been absorbed during the seed development. Theembryo apex has two minute leaf primordia. The features of theCuscuta seeds are discussed in relation to functional and environmentalconditions. Cuscuta pedicellata, Cuscuta campestris, seed, seed coat, cuticle, cell walls, endosperm, aleurone cells, galactomannan, embryo, TEM, SEM     

Modelling the Mechanical Properties of Xylem Tissue from Tobacco Plants (Nicotiana tabacum'Samsun') by Considering the Importance of Molecular and Micromechanisms

Tobacco plants (Nicotiana tabacum‘Samsun’) havexylem cell walls which are more sensitive to changes in theproperties of the matrix than was predicted using current cellwall mechanical models. A model is proposed which can accountfor the importance of the cell wall matrix in determining themodulus of the material. This model is based on a helical springsystem, with micromechanisms operating at the molecular scale.Xylem tissue fibre cells can behave as helical springs whenthe crosslink density of lignin is low, or they can behave likea composite sheet with fibres at an angle to the applied loadwhen the crosslink density is high. This highlights the importanceof molecular modelling when the properties of complex biologicalmaterials are being investigated.Copyright 1998 Annals of BotanyCompany Tobacco,Nicotiana tabacum, xylem tissue, Young's modulus, matrix polymer connectivity, plant biomechanics.     

WATER PERMEABILITY OF THE CELL WALL IN NITELLA

1. A method has been developed to measure the hydraulic conductivityof the wall of the internodal cell of Nitella flexilis.
2. Therate of water penetration through the cell wall varies linearlywith the hydrostatic pressure difference between the two sidesof the wall, showing that water permeability of the cell wallremains independent of the pressure difference applied.
3. Waterpermeability of the cell wall is inversely proportionalto itsthickness It is 30µµmin^–3{dot}atm^–3when the thickness of the wall is 10 µ.
4. Water permeabilityof the cell wall is the same for inward andoutward water flow.The polar water permeability of the entiremembrane system (walland protoplasmic part) of the living celldemonstrated by KAMIYAand TAZAWA (1) is, therefore, due tothe living protoplasmicpart.
5. The ratio of the inward to outward permeability constantsofthe protoplasmic layer alone is higher than that of the entiremembrane system composed of protoplasmic layer and cell wall.

¹ Dedicated to Prof. H. TAMIYA on the occasion of his 60th birthday.The present work was supported in part by a Grant-in-Aid forFundamental Scientific Research from the Ministry of Education. ² Present address: Sh?in Women's College, Kobe. (Received July 21, 1962; )     

The Kinetics of Tracheid Development in Tsuga canadensis Carr. and its Relation to Tree Vigour

Cell counts from samples taken at weekly intervals, from 14May to 22 October 1969, in a T. canadensis stand in Massachusetts,U.S.A., showed that the width of the annual ring was correlatedwith the rate of cell production and that only the least vigoroustrees (c. 20 tracheids year^–1) had a shorter growing season.The time required for completion of a cell-division cycle inthe cambial zone decreased during the course of the season,from 35 to 20 days for the less vigorous trees (25–45tracheids year^–1) and from 28 to 10 days for the morevigorous trees (45–100 tracheids year^–1). The timerequired for the completion of radial growth of the tracheidsdecreased from 18 to 9 days, with no evidence of any changeswith tree vigour. The actual radial growth-rate of the tracheidswas constant within the range 1.5–3 um day^–1. Thetime required for deposition of the secondary cell wall increasedfrom 10 to 50 days, with little evidence of any changes withtree vigour. The actual rate of deposition of cell wall materialwas about 0.15 µ² µ^–1 day^–1 and seemedto show little change during the course of the season. The timeperiod required for lysis of the cytoplasm was about 4 days,with no evidence of any changes with tree vigour and littleevidence of any changes during the course of the season.     

Role of elastin anisotropy in structural strain energy functions of arterial tissue

The vascular wall exhibits nonlinear anisotropic mechanical properties. The identification of a strain energy function (SEF) is the preferred method to describe its complex nonlinear elastic properties. Earlier constituent-based SEF models, where elastin is modeled as an isotropic material, failed in describing accurately the tissue response to inflation–extension loading. We hypothesized that these shortcomings are partly due to unaccounted anisotropic properties of elastin. We performed inflation–extension tests on common carotid of rabbits before and after enzymatic degradation of elastin and applied constituent-based SEFs, with both an isotropic and an anisotropic elastin part, on the experimental data. We used transmission electron microscopy (TEM) and serial block-face scanning electron microscopy (SBFSEM) to provide direct structural evidence of the assumed anisotropy. In intact arteries, the SEF including anisotropic elastin with one family of fibers in the circumferential direction fitted better the inflation–extension data than the isotropic SEF. This was supported by TEM and SBFSEM imaging, which showed interlamellar elastin fibers in the circumferential direction. In elastin-degraded arteries, both SEFs succeeded equally well in predicting anisotropic wall behavior. In elastase-treated arteries fitted with the anisotropic SEF for elastin, collagen engaged later than in intact arteries. We conclude that constituent-based models with an anisotropic elastin part characterize more accurately the mechanical properties of the arterial wall when compared to models with simply an isotropic elastin. Microstructural imaging based on electron microscopy techniques provided evidence for elastin anisotropy. Finally, the model suggests a later and less abrupt collagen engagement after elastase treatment.     

Influence of Environmental Factors on Cell Production and Differentiation in the Early Wood of Picea sitchensis

Three phases are identified in tracheid production by the mainstem of a 12th year forest-grown tree of Picea sitchensis: cellproduction, radial cell expansion and cell wall thickening.For 15 days during formation of early wood daily cell productionwas positively correlated (P=001) with daily solar radiation.Increases in cell production appear related to time-lagged increasesin numbers of cells undergoing expansion and then wall thickening. Radial diameter and wall thickness were measured for each individualcell along the post wall thickened section of radial files oftracheids. The radial diameters of neighbouring cells were positivelycorrelated, groups of 4–5 large cells tending to alternatewith groups of small cells. Furthermore, large cells had thickerwalls than small cells. It is suggested that this distinctivestructure is the result of fluctuations in the environ mentwhich induce changes in the rates of cell production. An increasein cell production increases the length of the queue of cellswaiting to pass successively out of the zones of cell expansionand wall thickening. Consequently cells spend longer in thedevelopment stages, they expand to a greater diameter and theirwalls thicken to a greater thickness than when fewer cells areproduced and the queue is shorter. We suggest that to investigate the influence which weather mayhave on the dynamics of tracheid pro duction it is necessaryto sample developing wood with a frequency twice as great asthat of the frequency of change of the weather variable underconsideration.     

Nitrate Supply and the Biophysics of Leaf Growth in Salix viminalis

The influence of nitrogen on leaf area development and the biophysicsof leaf growth was studied using clonal plants of the shrubwillow, Salix viminalis grown with either optimal (High N) orsub-optimal (Low N) supplies of nitrate. Leaf growth rate andfinal leaf size were reduced in the sub-optimal treatment andthe data suggest that in young rapidly growing leaves, thiswas primarily due to changes in cell wall properties, sincecell wall extensibility (% plasticity) was reduced in the LowN plants. The biophysical regulation of leaf cell expansion also differedwith nitrogen treatment as leaves aged. In the High N leaves,leaf cell turgor pressure (P) increased with age whilst in theLow N leaves P declined with age, again suggesting that foryoung leaves, cell wall plasticity limited expansion in theLow N plants. Measurements of cell wall properties showed thatcell wall elasticity (%E) was not influenced by nitrogen treatmentand remained constant regardless of leaf age. Key words: Salix, cell wall extensibility, nitrogen nutrition, biophysics of leaf growth     

Mechanisms of the acido- and thermo-phily of Cyanidium caldarium Geitler II. Physiological role of the cell wall

1) A partially disintegrated cell preparation of Cyanidium caldarium(i.e. the 100G fraction; cells from which the outer parts ofthe cell wall had been removed) was obtained by differentialcentrifugation of a cell suspension treated with a French press.2) The 100G fraction cells had completely lost the outer partsof their cell walls, but retained their subcellular structureunchanged and had Hill activity that was equally as high asthat of the intact cells. 3) The Hill activity of the 100G fractionshowed an optimum at pH 7.0 and at 35°C. This activity waslost under acid and high temperature conditions (e.g. pH 3,50°C) under which intact cells showed high activity. 4)The Hill activity of the 100G fraction was lost by heat treatment(55°C, 10min), acid-treatment (pH 3.0, 10 min) or pre-illumination(3xl0⁵lux, 30 min) though intact cells were not inactivatedby these treatments. However, no remarkable difference in sensitivitytowards inhibitors was found between the 100G fraction and intactcells. 5) We thus concluded that the cell wall plays an importantrole in the acido- and thermo-phily of Cyanidium cells. (Received August 19, 1974; )     

Structure and Function of Silica Bodies in the Epidermal System of Grass Shoots

Silica (SiO₂.nH₂O) is deposited in large quantities in the shootsystems of grasses. In the leaf epidermal system, it is incorporatedinto the cell wall matrix, primarily of outer epidermal walls,and within the lumena of some types of epidermal cells. This biogenic silica can be stained specifically with methylred, crystal violet lactone, and silver amine chromate. At theultrastructural level, the silica in lumens of silica cells,bulliform cells and long epidermal cells is made up of rodsabout 2.5 µm in length and 0.4µm in width. Ultimateparticles in the rods range from 1 to 2 nm in diameter. In contrast,silica in the cell wall matrix of trichomes and outer wallsof long epidermal cells is not rod-shaped, but rather, formsroughly spherical masses. Detailed analyses are presented on the frequencies of occurrenceof the different types of epidermal cells that contain silicain the leaves of representative C₃ and C₄ grasses. The C₄ grasseshave higher frequencies of bulliform cell clusters, silica cells,and long epidermal cells, whereas the C₃ grasses have higherfrequencies of trichomes. No correlation was found in the frequencyof occurrence of silica bodies in bulliform cells for C₃ grassesas compared with C₄ grasses. Of all the grasses examined, Coix,Oryza, and Eleusine had the highest densities of such bodies,and some taxa had no silica bodies apparent in their bulliformcells. The idea that silica bodies in bulliform cells and silica cellsmight act as "windows’ and trichomes might function as‘light pipes’ to facilitate light transmission throughthe epidermal system to photosynthetic mesophyll tissue belowwas tested. The experimental data presented do not support eitherof these hypotheses. C₂ and C₄ grasses, biogenic silica, light pipes, window hypothesis, silica staining, silica ultrastructure     

The Ultrastructure and Analytical Microscopy of Silicon Deposits in the Roots of Saccharum officinarum (L.)

Silicon accumulation in the endodermis of the ‘set’and ‘shoot’ roots of Saccharum officinarum (L.)were investigated by scanning electron microscopy and electron-probemicroanalysis. Silicon microassay was also carried out by meansof the Corinth analytical microscope (CORA). Aggregates arelargely associated with the inner tangential wall (ITW) of theendodermis and their formation is basically similar to thoseseen in Sorghum bicolor (L.) Moench. In contrast to Sorghumthe earliest deposits in Saccharum appear in wall strata wellwithin the cell wall cytoplasm interface. An additional layerof silicon was also located along the endodermal pericycle boundaryextending some distance along the middle lamella of the radialwalls. The results are discussed in relation to those of previous studiesof silicon accumulation in endodermal cells and to possiblefactors affecting such accumulations.     

Alteration of the Cell Surface during the Vegetative Cell Cycle of the Unicellular Green Alga Chlamydomonas reinhardtii

Alterations of the cell surface during the vegetative cell cycleof the unicellular green alga Chlamydomonas reinhardtii wereinvestigated using polyclonal antibodies against the purifiedand subsequently deglycosylated insoluble cell wall componentand against a 100 kDa polypeptide of the deglycosylated, chaotrope-solublewall fraction, respectively. Both antibodies recognized epitopeswithin the non-glycosylated domains of a ‘150 kDa’chaotrope-soluble glycoprotein (=GP3B) localized in the outerlayers of the C. reinhardtii cell wall. Immunofluorescence studiesindicated that both antibodies reacted with the surface of ‘late’sporangia (harvested 1 h before liberation of the zoospores),but not with the cell surfaces of released zoospores, growingcells and young sporangia, respectively. After pretreatmentwith aqueous LiCl, however, the cell surfaces of zoospores,growing cells and young sporangia became accessible to theseparticular antibodies. Highly purified preparations of the insolublewall fraction revealed strong immunofluorescence with both antibodiesbut not with the corresponding preimmune sera. Based on thesedata, we concluded that the antigenic sites of the insolubleglycoprotein framework of the C. reinhardtii wall are maskedby LiCl-soluble glycoproteins in single cell stages and youngsporangia, but not or to a lesser extent in the case of themother walls of ‘late’ sporangia. The conclusionwas supported by findings that (I) the multilayered structureof the mother-cell wall was disturbed in ‘late’,but not in young sporangia and that (II) the amounts of chaotropesolublecell wall glycoproteins present in the LiCl-extracts from intactsporangia decreased during ripening of the sporangia. (Received January 10, 1996; Accepted May 27, 1996)     

The Occurrence of Endogenous Rhythms in the Coleoptiles in Various Cereal Genera

The rate of growth of the coleoptiles was determined from photographstaken by infra-red radiation. CO₂ output was measured by meansof an infra-red gas analyser. The rhythm of CO₂ output from the coleoptile of Avena was inducedby a change from red light to darkness. It has a period of about24 hours and agrees in timing with the growth-rate rhythm previouslyrecorded. Some degree of rhythmicity in the growth-rate was found in Triticumvulgare (var. ‘Eclipse’) and in Secale cereale (var.Petkus). Very slight indications of rhythmicity were found inTriticum spelta and in Hordeum vulgare. Negative results wereobtained with Oryza sativa and with Zea mays. Where rhythmicityin the coleoptile is less strongly developed, the peaks comecloser together, the interval being about 18–20 hours.Cereals cannot be sharply separated into two groups accordingto the presence or absence of rhythmicity in the coleoptile.Of the genera examined, the most marked endogenous rhythms occurin Avena. It is doubtful if the ability of the coleoptile toexhibit an endogenous rhythm has any beneficial effect on thedevelopment of the seedling. Under normal conditions of germinationinduction of the rhythm would not occur.     

Ultrastructural Changes of the Egg Apparatus Associated with Fertilization and Proembryo Development of Soybean, Glycine max (Fabaceae)

As part of a study involving pod retention in soybean, Glycinemax (L.) Merr., we investigated changes occurring in the eggapparatus of non-abscised flowers from the time immediatelypreceding fertilization through early embryogeny. Prior to theentry of the pollen tube into the embryo sac, one of the synergidsbegins to degenerate as evidenced by increased electron densityand a loss of volume. This cell serves as the site of entryfor the pollen tube. The cytoplasm of the second, or persistentsynergid, remains unaltered until after fertilization. Bothsynergids contain, in addition to a filiform apparatus, a singleunidentified inclusion of flocculent material located in thechalazal portion of each cell. The zygote can be distinguishedfrom the egg by its consistently narrow wall; and it dividesto form a proembryo, a mass of cells not yet differentiatedinto embryo proper and suspensor. The basal cells of the proembryoare more vacuolate than the apical ones, characteristic of thebasal vacuolation of both egg and zygote. Cells of the proembryoare connected to one another via plasmodesmata, and with theexception of the basal-most cell, are isolated symplasticallyfrom the surrounding endosperm. Wall ingrowths frequently occurin certain cells of the proembryo, notably those cells in contactwith the degenerate synergid and embryo sac wall. At a laterstage of ontogeny, by which time the globular embryo properhas become distinct from the suspensor, the wall ingrowths areconcentrated in the suspensor. Glycine max, soybean, embryogeny, synergids     

The Effect of Artificial Wind on the Growth-rate of Plants in Water Culture: With Plate and three Figures in the Text

Young plants of Brassica napus (rape), Hordeum vulgare, andPisum sativum growing in water culture have been exposed tofour continuous wind speeds for from 4 to 5 days. The plantswere exposed to natural daylight, humidity, and temperaturein a wind tunnel in which only air movement was controlled.The wind speeds found among the plants in the four sectionsof this tunnel were approximately 0·3, 0·7, 1·7,and 4·0 m./sec. The results showed no significant changein relative growth-rate or net assinilation rate with wind speed. Previously published results, obtained with plants in soil orsand culture, have differed from those in the present experimentsin showing a fall in the amount of growth as wind speed increased.It is suggested that the reduction in growth found in theseprevious experiments was caused by partial drying out of theplants, whilst in the present experiments water-supply to theroots was abundant and this effect was considerably reduced.     

The Effect of pH on the Binding of Calcium to Pea Epicotyl Cell Walls and its Implications for the Control of Cell Extension

Cell walls were prepared from the epicotyls of dark-grown pea(Pisum sativum L.) seedlings. The walls were found to bind externally-added⁴⁵Ca²⁺, with a binding constant of 4 ? 10^–4 mol dm^–3and a maximum capacity of 1.5 ? 10^–8 g-ions of Ca²⁺ perg fresh weight of epicotyl. The binding capacity decreased asthe pH of the medium was decreased below 6.0, suggesting thatthe calcium was bound by an anionic group with an apparent pKof 4.7. More than half the calcium binding was due to polygalacturonicacid in the wall, since up to 60% of the calcium binding capacitywas removed by pre-incubation of the cell walls with polygalacturonase(E.C.3.2.1.15). Only small decreases in calcium binding wereseen following pre-incubation with protease, nucleases, phospholipaseand hemicellulase. These results indicate that calcium willbe displaced from the cell wall at hydrogen ion concentrationswhich are known to occur in the wall during wall extension.They are consistent with a mechanism by which calcium inhibitswall extension by forming ionic bridges between polygalacturonicacid molecules, and also with the hypothesis that calcium andhydrogen ions exert opposing influences on cell wall extensionby competing for the same binding sites on the polygalacturonicacid. Key words: Pea epicotyl, Cell wall, Calcium, pH