Studies in Stomatal Behaviour: II THE ROLE OF STARCH IN THE LIGHT RESPONSE OF STOMATA PART 4. VARIATION UNDER CONSTANT CONDITIONS

A quantitative study has been made of the extent and natureof the variation in starch-content and aperture of the stomataof Pelargonium. It is shown that distribution over the surfaceof a single leaf is substantially normal in form; that underexperimental conditions in common use there is a highly significantdifference between groups of stomata on the lower epidermisof a single leaf; that the variation between different leaveson the same plant, or between similar leaves on different plants,does not differ significantly from that between strips fromthe same leaf; that not less than 20 stomata should be measuredon each strip; and that pore-width is a satisfactory measureof the area of the stomatal aperture.     

Cellular Interactions in the Development of Stomatal Patterns in Vinca major L

The development of the leaf epidermis of Vinca major L. wasfollowed in situ by epi-illumination microscopy and evidencewas sought for cellular interactions. Stomata were often foundto be initiated in adjoining cells. The epidermal cells whichseparated such stomata when they had matured were formed fromthe same cells as the stomatal complexes themselves. The presenceof developing and mature stomata may influence only the orientationof divisions in neighbouring cells, and not the initiation andmaturation of stomata. There is great variability in the relativeorientation, timing and number of divisions which intervenebetween the first unequal division and the maturation of a stomaas well as the location of stomata relative to the spongy mesophylland minor veins. The results indicate that continuous short-rangeinteractions between the future guard cells and the adjoiningcells, rather than interactions between future stomata or afixed programme of development, are essential for the formationof the pattern of functional stomata in the mature leaf. Vinca major L., cell lineage, cellular interactions, development of stomata, epi-illumination microscopy, meristemoids, patterned differentiation, stomata     

Some Differences in the Responses of Stomata of the Two Leaf Surfaces in Cotton

Some differences in the responses of the upper and lower stomatain cotton (Gossypium hirsutuni) are presented. These differenceswere observed in the course of some studies in which the transpirationof the two leaf surfaces was measured under controlled environmentconditions and the transpiration data used as an estimate ofstomatal response. In darkness the upper stomata were more or less effectivelyclosed while the lower stomata were partially open. Upon illuminationof the leaf with non-saturation or saturation radiation theupper stomata were slower to open than the lower stomata. Thereductions in stomatal aperture which occurs with the increasein age of leaves commenced earlier in the upper stomata andproceeded at a faster rate than the lower stomata. Sudden exposureto saturation radiation caused the stomata of the two leaf surfacesto oscillate. These oscillations were not observed in youngleaves but in older leaves. During ageing of leaves oscillationsof the upper stomata commenced earlier than oscillations ofthe lower stomata. When the petiole was excised in darknessor light the upper stomata showed a transient increase but notthe lower stomata. Gossypium hirsutum, stomatal responses, transpiration     

Ochna pulchra Hook: Leaf Growth and Development Related to Photosynthetic Activity

Ochna pulchra Hook. is a deciduous broad-leaved tree in theMixed Bushveld vegetation of the Northern Transvaal. The growthand development of leaves taken from trees in the field werestudied from a stage shortly before bud break, in late spring,until they were fully expanded and at the peak of photosyntheticactivity. Leaf area was measured by photographing the leaf against a transparentmm² grid. Finally a constant relationship between leaf area(A) and the linear dimension of length (L) and breadth (B) wasestablished: A = b x LB, where coefficient b = 0.72. Transverse sections of the lamina of the youngest leaves showeda five-layered plate meristem with a few functional conductingelements in the midrib. During further leaf development, celldivision was followed by means of autoradiography using [³H]thymidine.It was most active during the week after bud break. Leaf cell increment following on cell division made the majorcontribution to leaf growth resulting in a lamina that was atleast 90% expanded 4 weeks after bud break. The histologicalchanges accompanying cell division were observed using lightand electron microscopy. Even in late stages of leaf development mature and differentiatingstomata occurred together, limited to the abaxial epidermisand the midrib. Scanning electron microscopy showed stomataldistribution, their increasing density and gradual opening.The structure of these sunken stomata could reduce the outwarddiffusion of water vapour and increase the diffusion resistanceto carbon dioxide. Carbon assimilation rates of the developing leaves were measuredusing an IRGA (infra-red gas analyser) and their chlorophyllvalues were calculated. Photosynthesis was first measured amonth after bud break when the leaves were fully expanded, over50 % of the stomata exposed and leaf mesophyll tissue differentiatedwith mature chloroplasts. Net photosynthetic rates and chlorophyllvalues peaked 1 month later. Ochna pulchra Hook., photosynthesis, leaf development, leaf area, stomata, chlorophyll, savanna     

Studies in Stomatal Behaviour: V. THE ROLE OF CARBON DIOXIDE IN THE LIGHT RESPONSE OF STOMATA

It was found that stomata on illuminated leaves, both of Pelargoniumand wheat, opened much wider where the leaf surface was enclosedin a small volume of air, as in a normal porometer cup, thanelsewhere. This was shown for both species by the infiltrationmethod, and for Pelargonium by Lloyd's method and direct microscopicalobservation also. The effect was shown not to be due to pressure of the porometercup or glass plate on the leaf, or to temperature differences,nor directly to the lack of movement or high humidity of theenclosed air. A considerable body of data was collected which appeared tosupport the hypothesis that the wide opening was due to accumulationof some volatile substance produced by the leaf, but all theresults were also consistent with the view that it was causedby reduction in the carbon dioxide content of the enclosed airbelow the normal 0·03 per cent. owing to photosynthesis.Further crucial experiments with both the porometer and infiltrationmethods left virtually no doubt that the latter hypothesis wascorrect. This extreme sensitivity of stomata to carbon dioxide concentrationwithin the range 0·03 per cent. to zero is discussedin relation to their operation in nature, and a possible biologicaladvantage is suggested. The bearing of the effect upon porometer investigations is alsodiscussed and it is concluded that for all quantitative or semi-quantitativeexperimentation it is essential to use a cup detached betweenreadings, or at least swept with air such as surrounds the restof the leaf, and to have the upper leaf surface above the cuparea freely exposed or similarly swept. For qualitative investigationof the light response of stomata the traditional form of cupmay be used. The importance is stressed of allowing porometer readings toreach equilibrium under one set of conditions before changingto another, when investigating the ‘closing’ or‘opening’ effects of external factors. Several subsidiary effects, observed in the course of the investigation,are discussed; in particular an effect of humidity upon therate of response to other factors.     

Ontogeny of Stomata on the Foliar and Floral Or{dot}gans of some Species of Crotalaria L.

The structure and development of stomata in eight species ofCrotalaria belonging to the family Papilionaceae are described.The study is mostly confined to the leaves but it has also beenmade on the floral organs of C. mysorensis, C. retusa, C. sericea,and C. triquetra. The stomata may be paracytic, anisocytic,anomocytic, diacytic, or with one subsidiary cell. The differenttypes occur individually or they are placed side by side evenon the same surface of an organ. In general the paracytic typeis by far the commonest, followed by anisocytic and anomocyticones. Diacytic stomata and those with one subsidiary cell arerelatively rare. Different types of stomata in various organsof the same plant develop mesogenously. The present investigationalso indicates that the inconstancy of stomata in the familyis due to (a) their diversity and (b) an increase in the numberof subsidiary cells either by their division or by the neighbouringperigenes becoming subsidiary cell-like.     

Variable Development and Cellular Patterning in the Epidermis of Ruscus hypoglossum

The general problem was the meaning of the variability of cellulardevelopment of the stomata-bearing epidermis of Ruscus hypoglossumin which 'immature stomata' occur, i.e. cells that have gonethrough part but not the final stages of stomatal development.The development of the epidermis was followed in vivo, by makingrepeated replicas of the same developing tissues using dentalimpression material. The development of stomata occurred overthe entire surface of large phyllodes and was not synchronous.The early development of future and 'immature stomata' couldnot be distinguished, neither by the form of the cells nor bythe timing of the initial, unequal divisions. The process ofstomatal development did not stop at any one, characteristicsstage. Statistical analyses indicated that the pattern of functionalstomata would have been less orderly if all stomatal initialshad developed into mature structures. The results suggest thatin Ruscus epidermal patterning occurs during, rather than preceding,stomatal development: many stomata are initiated, but they followa labile developmental program and cellular interactions selectthose that reach the mature functional state.Copyright 1993,1999 Academic Press Cellular patterns, epigenetic selection, immature stomata, Ruscus hypoglossum, spacing patterns, stomata, subsidiary cells, unequal divisions     

The Effects of Cytokinins and ABA on Stomatal Behaviour of Maize and Commelina

Epidermal strips and leaf fragments of Commelina and leaf fragmentsof maize were incubated on solutions containing naturally-occurringor synthetic cytokinins and/or ABA. The effects of these treatmentson stomatal behaviour were assessed. Cytokinins alone did notpromote stomatal opening in either species but concentrationsof both zeatin and kinetin from 10^–3 to 10^–1 molm^–3 caused some reversal of ABA-stimulated closure ofmaize stomata. The reversal of the ABA effect increased withincreasing cytokinin concentration. Cytokinins had no effecton ABA-stimulated closure of Commelina stomata. When appliedalone, at high concentration (10^–1 mol m^–3), toCommelina epidermis or leaf pieces both zeatin and kinetin restrictedstomatal opening. Key words: ABA, Cytokinins, Stomata, Maize, Commelina     

The Diffusive Conductivity of the Stomata of Wheat Leaves

A leaf chamber (described in detail) was used alternately witha resistance porometer to measure resistance to viscous flowof air through the leaf, and with a diffusion porometer to measurethe differential diffusive flow of hydrogen and air (V_H–V_A)through the leaf and the component of hydrogen flow (V'_H) movingstraight across the leaf. The resistance of the mesophyll isneeded for interpretation: estimates by three different methodsfor viscous flow did not agree very well, but two differentmethods for diffusive flow gave good agreement. For wheat leaves,only very large errors are important. Formal analysis is in three appendixes: I. Interpretation ofviscous and diffusive flow in small pores involves some problemsin molecular physics, complicated by the particular geometryof the wheat stoma. With some uncertainty, formal expressionsare derived for the viscous resistance of a single stoma, rv,and for the resistances to diffusion of hydrogen and air, andof water vapour and carbon dioxide, all expressed as r_s persquare centimetre of leaf surface. The analysis for hydrogen/airis the most uncertain; that for water vapour and carbon dioxideis more reliable. II. An indication is given of the flow characteristicsof the leaf-chamber system, from which rv can be derived, andof the basis for estimating mesophyll resistance. III. The methodof converting estimates of r_s into estimates of V_H–V_Aand V'_H is given. The results presented are expressed as nearly as possible interms of the quantities which were measured. For five leavesthe dependence of V_H–V_A on V'_H agrees well with theoreticalpredictions; the dependence of V_H–V_A (and V'_H) on rv,on average, agrees well with prediction, but involves the assumptionthat the stomata get shorter as they close. The agreement isgood enough to suggest that the formal expressions for r_s interms of stomatal dimensions and molecular gas constants arereliable enough to be carried forward into future transpirationand assimilation studies. The minimum value of ra for watervapour (c. 3 sec cm⁺¹) is close to values found elsewhere bydifferent techniques. At very small stomatal openings there was a large deviationfrom predicted behaviour, such as would occur if the imposedexcess air pressure further closed the stomata during viscousflow experiments.     

Epidermal Structures and Stomatal Ontogeny in Some Mimosaceae

The structure of trichomes and stomata on leaflets of 21 speciesof the Mimosaceae are described. Non-glandular trichomes inMimosa pudica are of three types: unicellular, with a roundedthick-walled base and a terminal unicellular body, and multicellular.Capitate, clavate, or cylindric, 3–6-celled glandularhairs are observed on leaflets of Mimosa pudica only. Leafletsare amphistomatic in all species except Adenanthera pavonina,Calliandra sp., Parkia biglandulosa, Pithecellobium dulce, andSamanea saman in which they are hypostomatic. Only paracyticstomata are found in Leucaena leucocephala and Mimosa pudica.In the rest stomata are of more than one type. In spite of thediversity, the most frequent type in these species is paracytic.Anisocytic stomata, in all cases, are secondarily derived fromparacytic ones by transverse or oblique wall formation in asubsidiary cell. Similarly some stomata with one subsidiarycell are also secondary derivatives of the paracytic ones becauseof one of the subsidiary cells assuming the form of an epidermalcell. The development has been traced in 14 species and thatof paracytic stomata may be mesogenous or mesoperigenous, thatof stomata with one subsidiary cell mesogenous but anomocyticstomata are ontogenetically perigenous. Occasionally a meristernoidis cut off from one of the subsidiary cells of a paracytic stoma.The organization of a stoma from such a meristemoid has beentraced.     

Distribution of Lithocysts, Trichomes, Hydathodes and Stomata in Leaves of Pilea cadierei Gagnep. & Guill. (Urticaceae)

In leaves of Pilea cadierei lithocysts, trichomes and hydathodesin the upper epidermis, and lithocysts, trichomes and anomocyticstomata in the lower epidermis are components of a regular distributionpattern in which spacing probably depends upon inhibitory interactionsbetween initial cells during leaf development. Anisocytic stomata,which also occur in the lower epidermis, show a clumped distributionpattern that results from their relatively late formation aftercell division in the epidermis is otherwise complete, and fromthe inhibitory influence of established lithocysts, trichomesand anomocytic stomata. Ordered spacing of anisocytic stomatawithin groups probably results from mutual inhibition betweeninitial cells at the time of inception. Lithocysts associatedwith primary and secondary veins are elongated along the veinaxis. Elsewhere in the leaf lithocyst, orientation is obliquerelative to the midrib and shows a correlation with vein orientationaround the time of lithocysts elongation. Lithocyst, cystolith, trichome, hydathode, stomata, Pilea cadierei     

Separation of Direct and Indirect Responses of Stomata to Light: Results from a Leaf Inversion Experiment at Constant Intercellular CO2 Molar Fraction

Previous work has shown that stomata respond directly to light,but it was not clear whether the only additional response isthrough CO₂, or whether some other metabolite is involved inthis response. Gas exchange experiments were done with normallypositioned and inverted leaves of Hedera helix to investigatethis problem. The macroscopic optical properties of the leavesand their anatomical structure were also studied. These experimentssnowed that there is no need to postulate the existence of amessenger other than CO₂ to explain the indirect response ofstomata to light. The experiments also showed that leaf inversionaffects both stomatal conductance and photosynthesis, and highlightthe difficulties involved in the interpretation of the effectof leaf inversion on stomata when stomatal conductance measurementsare not done concurrently with measurements of CO₂ flux densityand intercellular CO₂ molar fraction Key words: Hedera helix, ivy, gas exchange, leaf inversion, stomatal conductance, light, CO₂ flux density, photosynthesis     

Stomatal Features in the Leaf of Aganosma dichotoma (Roth) K. Schum

Paracytic and anisocytic types of mature stomata are found inthe leaf of Aganosma dichotoma. Stomata with one guard cell,stomata with degenerated guard cells, and contiguous stomataare common. Stomata with arrested pore development are alsofound in certain cases. A single guard cell without any porehas not been designated as a stoma with one guard cell in thepresent investigation. Ontogeny of contiguous stomata have beentraced. Subsidiary cells are, morphologically, just like theircontiguous guard cells. Subsidiary cells may retain their shapeand contents even when their contiguous stoma becomes mature,or may change their shape and lose their contents. They mayor may not divide. Subsidiary cells form a whorl of more thantwo subsidiary cells around a stoma by their divisions. Degenerationof guard cell(s)— their contents and nuclei—havebeen traced. In certain cases guard cells divide forming morethan two guard cells associated to a single pore. Cytoplasmicconnections are found between two guard cells of nearby stomata,and between a guard cell and an epidermal cell. Near the wound,the epidermal cells over the veins become meristermatic givingrise to new epidermal cells but no meristemoid.     

Stomatal Responses to Water Deficits and Abscisic Acid in Leaves of Sunflower Plants (Helianthus annuus L.) Grown under Different Conditions

The decrease in diffusive conductance of a leaf exposed to waterstress or to exogenous abscisic acid (ABA) was smaller in leavesof sunflower plants (Helianthus annuus L. cv. NK285) that hadbeen grown in a phytotron in humid air than in leaves of sunflowersgrown outdoors. Stomata of the phytotron-grown plants were slowerto close after detachment of a leaf than those of the outdoorplants. When stomata closed rapidly, as they did in detachedleaves and after treatment with ABA, the extent of closure wasvaried over the leaf's surface, in particular in the case ofphytotron-grown plants, and the extent of the heterogeneitywas greater in the phytotrongrown plants than in the outdoorplants. When stomata closed gradually, for example, under conditionsof limited moisture in the soil, closure occurred uniformlyover leaves of plants of both types. The smaller decrease indiffusive conductance of leaves from phytotron-grown plantsafter treatment with ABA resulted from the presence of patcheson the surface in which stomata remained open. The smaller decreaseof diffusive conductance in the phytotron-grown plants underconditions of limited moisture in the soil resulted from theuniformly lower responsiveness of stomata on a leaf to the decreasein water potential. When estimates are made of the intercellularconcentration of CO₂ (Ci) from gas-exchange measurements, heterogeneityin stomatal closure should be monitored when stomata close rapidly,in particular in plants grown in humid air, because heterogeneousstomatal closure can lead to overestimates of Ci. (Received April 18, 1994; Accepted May 25, 1995)     

Abscisic Acid and Water Relations of Rice (Oryza sativa L.): Effects of Drought Conditioning on Abscisic Acid Accumulation in the Leaf and Stomatal Response

The effects of a period of water stress (drought conditioning)on responses to a second (challenge) stress were examined inyoung vegetative rice (Oryza sativa L.) plants. Drought conditioningdid not affect the rate of subsequent stress development, nor,in a first experiment, did it influence relations between turgor(_p) and total () leaf water potential. However, conditioningdid extend the range of _p over which stomata remained open andsignificantly reduced the amount of ABA which accumulated inthe leaf at a given _p. The change in stomatal behaviour (stomataladjustment) was quantitatively accounted for by the change inleaf ABA accumulation. The reduction in ABA accumulation due to conditioning did notinvolve a change in the potential capacity to produce ABA, asABA accumulation in partially dehydrated detached leaves wasnot reduced by conditioning. It is suggested that effects ofconditioning on leaf ABA content in the intact plant involvechanges in the rate of ABA export from the leaf. Oryza sativa L, rice, drought conditioning, stomata, water stress, abscisic acid     

Stomatal Pattern in Four Species of Monocotyledons

The distribution of stomata has been investigated in the leafepidermis of Chlorophytum comosum, Galanthus nivalis, Schizostyliscoccinea and Scilla lancifolia. The epidermis was consideredto consist of units of construction of two kinds: type A, along epidermal cell with a stoma at its distal end, and typeB, a long epidermal cell without an associated stoma. Exceptin Scilla, the probability of an epidermal unit being type Aincreases approximately with its length. Considering the epidermisas rows of units, alternating sequences of type A and type Bdo not occur randomly along the rows. In Chlorophytum, Galanthusand Schizostylis, both type A and B units tend to be aggregatedinto longer sequences than would be expected on a random basis.It is suggested that homoeogenetic induction (i.e. of like bylike) may be occurring during development. No case can be madefor homoeogenetic induction of units in Scilla. There is a slighttendency to periodicity of distribution of type A units in Galanthus,Schizostylis and Scilla, but this does not seem to representa primary element of pattern. There is interaction between rowsin the sense that unit ends (transverse walls) tend to avoidthose in neighbour rows; this affects the relative distributionof stomata, but there is no evidence of any direct interactionbetween stomata in different rows. Chlorophytum comosum, Galanthus nivalis, Schizostylis coccinea, Scilla lancifolia, leaf, epidermis, stomata, pattern     

Structure and Development of Stomata on the Vegetative and Floral Organs of Some Amaryllidaceae

The structure and development of stomata in 10 Amaryllidaceousmembers are described. The study is made on the leaves and floralorgans of Amaryllis belladona, Crinum asiaticum, C. bulbispermum,C. latifolium, C. pratense, Pancratium sp., Polianthes tuberosa,and Zephyranthes atamsco, and on leaves of Agave sp. and Curculigoorchioides. In most of the organs of the different plants studiedmore than one type of stoma occur, even on the same surface.However, the stomata are largely anomocytic in Amaryllis, Crinum,Pancratium, Polianthes, and Zephyranthes, but they are tetracyticin Agave and Curculigo. An increase in the number of subsidiarycells in teracytic, tricytic, paracytic, and stomata with onesubsidiary cell has been noted. It may be due to wall formationin the subsidiary cells or by the neighbouring perigenes assumingtheir form. The range of variation in different types of stomatain each organ has been studied. Abnormal features such as juxtaposedor superimposed contiguous stomata, connections between guardcells of nearby stomata, transformation of meristemoids intoepidermal cells are observed. The development of different typesof stomata in different organs of the same plant is perigenous.The systematic position of different genera is discussed inlight of the present findings. A method of preparing epidermalimprints by the use of Depex, a mounting medium, and white ofan egg is also described.     

Positive and Negative Messages from Roots Induce Foliar Desiccation and Stomatal Closure in Flooded Pea Plants

Flooding the soil for 5–7 d caused partial desiccationin leaves of pea plants (Pisum sativum. L. cv. ‘Sprite’).The injury was associated with anaerobiosis in the soil, a largeincrease in the permeability of leaf tissue to electrolytesand other substances, a low leaf water content and an increasedwater saturation deficit (WSD). Desiccating leaves also lackedthe capacity to rehydrate in humid atmospheres, a disabilityexpressed as a water resaturation deficit (WRSD). This irreversibleinjury was preceded during the first 4–5 d of floodingby closure of stomata within 24 h, decreased transpiration,an unusually large leaf water content and small WSD. Leaf waterpotentials were higher than those in well-drained controls.Also, there was no appreciable WRSD. Leaflets detached fromflooded plants during this early phase retained their watermore effectively than those from controls when left exposedto the atmosphere for 5 min. Stomatal closure and the associated increase in leaf hydrationcould be simulated by excising leaves and incubating them withtheir petioles in open vials of water. Thus, such changes inflooded plants possibly represented a response to a deficiencyin the supply of substances that would usually be transportedfrom roots to leaves in healthy plants (negative message). Ionleakage and the associated loss of leaf hydration that occurswhen flooding is extended for more than 5 d could not be simulatedby isolating the leaves from the roots. Appearance of this symptomdepended on leaves remaining attached to flooded root systems,implying that the damage is caused by injurious substances passingupwards (positive message). Both ethylene and ethanol have beeneliminated as likely causes, but flooding increased phosphorusin the leaves to concentrations that may be toxic. Key words: Pisum sativum, Flooding, Foliar desiccation, Stomata, Ethylene     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant II. Water relations, stomatal conductance, abscisic acid content in leaves and xylem sap of plants subjected to water deficiency

The response of w-1, a wilty sunflower (Helianthus annuus L.)mutant, to water stress is described in comparison with thecontrol line (W-1). Detached leaves of w-1 strongly dehydratedduring the first 30 min without significant changes in leafconductance, whereas W-1 responded rapidly to water loss byreducing stomatal aperture. After 2 h stress ABA increased slightlyin w-1, while W-1 leaves showed a 20-fold increase. When waterstress was imposed to potted plants by water withholding, w-1quickly dehydrated, and lost turgor, while W-1 maintained positiveturgor values for a longer period. Wild-type plants respondedto small changes in leaf water potential by accumulating ABAand by closing stomata, whereas in the mutant significant changesin ABA content and in stomatal conductance were found only atvery low water potentials. In another experiment in which waterwas withheld under high relative humidity, when soil water contentstarted to decrease W-1 rapidly closed stomata in the absenceof any change in leaf water status and the reduction in conductancewas paralleled by a rise in xylem sap ABA concentration. Bycontrast the mutant started to accumulate ABA in the xylem sapand to close stomata when soil water content and leaf waterpotential were dramatically reduced. The low endogenous ABAlevels and the inability to synthesize the hormone rapidly eitherin the leaves or in the roots seem to be responsible for thehigh sensitivity of w-1 to water stress. Key words: ABA, Helianthus annuus L, water relations, stomatal conductance, drought, wilty mutant     

In situ Observations of Stomatal Movements

Kappen, L., Andresen, G. and L?sch, R. 1987. In situ observationsof stomatal movements.—J. exp. Bot. 38: 126–141. A device is described by which stomatal movements in situ canbe observed and recorded continuously in light and in darkness.It is mounted in a conditioned CO₂ exchange measuring chamberso that stomatal movements can be observed whilst CO₂ exchange(photosynthesis and respiration) of the same leaf is measured.Advantages and limitations are discussed. By this method itwas shown that stomata of Vicia faba although responding inthe same direction to environmental stimuli exhibited a widerange of pore widths. Responses to changes of air humidity andof CO₂ content were clearly evident when the leaves were exposedto light. Before stomata closed due to decreasing water vapourpressure differences between leaf and air they showed a markedwidening of the pore. An inverse response occurred when watervapour pressure deficit decreased. In darkness stomata did notrespond to such changes. Key words: Stomata, leaf gas exchange, microscopic observation