Effect of CO2 Concentration on Growth of Sugar-beet, Barley, Kale, and Maize

Increasing the concentration of CO₂ in the air from the usual300 ppm to 1, 000 ppm in growth rooms with temperatures of 20°C during the 16-h light period and 15° C during the 8-hdark period increased the total dry weight of sugar-beet, barley,and kale by about 5o per cent. A further increase in CO, concentrationto 3, 300 ppm increased dry weight slightly more. These effectsoccurred with light intensities ranging from 3.7 to II.6 caldm–² min–¹ of visible radiation supplied by a mixtureof fluorescent and tungsten lamps, and were only slightly greaterwith the brighter light. Extra CO₂ also increased leaf area,though relatively less than dry weight, and the number of barleyshoots but not of sugar-beet or kale leaves; it decreased leaf-arearatio, specific leaf area, and the ratio of tops to roots. Maizewas taller with extra CO₂. Net assimilation rates in 1, 000 and 3, 300 ppm CO₂ were about20 and 30 per cent respectively greater than in 300 ppm. Uptakeof CO₂ in the light by complete tops and single leaves alsoincreased with increase in CO₂ concentration. Photosynthesisof leaves of plants recently transferred to a new CO₂ concentrationdepended only on that concentration and not on the originalone. Doubling the light intensity from 3.7 to 7.7 cal dm–²min–¹ affected dry weight, leaf area, net assimilationrate, etc., similarly to a tenfold increase in CO₂ concentration.     

The effect of atmospheric humidity on photosynthesis,transpiration and water use efficiency of leaves of several plant species

The effect of humidity on the gas exchange of leaves of the dicotyledons soybean (Glycine max (L.) Merrill), sunflower (Helianthus annuus L.), jojoba (Simmondsia chinensis (L.) Schneider), and saltbush (Atriplex halimus L.) and the monocotyledons wheat (Triticum aestivum L.), barley (Hordeum vulgare L.) sorghum (Sorghum bicolor (L.) Moench) and barnyard grass (Echinochloa crus-galli (L.) Beauv.) was examined under conditions of adequate soil moisture in a controlled environment. Photosynthesis and stomatal and internal diffusion resistances of whole, attached, single leaves were not affected by changes in humidity as the vapour pressure deficit between the leaf and atmosphere ranged from 8 to 27 mb. Transpiration increased linearly with increasing vapour pressure deficit. Whole plants of barley exhibited a different response. As humidity was increased, photosynthesis increased, transpiration expressed per unit of vapour pressure difference increased, and diffusion resistances became smaller. Reasons for the different behaviour of single leaves and whole plants are suggested. An index for water use efficiency, expressed per millibar of vapour pressure deficit, was calculated for single leaves of each species used in the experiments. This showed that water use efficiency was highest in the C₄ xerophytes and lowest in the C₃ mesophytes. The effect of environment on water use efficiency is examined using data from the literature.     

Effects of Nitrogen Fertilizer on Growth and Yield of Spring Wheat

Nine amounts of nitrogen fertilizer, ranging from 0 to 200 kgN ha^–1, were applied to spring wheat cv. Kleiber in the3 years 1972-1974. In 1972 grain dry weight with 125 kg N ha^–1or more was 100 g m^–2 (23 per cent) greater than withoutnitrogen. Grain yield was unaffected by nitrogen in the otheryears. Leaf area at and after anthesis was increased throughoutthe range of nitrogen tested, most in 1972 and least in 1973.Consequently, the addition of 200 kg N ha^–1 decreasedthe amount of grain produced per unit of leaf area by approximately25 per cent in all years. The dry weight of leaves and stems at anthesis and maturitywas increased by nitrogen in all years, similarly to leaf area.However, the change in stem dry weight between anthesis andmaturity was not affected by nitrogen; stems increased in dryweight for about 20 days after anthesis and then decreased tovalues similar to those at anthesis. The uptake of CO₂ per unit area of flag leaf or second leaf(leaf below the flag leaf) was slightly decreased by nitrogenwhen the increase in leaf area caused by nitrogen appreciablydecreased the light intensity at the surface of these leaves.In spite of such decreases the CO₂ absorbed by flag and secondleaves per unit area of land was always increased by nitrogen,and relatively more than was grain yield. It is suggested that increases in respiratory loss of CO₂ withincreasing nitrogen fertilizer may explain why nitrogen increasedvegetative growth and leaf area relatively more than grain yield.     

Photosynthesis of Ears and Flag Leaves of Wheat and Barley

Immediately after anthesis ears of spring wheat absorbed lessthan 0.5 mg CO₂, per hour in daylight and later evolved CO₂,in the light and in the dark. The rate of apparent photosynthesisof the combined flag-leaf lamina and sheath and peduncle (collectivelycalled flag leaf) of two spring wheat varieties, Atle and JufyI, was 3–4 mg per hour; the rates of the flag leaf andthe ear of two spring barleys, Plumage Archer and Proctor, wereeach about 1 mg per hour. The gas exchange of ears and flag leaves between ear emergenceand maturity accounted for most of the final grain dry weight.The CO₂, fixed by the wheat ear was equivalent to between 17and 30 per cent of the grain weight, but more than this waslost by respiration, so assimilation in the flag leaf was equivalentto 110–20 per cent of the final grain weight. In barley,photosynthesis in the flag leaf and the net CO₂ uptake by theear each provided about half of the carbohydrate in the grain. Barley ears photosynthesized more than wheat ears because oftheir greater surface, and flag leaves of wheat photosynthesizedmore than those of barley because they had more surface anda slightly greater rate of photosynthesis per dm².     

Nitrogen Fixation in the Phyllosphere of Tropical Plants: Occurrence of Phyllosphere Nitrogen-Fixing Micro-organisms in Eastern India and their Utility for the Growth and Nitrogen Nutrition of Host Plants

Of the 560 leaf samples belonging to 259 species of green plantsexamined more than 50 per cent of the Angiosperms and 25 percent of the Pteridophytes and Gymnosperms revealed the presenceof N₂-fixing micro-organisms in their phyllosphere. Plants particularlyremarkable in this respect are orchids and several other epiphytes,Scindapsus officinalis, Ficus and cucurbits. Most of the isolatesappear to be biotypes of Klebsiella pneumoniae. The more activestrains fixed more than 5 mg N g^–1 glucose utilized andreduced more than 100 nmol C₂H₂ mg^–1 cell d. w h^–1. The efficacy of the phyllosphere N₂-fixing isolates for N-nutritionof host plants was studied by spraying suspensions of the culturesgrown on N-free media on rice and wheat seedlings. In IR-26rice or Sonalika and Janak wheat grown on soil in wooden flatsor earthenware pots, 22 per cent of the 161 cultures studiedcaused increased height and about three-quarters of the culturesenhanced dry weight by more than 50 per cent; chlorophyll andN-contents were enhanced more than 50 per cent by about halfand two-thirds of the cultures respectively. In N-free sandculture 26 of the 50 promising strains doubled N-content, and30 doubled dry weight of the tested plants. In some cases dryweight, number of grains per panicle, and 1000 grain weightwere increased by 300, 70–83 and 126–158 per centrespectively; N-content of straw and seed was increased three-or fourfold. In several cases the beneficial effects were foundto match closely the performance of plants receiving ammoniumsulphate. Nitrogen-fixing micro-organisms, nitrogen nutrition, phyllosphere, rice, tropical plants, wheat     

Is stomatal conductance in a tomato crop controlled by soil or atmosphere?

Summary The effects of soil water deficits and air vapour pressure deficits on stomatal conductance of tomato leaves were analysed separately under field conditions in central Portugal. Three conditions were created: low soil and air humidity (A), high soil and air humidity (B) and low soil but high air humidity (C). The results show that the effect of air vapour pressure deficit on stomatal behaviour is more important than the effect of soil water deficit when the predawn leaf water potential is above –0.4 MPa.     

The Effect of Sodium on Growth of Water-stressed Sugar beet

Sugar beet grown in solution culture, with or without a supplementof 16 millequivalents per litre of sodium, were subjected towater stress with polyethylene glycol solutions of –0.4,–3, and –8 bar osmotic potential. With the –0.4bar solution leaf water potential was between –6 and –8bar and leaf relative water content about 90 per cent. Decreasingthe solution osmotic potential to –8 bar decreased leafwater potential to about –15 bar and relative water contentto 75 per cent; leaves stopped expanding and transpiration andcarbon dioxide uptake were decreased by 80 and 50 per cent respectively.Net assimilation rates were only slightly decreased becauseleaf growth was decreased more than carbon dioxide assimilation.Relative growth rates of the plants were decreased by 8 percent at –3 bar and by 15 per cent at –8 bar. Sodium absorbed by the plant accumulated mainly in the leavesand petioles; it increased the water content of the leaves andstorage root and the plant fresh weight. Sodium decreased theleaf osmotic potential, slightly increased leaf water potential,and significantly increased turgor. It had no effect on carbondioxide uptake, transpiration, net assimilation rate, or relativegrowth rate. Sodium increased the rate at which the leaf areagrew and it is concluded that it did so by altering the leafwater balance.     

The Control of Water and Carbon Dioxide Flux in Water-stressed Sugar Beet

Changes in leaf and canopy water potential of sugar beet growingin soil of decreasing water content depended on soil water potentialand were independent of water flux from the plant when thiswas varied by changing the water vapour content of the air.The calculated hydraulic conductance of the plant increasedas flux increased and decreased as leaf water potential decreasedand as the plant aged. The conductances to water vapour of individualleaves and of the canopy decreased as leaf water potential decreasedand increased with increasing humidity of the air. The lattereffect was independent of changes in leaf water potential. Theconductances were not affected by the rate of evaporation orleaf temperature. The rate of photosynthesis was directly relatedto leaf conductance except in severely stressed, mature leavesin which leaf water potential had a more direct effect on photosynthesis.Stomatal conductances, transpiration, and photosynthesis weregreater in young leaves than mature leaves on the same plantand at the same leaf water potential. These results are discussedin relation to current agricultural irrigation practices usedfor sugar beet.     

The Production and Distribution of Dry Matter in Maize after Flowering

An experiment in which different groups of leaf laminae wereremoved, or ears shaded, shortly after silking showed that mostof the dry-matter increase after flowering was produced by upperleaves. The top five, the middle four, and the bottom six laminaeaccounted, respectively, for 26 per cent, 42 per cent, and 32per cent of the leaf area duration (D) of the laminae afterflowering; the estimated contributions of the three groups todry-matter production by the laminae after flowering were about40 per cent, 35–50 per cent, and 5–25 per cent,respectively. The sheaths provided about one-fifth of the totalleaf area and probably contributed about one-fifth, and laminaefour-fifths, of the total dry matter produced after flowering.The contribution from photosynthesis by the ear was negligible,presumably because its surface area was only 2 per cent of thatof the leaves. Leaf efficiency (dry matter produced per unitarea) decreased greatly from the top to the base of the shoot.When laminae were removed, the grain received a larger fractionof the dry matter accumulated after flowering, less dry matterremained in the stem, and the photosynthetic efficiency of theremaining leaves was apparently increased. When alternate laminae were removed at the time of silking (half-defoliation)D was decreased by 40 per cent, and the subsequent productionof dry matter decreased nearly proportionately, so that netassimilation rate (E) was not affected but grain dry weightwas decreased by only 32 per cent. At the final harvest, thegrain of half-defoliated plants constituted 80 per cent of thedry matter accumulated after flowering, compared with 65 percent for intact plants. Stem weight decreased from two weeksafter flowering in half-defoliated plants, but remained nearlyconstant in intact plants. When pollination was prevented andno grains formed, E during the first month after flowering wasunaffected; the dry matter that would have passed into the grainaccumulated in the stem and husks, not in the leaves. The decrease in stem weight caused by defoliation suggests thatpreviously stored dry matter was moved to the grain. That suchmovement is possible was shown by keeping prematurely harvestedshoots in the dark for two weeks with their cut ends in water;the dry weight of the grain increased and that of the stem,laminae, husks, and core decreased. Nevertheless, dry-matterproduction after flowering was more than sufficient for graingrowth, and previous photosynthesis probably contributed littleto the grain.     

Responses to Saturation Deficit in a Stand of Groundnut (Arachis hypogaea L.). 2. Growth and Development

Stands of groundnut were grown in four glasshouses with themaximum saturation deficit (D) of the air limited to 1.0, 2.0,2.5 or 3.0 kPa. The soil was near field capacity when plantsemerged and no water was applied thereafter. In a fifth glasshouse,a stand was grown at low D on soil irrigated to field capacityevery few days. Developmental processes such as timing of flowering, peggingand pod formation were unaffected by D, but the numbers of branches,flowers and pegs were reduced in the drier treatments. Measurementsduring the first 30 d showed that in the drier treatments leafgrowth was reduced, and the partitioning of dry matter intoroots was enhanced. In the unirrigated stands, dry matter production in shoots wasreduced by 40 per cent as the maximum D increased from 1.0 to3.0 kPa. Growth was affected through reductions both in leafarea (and therefore light interception) and in the productivityper unit of light intercepted. These responses to D and soilwater were linked to changes in bulk water potential of leaves. Productivity per unit of water transpired (q) decreased withincreasing D. The product of q and the mean daytime value ofthe difference in vapour pressure between leaf and air was moreconservative than q, and ranged from 3.1 to 5.6 g kPa kg^–1. Groundnut, Arachis hypogaea L., saturation deficit, growth, development, light interception, water use efficiency     

Studies in the Nutrition of Apple Rootstocks: II. Effects of Concentration of Iron in the Nutrient Solution on Growth and Chlorophyll Content

Rooted one-year shoots were grown for one season by sprayingtheir roots with nutrient solution. Iron supplied as Fe-EDTAat four concentrations resulted in plants which were respectively(a) severely chlorotic, (b) mildly chlorotic, (c) dark greenand healthy (controls), and (d) dark green but with slight reductionin growth. Severely deficient plants showed 40–70 per cent reductionsin growth as measured by fresh weight, shoot length, diameterincrease, leaf area, net assimilation and relative growth-rates.Dry weights were reduced 70–80 per cent and of the totaldry-weight increment a greater proportion remained in the leaves,which had a lower dry weight and higher water content per unitarea. However, because the initial old stem formed a greaterproportion of the total dry weight, the leaf area ratio remainedabout 11 per cent lower than in the controls. Severely deficientplants had, per unit of chlorophyll, a higher dry-weight increaseand net assimilation rate than the controls. Mild deficiency caused 10–20 per cent reductions in growthand net assimilation rate; the leaf area ratio was normal. Possible mechanisms of the effects of low iron supply are discussed,while the small growth reduction at the highest Fe-EDTA concentrationis attributed to chelate toxicity     

Developmental Physiology of Sugar Beet: I. THE INFLUENCE OF LIGHT AND TEMPERATURE ON GROWTH

Sugar beet plants were grown for 12 weeks from emergence ingrowth rooms at temperatures of 10, 17, 24 and 31 °C and20, 50, 80, and 110 cal visible radiation cm^-2d^-1, and the changeswith time in their dry weight, leaf area, leaf numbers, andstorage root sugar determined. The first stage of growth wasdominated by the development of the shoot, but the storage rootgradually assumed increasing importance and eventually grewat a faster rate and to a greater weight than the shoot. Therelative growth rate and final yield of dry matter of the shootwere greatest at 24 °C and of the root between 17 and 24°C. The relative rate of expansion and the final area ofthe leaf surface were also greatest at 24 °C, whilst therates of production and of unfolding of leaves were greatestat about 17 °C. All these attributes were increased withincreased radiation. Net assimilation rate increased almostproportionately with radiation and was not significantly affectedby temperature.The relationships of total leaf area with plantdry weight, root dry weight with shoot dry weight, and totalleaf number with plant dry weight were scarcely affected bychanges in radiation, but were much influenced by temperature.Plants of the same dry weight generally had bigger roots andsmaller areas of leaf surface as temperatures departed from24 °C and had most leaves at 17 °C. Sugar concentrationsin the storage root were greatest at 17 °C, but the totalamount of sugar was about the same at 17 and 24 °C. Theconcentration of sugar in the storage root depended on rootsize.Thus, temperature affected both the rate and pattern ofdevelopment, and radiation affected the rate but not the patternof development.     

Identifying soybean lines differing in gas exchange sensitivity to humidity

The ratios of root length and root weight to leaf area differed within and between cultivars of soybean. Plants with low ratios of root length or weight to leaf area had leaf conductances and net photosynthetic rates more reduced by a given increase in the leaf to air water vapour pressure difference around a single leaf than plants with high ratios. Plant and root system conductances to water were estimated as transpiration rate per unit leaf area divided by the difference between substrate and leaf water potentials, and by the rate of water flow through pressurised root systems. These conductances were greater in plants with large, as compared with small, root systems per unit leaf area. Cultivar rankings in sensitivity of gas exchange to humidity were consistent in controlled environment chambers and in field tests.     

Effect of Post-anthesis Drought on Cell Division and Starch Accumulation in Developing Wheat Grains

Wheat plants (Triticum aestivum L., cv. Warigal) were subjectedto 20 d of water deficit during the period of endosperm celldivision. Drought accentuated the differences in final grainweight between spikelets and between grains within spikelets.The distal grains of top spikelets were most affected by drought.The maximum number of endosperm cells was, respectively, 30and 40 per cent lower in basal grains and distal grains of draughtedplants. In basal grains of middle spikelets, the number of largestarch granules per cell was unaffected but the number of smallstarch granules per cell was 45 per cent lower in grains ofdraughted plants. The initiation of small starch granules wasmore affected than cell division because severe water deficitoccurred earlier during the former process than the latter.Final dry weight appeared to correlate well with the maximumnumber of endosperm cells, but depended also on the number ofstarch granules per cell. Consequently, the amount of dry matterper cell was not constant in both treatments. The concentration of sucrose per endosperm cell was lower onlyin the droughted distal grains of top spikelets. The supplyof sucrose to endosperm cells did not regulate the initiationof small starch granules. Triticum aestivum L., wheat, drought, grain growth, cell division, starch     

The effect of temperature and water stress on the timing of leaf death in Vicia faba

The timing and pattern of leaf death was studied in three field experiments from observations of the area and dates of death of individual leaves. Two linear regression lines adequately described the pattern of leaf death on the plant in all experiments and treatments. These represented an initial phase with a slow rate of leaf death changing to a second phase with a more rapid rate of leaf death. The change from the first to second phase of leaf death occurred when the maximum seed dry weight was reached. The rates of both the first and second phases of leaf death increased with temperature. This was confirmed in an experiment in growth rooms. The rate of leaf death in both phases of plants grown at three constant temperatures increased linearly with temperature from 14 °C to 22 °C. The rate of leaf death in phase 2 increased with increasing solar irradiance and vapour pressure deficit. The rate of leaf death in both phases increased with increasing water stress in three irrigation treatments in the field.     

The Function of the Hilum in Some Papilionaceae in Relation to the Ripening of the Seed and the Permeability of the Testa

In seeds of Trifolium repens, T. pratense, and Lupinus arboreus,the hilum is a hygroscopically activated valve in the impermeableepidermis of the testa. When relative humidity was low the fissurein the hilum opened permitting the seed to dry out; when therelative humidity was high the fissure closed obstructing theabsorption of moisture. During seed-ripening the moisture contentfell readily to approximately 25 per cent., and thereafter moreslowly until the epidermis became impermeable at approximately14 per cent, moisture content. Further drying of the seed tookplace only by diffusion of water vapour through the hilum. ‘Hard’seeds tended to have a moisture content in equilibrium withthe lowest relative humidity to which they had been exposed.They absorbed moisture under conditions of gradually increasingrelative humidity such that the hilar fissure remained open.The duration of the impermeable condition increased with thedegree of desiccation brought about by loss of water throughthe hilum.     

The responses of stomata and leaf gas exchange to vapour pressure deficits and soil water content

Summary The responses of photosynthesis, transpiration and leaf conductance to changes in vapour pressure deficit were followed in well-watered plants of the herbaceous species, Helianthus annuus, Helianthus nuttallii, Pisum sativum and Vigna unguiculata, and in the woody species having either sclerophyllous leaves, Arbutus unedo, Nerium oleander and Pistacia vera, or mesomorphic leaves, Corylus avellana, Gossypium hirsutum and Prunus dulcis. When the vapour pressure deficit of the air around a single leaf in a cuvette was varied from 10 to 30 Pa kPa^-1 in 5 Pa kPa^-1 steps, while holding the remainder of the plant at a vapour presure deficit of 10 Pa kPa^-1, the leaf conductance and net photosynthetic rate of the leaf decreased in all species. The rate of transpiration increased initially with increase in vapour pressure deficit in all species, but in several species a maximum transpiration rate was observed at 20 to 25 Pa kPa^-1. Concurrent measurements of the leaf water potential by in situ psychrometry showed that an increase in the vapour pressure deficit decreased the leaf water potential in all species. The decrease was greatest in woody species, and least in herbaceous species. When the vapour pressure deficit around the remainder of the plant was increased while the leaf in the cuvette was exposed to a low and constant vapour pressure deficit, similar responses in both degree and magnitude in the rates of transpiration and leaf conductance were observed in the remainder of the plant as those occurring when the vapour pressure deficit around the single leaf was varied. Increasing the external vapour pressure deficit lowered the water potential of the leaf in the cuvette in the woody species and induced a decrease in leaf conductance in some, but not all, speies. The decrease in leaf conductance with decreasing water potential was greater in the woody species when the vapour pressure deficit was increased than when it remained low and constant, indicating that changing the leaf-to-air vapour pressure difference had a direct effect on the stomata in these species. The low hydraulic resistance and maintenance of a high leaf water potential precluded such an analysis in the herbaceous species. We conclude that at least in the woody species studied, an increase in the vapour pressure deficit around a leaf will decrease leaf gas exchange through a direct effect on the leaf epidermis and sometimes additionally through a lowering of the mesophyll water potential.     

Varietal Differences in Photosynthesis of Ears and Leaves of Barley

Rates of apparent photosynthesis of ears and of the combinedflag leaf and sheath and peduncle of Proctor barley grown inpots or in the field were similar to those of Plumage Archer,or slightly smaller when the dimensions of the ear and leafarea of Proctor were less than those of Plumage Archer. Thephotosynthesis rate of the ear—about 1.0 mg. CO₂ per earper hour—was similar or slightly less than the rate ofthe flag leaf and sheath and peduncle. These rates of photosynthesisindicated that 40-50 per cent, of the carbohydrate in the grainwas provided by photosynthesis in the shoot and about 40 percent, by photosynthesis in the ear. The total CO₂ fixed by theear was equivalent to about 60 per cent, of the grain weight,20 per cent, being lost by respiration. Shading the ear underestimatedthe total amount of CO₂ fixed by the ear and decreased dry weightof grain per ear of both Proctor and Plumage Archer by 26 percent., as in pots. The contribution of ear photosynthesis toyield of grain per acre was greater for Proctor than for PlumageArcher because Proctor had more ears. The rate of apparent photosynthesis per dm.² of leaves of Proctorwas similar to that of Plumage Archer both before and afterear emergence. Before ear emergence, the photosynthesis rateof a particular leaf decreased linearly with time and was slowerfor lower than for higher leaves on the shoot. Respiration ratesper g. dry weight of ears of Proctor and Plumage Archer weresimilar; in one experiment the leaves of Proctor respired slightlyfaster than those of Plumage Archer.     

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.