Nature of 'Pollinator' Effect in Potato (Solanum tuberosum L.) Haploid Production

Haploids (2n =24) of the common tetraploid (2n=48) potato (SolanumtuberosumL.) provide promising material for attacking many problemsconcerned with the genetics, cytogenetics and breeding of thisspecies. Interspecific 4xx2xcrosses betweenSolanum tuberosumgp.Andigenaorgp.Tuberosumcultivars as pistillate parents andSolanum tuberosumgp.Phurejaassource of pollen (hereafter ‘pollinator’) have beenused to produce maternally derived haploids through parthenogenesis.This paper discusses the nature of the ‘pollinator’effect in haploid extraction. The ‘pollinator’ hada significant effect on haploid frequencies following 4xx2xcrosses.The ‘pollinator’ effect seems to operate via theendosperm, in which haploid (n=2x) embryos are associated withhexaploid endosperm. A superior ‘pollinator’ appearsto have its effect by contributing two haploid (n) gametes tothe central cell. 2n pollen; double fertilization; endosperm; ploidy manipulations; Solanum tuberosum     

Effect of Inorganic Phosphate on the Biosynthesis of Purine and Pyrimidine Nucleotides in Suspension-Cultured Cells of Catharanthus roseus

The levels of purine and pyrimidine nucleotides in suspensioncultures of Catharanthus roseus were determined 24 h after stationary-phasecells were transferred to fresh complete (‘+Pi’)or phosphate-deficient (‘–Pi’) Murashige-Skoogmedium. The levels of ATP, GTP, UTP and CTP were from approx.3 to 5-fold greater in the cells grown in ‘+Pi’medium than in the cells grown in ‘–Pi’ medium.The levels of almost all other nucleotides were slightly higherin the cells in ‘+Pi’ medium. The rates of de novoand salvage biosynthesis of purine and pyrimidine nucleotideswere estimated from the rates of incorporation of radioactivityfrom [¹⁴C]formate, [2–¹⁴C]glycine, NaH¹⁴CO₃, [6–¹⁴C]orotate,[8–¹⁴C]adenine, [8–¹⁴C]adenosine, [2–¹⁴C]uraciland [2–¹⁴C]uridine. The results indicated that the activityof both the de novo and the salvage pathway was higher in thecells in ‘+Pi’ medium than in the cells in ‘–Pi’medium. The rate of degradation estimated from the rate of releaseof ¹⁴CO₂ from labelled purines and pyrimidines indicated thatdegradation of uridine was significantly reduced in the cellsin ‘+Pi’ medium, but no significant difference wasfound in the degradation of adenine, adenosine and uracil. Thepossible role of Pi in the control of the biosynthesis of nucleotidesand in the degradation of uridine is discussed. Catharanthus roseus, Madagascar periwinkle, suspension culture, inorganic phosphate, nucleotides, purines, pyrimidines, biosynthesis, degradation     

The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 1. Grown as Simulated Swards During a Regrowth Period

Simulated swards of each of two selection lines of Lolium perennecv. S23 with ‘fast’ and ‘slow’ ratesof ‘mature tissue’ respiration were establishedin growth rooms at 20/15 °C day/night temperatures and studiedover four successive regrowth periods of 46, 30, 26 and 53 daysduration. The ‘slow’ line outyielded the ‘fast’,both in harvestable shoot (above a 5 cm cut) and in root andstubble. Its advantage increased over successive regrowth periodsto 23 per cent (total biomass). Gas analysis measurements onthe entire communities (including roots), during the final regrowthperiod, showed that the ‘slow’ line had a 22–34per cent lower rate of dark respiration per unit dry weight.This enabled it to maintain its greater mass of tissue for thesame cost in terms of CO₂ efflux per unit ground area. Halfthe extra dry weight produced by the ‘slow’ line,relative to the ‘fast’, could be attributed to itsmore economic use of carbon. The rest could be traced to a 25per cent greater tiller number which enabled the ‘slow’line to expand leaf area faster (though not at a greater rateper tiller), intercept more light and fix more carbon, earlyin the regrowth period. Lolium perenne L., ryegrass, respiration, maintenance respiration, tiller production, simulated swards, canopy photosynthesis, carbon economy     

Pressure and Solute Potentials in Stomatal Cells of Tradescantia virginiana

The postulated mechanical advantage of subsidiary cells overguard cells has been estimated using leaves of Tradescantiavirginiana. The turgor pressures of subsidiary cells were adjusted to bezero or maximal by plasmolytic treatments, and the resultingstomatal apertures were measured. The ‘mechanical advantage’was calculated from two mathematical models which define itas an ‘antagonism ratio’. The discussion deals withmethods of preparing the tissue, the validity of the plasmolytictreatments, and the function of the antagonism ratio in relationto the Spannungsphase     

Pyrimidine Nucleotide Biosynthesis in Vinca rosea Cells: Changes in the Activity of the de novo and Salvage Pathways during Growth in a Suspension Culture

Marked changes in the activity of the ‘de novo’and ‘salvage’ pathways of pyrimidine biosynthesisduring growth of Vinca rosea cells in a batch suspension culturewere observed. The activity of these pathways was investigated by determiningthe contribution of ¹⁴C of [2-¹⁴Cluracil, 12-¹⁴Cluridine. and[6-¹⁴Clorotate to the cell constituents and by measuring theactivity of the several enzymes of these pathways. During the lag phase of the culture, ‘uracil-’ and‘uridine-salvage’ pathways made the predominantcontribution to nucleotide biosynthesis, but, following theinitiation of cell division, the ‘de novo’ pathwayfor nucleotide biosynthesis operated appreciably. These results suggest that nucleotide synthesis during cellgrowth in a suspension culture can be divided into two stages:a ‘turnover stage’, during the lag phase of cellgrowth, and a ‘true biosynthetic stage’, which isinitiated in the cell division phase.     

Primary and Secondary Induction Requirements for Flowering of Contrasting European Varieties of Lolium perenne

The flowering requirements of six European varieties of Loliumperenne L. were studied in controlled environments. In experimentson primary induction, flowering was recorded after transferto long days (LD) in a greenhouse at 12–24°C. In experimentson secondary induction, primary induction was first accomplishedat 6°C/10 h daylength for 12 weeks. When evaluated by the50% heading criterion, the requirement for duration of primaryinduction at 6°C/8 h daylength was <3 weeks in Mediterranean,5–6 weeks in Central European and 7–8 weeks in Scandinavianvarieties. While ‘Veyo’ (Italy) flowered profuselyregardless of temperature or daylength during primary induction,critical temperatures for primary induction in SD and LD were15 and 11°C in ‘Baca’ (Czech Republic) and 11and 7°C in ‘Falster’ (Denmark). The criticalphotoperiod for secondary induction at 15°C ranged from12 h in ‘Veyo’ and 14 h in ‘Baca’ to16.5 h in ‘Falster’ and 17.5 in ‘Kleppe’(Norway). The critical number of LD cycles varied correspondingly.While the Central and North European varieties required fewerLD cycles for 50% heading at 18 than at 12°C, ‘Veyo’showed the opposite response. It is concluded that the requirementsfor both primary and secondary induction of Lolium perenne increasewith increasing latitude of origin of the germplasm. In oneexperiment, 39–87% of the inflorescences came from tillersthat were not visible on transfer from primary to secondaryinduction, thus it is also concluded that there is no juvenilestage in tillers of Lolium perenne. Copyright 2000 Annals ofBotany Company Daylength, flowering, juvenility, perennial ryegrass (Lolium perenne L.), primary induction, secondary induction, temperature, varieties, vernalization     

The Growth and Carbon Economy of Selection Lines of Lolium perenne cv. S23 with Differing Rates of Dark Respiration: 2. Grown as Young Plants from Seed

Young plants of two selection lines of Lolium perenne cv. S23with ‘fast’ and ‘slow’ rates of ‘maturetissue’ respiration were individually grown from seed,together with plants of S23, their common parent, in 9.2 cmpots in a controlled environment at 20/15 °C day/night temperatures. No significant differences were found between the genotypesin leaf extension and tiller production during this early stageof their growth. They did differ however, by an average of 26%,in the rate of dark respiration of fully expanded leaf laminae.The use of a simple model demonstrated that such a differencein respiration could alone account for the different rates ofdry matter production shown by the selection lines when grownas young crops from seed. Possible penalties of ‘slow’respiration are also considered. Lolium perenne L., ryegrass, respiration, maintenance respiration, stimulated swards, leaf growth, tiller production, carbon economy     

Diurnal Phototropism in Leaves of Lavatera cretica L. under Conditions of Simulated Solar-Tracking

Diurnal laminar reorientation was followed in solar-trackingleaves of Lavatera cretica L. under simulated conditions. Asimulated ‘sun’ was moved over the lamina in a 180?arc in the vertical plane of the mid-vein, at an angular velocityof 15? h^–1 in a regime of 12-h photoperiods. In one groupof plants the petioles of the experimental leaves were arrangedto face ‘sunrise’, while in the other they werearranged to face ‘sunset’. At ‘sunrise’,the laminae in both groups, which were inclined towards theanticipated direction of ‘sunrise’, changed theirelevation towards the rising ‘sun’, resulting inprogressive reduction in the angle of incidence (AI) of lighton the laminar surface (AI= differential between laminar and‘solar’ elevation). As a result, laminar and ‘solar’elevation converged, and laminar reorientation gradually ceased,until the ‘solar’ elevation had passed the normalto the laminar surface (AI=0?). laminar reorientation was thenresumed, but its direction was reversed to follow the directionof ‘solar’ reorientation. During most of the remaining‘day’, laminar elevation (LE) trailed that of the‘sun’ by an average of 11?-14?. Laminar reorientationthen anticipated ‘sunset’ by starting to slow down60 to 90min in advance. During the 12-h dark period, the laminareoriented towards the anticipated direction of the subsequent‘sunrise’. The time-course of nocturnal reorientationwas qualitatively different in the two groups of experimentalplants. The time-course of diurnal phototropism under naturaland simulated conditions is analysed and compared and differencesand similarities between them are discussed. Key words: Diurnal phototropism, solar-tracking, vectorial excitation     

Effects of Inorganic Phosphate on the Utilization of Sucrose by Suspension-cultured Catharanthus roseus Cells

The metabolic fate of [U-¹⁴C]sucrose in suspension culturesof Catharanthus roseus cells was monitored for 96 h after thecells were transferred to fresh complete (‘+Pi’)or to phosphate-deficient Murashige and Skoog (‘–Pi’)medium. Sucrose was hydrolysed extracellularly to glucose andfructose. The rate of uptake of sugars by the cells was 1.5–3times higher in ‘+Pi’ culture than in ‘–Pi’culture. Little difference in the rate of incorporation of radioactivityinto the ethanol-soluble fraction was found between the ‘+Pi’and ‘– Pi’ cultures during the initial 24h of culture, but after 48 h the rate in ‘ +Pi’cultures was higher than that in ‘–Pi’ cultures.Incorporation of radioactivity into ethanol-insoluble macromoleculeswas always significantly higher in the cells in ‘+Pi’cultures than in those in ‘–Pi’ cultures.The results suggest that Pi strongly affects the utilizationof sugars by cultured plant cells through the stimulation oftransport of sugars as well as through the activation of metabolism. Catharanthus roseus, Madagascar periwinkle, suspension culture, inorganic phosphate, sucrose, transport, metabolism     

Morphology and Anatomy of Stems and Pedicels of Spring Flush Shoots Associated with Citrus Fruit Set

Flowering and vegetative shoots of ‘Shamouti’ orange[Citrus sinensis (L.) Osbeck] and ‘Marsh’ seedlessgrapefruit (Citrus paradisi Macf.) were examined for correlationof their morphology and anatomy with fruit set. Fruit set isfavoured on leafy inflorescences whereas abortion is nearlycomplete on leafless inflorescences. Leafless inflorescencesof ‘Shamouti’ with one flower were found to havea very thin stem which contained few vascular bundles, whereasthose with three flowers had better-developed vascular systems.The vascular system of leafy inflorescences is significantlydifferent from that of leafless ones and contains a distinctcentral xylem cylinder. The vascular area of leafless inflorescencesis only about one-quarter of that of the leafy ones. The vascularsystem of grapefruit resembles that of the ‘Shamouti’orange. This study emphasizes the importance of the dimensionof the vascular system for fruit set and provides a possibleexplanation for the better fruit set on both leafy and leaflessinflorescences with several flowers compared with single-floweredinflorescences. Anatomy; citrus; fruit set; leafless inflorescence; leafy inflorescence; pedicel; vascular system; vegetative shoot     

Initial Anatomical Changes Associated with Tuber Formation on Single-node Potato (Solanum tuberosum L.) Cuttings

One-node potato (Solanum tuberosum L. cv. ‘Katahdin’)cuttings were used to study early anatomical changes associatedwith tuberization. Starch deposition and the percentage frequencyof cells in mitosis increased in the medullary region of thebud within 1 d after cutting, whereas increases in average cellsize were not detected until 4 d after cutting. Starch depositionand mitosis were the earliest detectable changes in anatomyassociated with tuber initiation. Potato, Solanum tuberosum L., tuber initiation, cuttings, cell enlargement, mitosis, starch deposition     

The Nitrogen Content of Plants and the Self-thinning Rule of Plant Ecology: A Test of the Core-skin Hypothesis

The ‘core-skin’ hypothesis postulates that secondarilythickened plants behave energetically as an inert ‘core’covered by an active ‘skin’, the ‘skin’being two-imensional, the ‘core’ three-dimensional.This would explain the ‘self-thinning ‘or‘–3/2’ rule of plant ecology, that is, the tendencyfor log (dry weight per plant) and log (number of plants perunit area) to progress along a straight line relationship, withslope = – 3/2’. The hypothesis was tested as follows. Plant nitrogen contentwas used as an estimate of the mass of ‘skin’ perplant, and dry weight as an estimate of the mass of the ‘core’.As plants mature the slope of the relationship between y = log(mass of nitrogen per plant) and x = log (mass of dry matterper plant) is expected to decline from an initial value of 1.0towards a final value of 0.66. The intercept of the relationshipis expected to reflect the intrinsic content of ‘skin’per unit of ‘core’. Genotypic variation in thisparameter should cause genotypic differences in the maximumattainable yield of biomass per unit area. The expectations were investigated by fitting the function y= p+qx+r exp – x to 30 sets of data on plant nitrogencontent, plant weight and time in 18 different vegetables. Simplelinear regressions of y on x were fitted to more limited setsof data on weights and nitrogen contents of mature trees. Theexpectations were, with some minor exceptions, confirmed. Nitrogen, yield, plant competition, self-thinning     

The Sensitivity of Net Photosynthesis in Several Plant Species to Short-term Fumigation with Sulphur Dioxide

The effects of exposure of up to 2 h with sulphur dioxide ona range of plant species was observed by measuring changes inthe rate of net photosynthesis under closely controlled environmentalconditions. Ryegrass, Lolium perenne ‘S23’ was thespecies most sensitive to SO₂; significant inhibition was detectedat 200 nl l^–1. Fumigations at 300 nl l^–1 also inhibitedphotosynthesis in field bean (Vicia faba cv. ‘Three FoldWhite’ and ‘Blaze’) and in barley (Hordeumvulgare cv. ‘Sonja’). No effect was detected inwheat (Triticum aestivum cv. ‘Virtue’) at concentrationsup to 600 nl l^–1 SO₂, or in oil-seed rape (Brassica napuscv. ‘Rafal’) except at 800 nl l^–1 SO₂). Recoverycommenced immediately after the fumigation was terminated andwas complete within 2 h when inhibition had not exceeded 20%during the SO₂ treatment. Key words: Sulphur dioxide, short-term fumigation, photosynthesis     

The Developmental Anatomy of the Root System in Yam Bean, Pachyrhizus erosus Urban

Investigations revealed that the anatomy of the primary radicularroot of yam bean (Pachyrhizus erosus L.) was typically dicotyledonousexcept that the xylem was not completely developed centripetally.Most of the roots had tetrarch xylem, although a few triarchand pentarch roots were also observed. In both tuberous andnon-tuberous roots, secondary thickening occurred by the formationof the meristematic vascular cambium which formed secondarytissues in a normal fashion. Subsequently, tuberization wasinitiated in the secondary xylem by the development of anomalous‘secondary’ cambia from parenchyma cells surroundingvessel elements. Anomalous ‘secondary’ cambia alsodeveloped from parenchyma cells not associated with vessels.Subsequently, anomalous ‘tertiary’ cambia differentiatedfrom tissues produced by the anomalous ‘secondary’cambia. Activities of these anomalous cambia resulted in theproduction of parenchyma storage cells and were chiefly responsiblefor the growth of the mature tuber. Pachyrhizus erosus L., yam bean, tuberous root, anatomy, anomalous ‘secondary’ cambia, anomalous ‘tertiary’ cambia, centripetal xylem development     

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     

Development and Evaluation of an In Vitro System to Study Strawberry Fruit Development

Growth and ripening of strawberry (Fragariaananassa Duch.)fruit harvested at immature stages of development was accomplishedby placing the peduncles of individual fruit in solutions composedof hydroxyquinoline hemisulphate (HQS) and sucrose. Fruit cultivarand developmental stage at harvest were the major determinantsof in vitro performance. ‘Pajaro’ fruit harvestedat 50 to 60% maturity exhibited the greatest and most uniformweight gain when placed in solutions containing 200 mol m^–3HQS and 88 mol m^–3 sucrose. Although the final fruit weightof in vitro-ripened fruit was less than that of field-ripenedfruit, colour development in vitro occurred at the same rateand to the same extent as field-grown fruit. Key words: Ripening, non-climacteric fruit     

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     

CORRECTIONS, VOLUME 29

Part 1, under the frontispiece portrait of Dr. N. B. Eales,the words ‘President 1948–1951’ should havebeen added. Page 103, line 49, for ‘Newton Collection’ read‘Norman Collection (Canon Norman)’. 185, line 37, for ‘capillaris’ read ‘capillacca’. 188, Table 1, for ‘bemoralis’. read ‘nemoralis’. 188, Table 2, for ‘Cochlicella acuta (Müll)? ventrosa(Fér.)’ read ‘Cochlicella ventrosa (Fér.)’. 191, line 24, for ‘araheo-’ read ‘archeo-’.     

Novel 'homeotic' CMS patterns generated in Nicotiana via cybridization with Hyoscyamus and Scopolia

Cytoplasmic hybrids (cybrids) that contain nuclear genetic materialfrom Nicotiana tabacum and cytoplasms from Hyoscyamus nigeror Scopolia carniolica were constructed by protoplast fusions.Both types of hybrids exhibited cytoplasmic male sterility (CMS).Furthermore, unusual floral morphogenesis marked by ‘greenflowers’ deprived of corolla and stamens occurred in themajority of the lines. Backcrosses of these plants with wild-typetobacco demonstrated a maternal inheritance of the ‘greenflower’ trait. After repeated transfer of cytoplasm (‘donor-recipientfusion’) from cytoplasmic hybrid N. tabacum (+H. niger)to albino plastome mutant N. tabacum DSR A15, male sterile tobaccoplants with two types of flowers were recovered (‘greenflowers’ and corolla-containing flowers with transformedstamens). RFLP analysis confirmed that N. tabacum (+ H. niger)and N. tabacum (+S. carniollca) as well as their sexual progeniescontained plastids from H. niger and S. carniolica, respectively.Mitochondrial DNA within the hybrids N. tabacum (+H. niger)originated from H. niger, but was obviously altered. Repeatedparasexual transmission, cybrids in the combination of N. tabacum+N.tabacum (+H. niger), reflected similar characteristics. Cybrids,N. tabacum (+S. carniolica) and their sexual progeny, whichresulted after pollination with wild-type tobacco, containeda modified mtDNA generally originating from tobacco. Furtherhistological analysis established the dramatic difference inthe composition of ‘green flowers’ and flowers ofwild-type tobacco. Therefore, the construction of tobacco cybridswith foreign cytoplasms provides a functional method for thede nova generation of alternative CMS types. Key words: Nicotiana, Hyoscyamus, Scopolia, cybrids, CMS, homeotic patterns     

Factors Affecting the Abscission of Reproductive Organs in Yellow Lupins (Lupinus luteus L.): III. ENDOGENOUS GROWTH SUBSTANCES IN VIRUS-INFECTED AND HEALTHY PLANTS AND THEIR EFFECT ON ABSCISSION

Extracts of small and mature-size lupin pods yielded four substancesaffecting the growth of wheat-coleoptile sections: one acidpromotor (A), two acid inhibitors(B and X), and one neutralinhibitor(Y). Inhibitor B was extremely active, however, coleoptile sectionsshowed no signs of toxic effects; they resumed growth at a rapidrate after rinsing them and adding ß-indolylaceticand (IAA) to the medium. 1 µg of IAA was required to counteractthe effect of ‘B’ extracted from 230 mg. Of tissue.On an equal fresh weight basis the inhibiting action of ‘B’in lupin pods was 500–1,500 times more potent than thatof ‘inhibitor ß’ in etiolated pea seedlings. Small pods of plants infected with pea-mosaic virus yielded3 times the amount of ‘A’ of healthy plants (equivalentto 1 µg. IAA 0.3 µg. IAA per 25 g. of tissue respectively),and approximately the amount of ‘B’. Mature podsof virus-infected plants again yielded more‘A’,but also 2? times more ‘B’ than pods of healthyplants. Healthy pods yielded more ‘A’ than virus-infectedpods, and there was no difference in ‘X’. A lupin abscission test was developed and the effects of proximaland distal application of -naphthyl acetic acid (NAA) are presented,and discussed with respect to results of other abscission tests. ‘A’ accelerated abscission when applied proximally,and delayed or prevented it when applied distally. ‘B’strongly accelerated abscission when applied in either way.A possible mechanism explaining the abscission-inducing effectof developing pods on later flowers is discussed in terms ofthe substances ‘A’ and ‘B’. The partlyprevented abscission observed on virus-infected plants was foundto agree well with the proposed mechanism.