Field experiments to determine the optimum fertilizer treatment and ratio of wheat to chickpea in late-sown irrigated intercropped populations were done during the winter seasons of 1983–85 on sandy loam soils at the research station of the Directorate of Pulses Research, Kanpur, India. Of the three populations tested (2:2, 2:1 and 3:1 row ratios of wheat: chickpea), the 2:2 row ratio allowed more light interception and transmission to the lower canopy and gave significantly higher yield (4155 kg/ha wheat equivalent) and land equivalent ratio (LER) than the other treatments. Chickpea yields were reduced more than those of wheat under all intercropping systems and the reduction was largest at the 3:1 row ratio. Wheat (cv. Sonalika) was more productive than chickpea (cv. BG261). Application of the normally recommended rate of fertilizer to the wheat only was as effective as applying fertilizers, at the appropriate rates, to both the component crops; indicating that fertilizers applied only to wheat were adequate to meet the requirements of the associated chickpea also.

Winter wheat cultivars Apollo, Hornet, Longbow and Norman were each sown at 50, 100, 200, 400, 800 and 1600 seeds/m2 in a field experiment conducted in Northern Ireland over the 1989/90 crop year. No growth regulators were applied and the wheat received 178 kgN/ha top-dressing in the spring. Hourly rainfall, windspeed and wind direction data were recorded and lodging was visually assessed from the end of May to harvest. Lodging first occurred in the 1600 seed-rate plots as the ears were emerging in early June and then progressively increased in the 800, 400 and 200 plots during June, July and August. Lodging did not occur suddenly but took several hours, with the stems first lying at an angle before lodging completely. Stem buckling or breakage did not appear to be the principal form of structural failure. The longest strawed cultivar, Longbow, lodged most severely. The shorter-strawed Norman also lodged badly in contrast with Hornet which had a similar straw length. Apollo, which was taller than Norman and Hornet and produced more ears per square metre than the other cultivars, lodged least but tended to lean at c. 30° from the vertical. Lodging occurred during or within 24 h of periods of rainfall which, in many cases, coincided with windspeeds at crop height averaging > 25 km/h and occasionally > 50 km/h. Lodging also occurred following rainfall when the windspeed did not exceed 16 km/h. The grain yield was negatively correlated with the average lodging from ear emergence to harvest, there being a 1 t/ha decline in yield for each 10% increase in average area lodged. The 50 and 100 seed-rate plots yielded 10 t/ha and had little or no lodging. The decline in yield with increased lodging and seed rate was attributed to the effect of lodging rather than to seed rate and was associated with a fall in the number of grains/ear and 1000-grain weight from 56 and 53·5 g at the lowest seed rate to 15 and 42·7 g at the highest, respectively. A comparison of the plants from lodged and unlodged plots of the 1600 and 800 seed rates, and subsequently of the 800 and 400 seed rates, indicated that at the higher seed rate, lodged plots had less fresh weight per unit area, basal internodes with smaller diameters, fewer support roots per stem, and a lower root dry weight per stem.

A long-term field experiment on permanent grass at Rothamsted (UK) was studied between 1988 and 1991 to determine the relationship between soil and plant K and to identify critical soil and plant concentrations for grass yield. Maximum dry matter yield was obtained at a soil exchangeable K concentration (Kex) of 114 mg/kg dry soil (45 μм-K in the soil solution). Grass well-supplied with K maintained an average concentration of c. 200 m м-K in the tissue water of whole shoots in spring, although individual measurements varied in the range 140–320 mм, depending largely on the soil water availability. These tissue concentrations were not exceeded in soils with Kex values up to 800 mg/kg. Yield was reduced by 60% on plots low in K (Kex in the range 60–90 mg/kg). The yield reductions were probably due to low turgor because tissue osmotic pressures were low and potassium concentrations in tissue water (which averaged 64 mм) were probably too high to disrupt the biochemical functions of K. The results suggest that there is no universal critical tissue water concentration for K in the biophysical role of turgor maintenance, as other cations and solutes are able to substitute for it. The ultimate determinant of turgor-dependent yield loss is tissue osmotic pressure. The critical tissue osmotic pressure in grass shoots in a typical season at Rothamsted was c. 400 mosm/kg. No universal critical osmotic pressure can be expected however, as this will depend on the availability of water and therefore on growing conditions.

Fifteen cultivars of orchardgrass (Dactylis glomerata L.) were grown in the field at Hiroshima University, Japan, to investigate seasonal changes in leaf water relations and cell membrane stability (CMS) measured by the polyethylene glycol (PEG) test. Leaf water potential and osmotic potential were measured from August 1988 to August 1989. Solute concentration in leaf cell sap was also estimated.

Cell membrane stability increased, leaf water potential and osmotic potential decreased and turgor potential increased with decreasing environmental temperatures during autumn and winter. The significant increases observed in CMS may enable plants to tolerate freezing temperatures during winter. Decrease in leaf water potential may be a result of water-deficit effects due to soil freezing at low temperatures and the decrease in osmotic potential may help plants to maintain turgor and tolerate freezing conditions. Plants maintained higher turgor as the osmotic potential decreased to values as low as – 3·98 MPa during winter; the maintenance of turgor helps to maintain water uptake under water deficit conditions at low temperatures.

Sugar and K were the major osmotic contributors in orchardgrass leaves. Sugar and Ca concentrations increased and Mg and P concentrations decreased at cold temperatures. K concentration increased in six cultivars and decreased in nine others at cold temperatures. Sugar concentration in cell sap was negatively correlated with osmotic potential. It was concluded that seasonal changes in CMS may be mainly associated with the osmotic potential of the leaf tissues.

The seed yields and maturity dates of an indeterminate cultivar (Lunoble) and a determinate line (CH304/70) of Lupinus albus L. were measured at three locations in France (Lusignan, Dijon and Gotheron) and at Rothamsted, UK, in 1989/90 and 1990/91. Different combinations of sowing dates, plant densities and irrigation treatments were tested at some sites.

Averaged over all sites, CH304/70 yielded more than Lunoble (3·26 v. 2·98 t/ha) but there were significant genotype × location interactions for yield and date of maturity. Both genotypes gave similar yields at the three locations in France (3·13 and 3·06 t/ha, respectively). But under the cool and wet conditions at Rothamsted, CH304/70 yielded 4·26 t/ha compared with only 202 t/ha for Lunoble. Yields of CH304/70 were more stable than those of Lunoble across locations and years; the coefficients of variation about the overall means were 14 and 22%, respectively. CH304/70 ripened up to 16 days sooner than Lunoble in France, and 6 weeks earlier than Lunoble in the cooler conditions of the UK.

In wet growing conditions (Lusignan 1989/90, Rothamsted 1990/91) Lunoble had increased vegetative growth which decreased the proportion of yield produced by mainstem pods and correspondingly increased the proportion borne on second-order branches. This continued branching also delayed harvest, especially in the UK where the crop was not harvestable until early November. In these conditions, CH304/70 outyielded Lunoble at Lusignan (3·4 v. 2·7 t/ha) and at Rothamsted (4·3 v. 2·0 t/ha) and was harvested acceptably early in mid-September. However, under warm, dry conditions, Lunoble produced only one order of branches at Gotheron and two orders at Lusignan in 1990/91. At Gotheron, both genotypes matured at the same time but CH304/70 yielded slightly better (3·1 v. 2·8 t/ha) because it produced a higher proportion of its yield on the mainstem, whereas at Lusignan, Lunoble yielded better than CH304/70 (4·1 v. 3·4 t/ha) because of extra yield from pods on second-order branches.

CH304/70 produced seeds of larger mean size and had greater stability of seed size across years and locations than Lunoble did. This was apparently the result of CH304/70's determinate structure, in which c. 75–100% of the yield was from pods on the mainstem and first-order branches, whose seeds were larger and less variable in size than those from pods on higher-order branches of indeterminates.

The development of autumn-sown genotypes with a determinate structure that combine adequate yield and early harvest under cool conditions offers the prospect of extending the geographical range of lupins, grown for grain, both to the cool regions of northern Europe and to hotter and drier regions in southern Europe where summer drought limits the yield of conventional indeterminate types.

The population dynamics of the potato tuber moth (PTM) Phthorimaea operculella (Zeller) and the influence of potato cultivar, planting date, planting depth, hilling-up, irrigation intervals and mulching on insect infestation and on the greening of tubers in the field were studied at Shambat, Khartoum North, Sudan in the 1987/88 and 1988/89 winter seasons.

The insect population initially increased slowly, then more rapidly after crop establishment and throughout the harvest period, after which it declined sharply. Tuber shape and skin characteristics had no effect on the degree of PTM infestation. Planting in the second week of November resulted in less insect damage and a significantly greater total yield compared with crops planted 3 weeks later. Greater depth of planting and more frequent hilling-up significantly lowered infestation levels and reduced the greening of tubers; for example, insect damage in early planted crops ranged from 3·3% (planting 10 cm deep and hilling-up thrice) to 16% (planting at 5 cm depth and hilling-up once).

Irrigation and mulching both significantly reduced PTM damage and the greening of tubers.

Light irrigation every 4 days and mulching with neem (Azadirachta indica) leaves during the last 4 weeks before harvest were the most effective treatments.

Significant reductions in losses inflicted by PTM and by the greening of tubers were therefore achieved by the manipulation of crop husbandry practices.

A bivariate normal model is proposed for the joint distribution of potato tuber size and weight. Parameters in the model are estimated from the yields and numbers of tubers in a range of riddle-size categories. The method is illustrated using data from a field experiment; parameters are estimated for each plot, and subjected to analysis of variance. The result is a more succinct summary of treatment effects than that produced in the traditional analysis, where data from each riddle size are analysed separately.

A decline in supplies of organic manures and the increased use of chemical fertilizers has led to a decrease in the micronutrient content of the soil, with attendant effects on yield.

Field studies were conducted in 1986 and 1987 at the Giza Agricultural Research Centre, Egypt, to determine the effect of Cu-EDTA and Mn-EDTA foliar sprays on growth, mineral content, yield components and fibre properties of the Egyptian cotton cultivar Giza 75 (Gossypium barbadense L.).

All combinations used significantly increased the uptake of Cu and Mn and the dry matter yield. The earliness of harvest and the value of yield components were all increased by the application of Cu or Mn; 25 mg/1 of both Cu and Mn gave the highest values. Lint percentage and fibre properties were not significantly affected.

This study indicated that the yield of cotton, grown under otherwise standard conditions, may be increased by foliar sprays of Cu-EDTA and Mn-EDTA combined.

Erect type groundnut germplasm belonging to the Valencia and Spanish botanical groups was examined in the field for five consecutive cropping seasons (wet 1988, 1989 and 1990 and dry 1989 and 1990) at Junagadh, India, in order to identify groundnut genotypes with the ability to grow in calcareous soil and tolerate iron deficiency (lime-induced iron chlorosis). On the basis of the visual chlorotic rating scores of their leaves, 17 and 19 genotypes of the Valencia and Spanish groups, respectively, which showed the least or no chlorosis were classified as tolerant (iron-efficient) to iron chlorosis. These genotypes may now be used as donor parents in groundnut breeding programmes for the development of iron chlorosis-tolerant varieties.

Almost all the iron-efficient groundnut genotypes had higher pod and seed (kernel) yields and Harvest Index (HI) than the susceptible types. The seed yield of 12 iron-efficient groundnut genotypes was either equal to or higher than a commercial cultivar which showed tolerance to iron chlorosis and, hence, these genotypes could profitably be grown in those areas where chlorosis due to iron deficiency is a major problem.

Between 1980 and 1986, six field experiments were conducted to investigate the relations between planting density, total dry matter and pod yield of groundnut (Arachis hypogaea L. cv. TMV2) grown at different levels of irrigation and rainfall at two sites in central India. In general, the relationship between total dry matter and planting density for most treatments was well described by the function:

where W is the crop dry weight per unit ground area, wm is the maximum weight per plant, Wm is the maximum crop weight per unit ground area and P is the plant population. Because the harvest index, h, was constant for each treatment irrespective of plant population, a similar equation described the relationship between pod yield and planting density. When nine of the eleven treatments planted in a square (i.e. 1:1) arrangement were compared, the asymptotic value Wm varied between treatments depending on available soil water and atmospheric demand. To quantify the effects of plant and environmental factors on crop productivity, a ‘transpiration equivalent’ (ωw; (g/kg)/kPa), i.e. the product of the dry matter/water ratio and mean seasonal saturation deficit D, was used as a crop constant to calculate productivity at each site or season from a knowledge of seasonal rainfall and/or irrigation and soil water-holding capacity. Thus, total crop productivity, W'8, was calculated using the equation W'8 = ωwS/D where S (mm) is a soil supply term dependent on soil water-holding capacity and monthly values of rainfall and/or irrigation. When values for Wm and W'8 were plotted against each other, a linear regression was obtained with a slope = 1·02 (R2 = 0·78). The mean harvest index of 0·38 was used to predict pod yield from a knowledge of W'8. It was concluded that of all the climatic, soil and management factors that influence crop growth in semi-arid situations, it is the interaction between the supply of and demand for water that ultimately determines total productivity, pod yield and optimum plant population.

In the autumns of 1985, 1986 and 1987, a total of 84 fields in England and Wales, which had received a recent dressing of organic manure or had recently been ploughed out of grass, was sampled to 90 cm depth to measure soil mineral nitrogen (Nmin i.e. NH4-N and N03-N). Amounts of Nmin varied widely between sites, indicating the difficulty in estimating soil supply following ploughing grass or manure application although type of manure and Nmin were related. Soils contained much inorganic nitrogen where manures containing a large proportion of readily available N (pig slurry or poultry manure) had been applied. Where manures containing straw were applied, Nmin was greater in the second year after application than the first. The relationship between Nmin and response of winter wheat to fertilizer was measured on 62 of the 84 sites sampled; there was no simple relationship between N min and response to N. For fields just ploughed out of grass, the relationship was particularly poor. However, for fields with a history of arable cropping, there was no response to N fertilizer when Nmin was > 300 kg/ha N in either autumn or spring. Below this amount, response to fertilizer N varied considerably even for sites with similar amounts of Nmin. Nmin measurements are of value in identifying soils containing sufficiently large N residues to allow fertilizer applications to be reduced, or even omitted, with confidence. Where residues are smaller, models need to be developed which take into account other factors, such as mineralization of organic nitrogen reserves, which modify nitrogen supply.

The production of biomass and its partitioning into stem, leaf, ear and dead material were monitored weekly in crops of wheat and oats from 16 May 1990, which corresponded approximately to ear emergence in the wheat, until ensilage on 29 June, 7 July and 19 July 1990 using the Ag-Bag system. An enzyme/inoculant additive was used on both crops on each ensilage date, although a portion of oats was ensiled without additive on the first two cutting dates, giving a total of eight silages. Changes in rumen degradability of dry matter were assessed for both crops throughout the monitoring period using the synthetic fibre bag technique. The digestibilities of gross energy, modified acid detergent fibre, organic matter and crude protein were determined in vivo using sheep.

Over the total period of monitoring, the daily rates of increase in dry matter (DM) yield were 15·1 (± 1·6) and 16·5 (±1·9) g DM/m2 for wheat and oats respectively. From 29 June to 19 July the rates of increase were 11·3 (± 3·1) and 23·1 (±6·0) g DM/m2. The increase in weight of the ears accounted for almost all of this increase and, by the end of the monitoring period, the ears contributed approximately half of the dry matter of each crop. In both crops the portion of dead material was small until 19 July, when there was a rapid increase in the amount of dead stem. The DM content of both crops remained < 30% up to day 33 (18 June) but increased from c. 33% to c. 52% between days 53 and 65 (8–20 July). Rumen degradability of both crops decreased rapidly from c. 66% on 16 May until 16 June, when it was c. 56% for wheat and 55% for oats; it remained constant at these values thereafter.

For silages made on the three successive ensilage dates, there was an increase in DM and starch contents but decreases in digestible energy, digestible fibre and digestible crude protein contents. Digestible organic matter was similar for the first two ensilage dates but lower in silage made on the third date.

The use of an additive with the oat crop resulted in decreases in the digestible energy, digestible organic matter and digestible crude protein contents of the silages compared with untreated oats.

Maximum yield of DM from wheat (18·6 t/ha) was obtained with the third cutting date, but optimum yield of energy (170 GJ/ha) and digestible crude protein (746 kg/ha) were found at the second cutting date. For oats, maximum yield of DM (17·3 t/ha) was again found with the third cut and maximum yield of energy (159 GJ/ha, untreated; 140 GJ/ha with additive) with the second cut. Digestible crude protein yield was greatest with the second cut of oats when no additive was used (708 kg/ha) but with additive it was greatest with the first cut (661 kg/ha).

It was concluded that, under these conditions, the optimum date for ensilage of wheat or oats was 7 July and that the use of the additive was detrimental to the quality of the oat silage.

Monensin, an ionophore, and Aspergillus oryzae fermentation extract (AO), a fungal feed additive, are growth promoters which modify rumen fermentation. The effects of combining these additives were determined in the rumen simulation technique (Rusitec). Sixteen vessels received 20 g/day of a diet of hay, barley, molasses, fishmeal and a mineral/vitamin mixture (500, 299·5, 100, 91 and 9·5 g/kg DM respectively). AO, monensin (M) and AO + monensin (AO/M) were each added to four vessels at 500, 10 and (500 + 10) mg/day respectively. Both M and AO/M increased propionate (14·5 and 138 v. 91 mmol/day in controls; P < 0·001) and reduced butyrate production (6·6, 5·2 and 9·1 mmol/day respectively; P < 0·01), whereas AO had no effect. AO increased bacterial numbers by 70% (P < 0·055), while M had no effect, and in the presence of M, AO also had no effect. Proteolytic activity in samples from the vessels decreased (P < 0·05) in all treatments (1·06, 0·71, 0·60 and 0·47 mg 14C-casein/mg protein/h for control, AO, M and AO/M respectively). In contrast, deamination of amino acids increased (P < 0·001) with AO and AO/M but decreased slightly with M alone (482, 646, 434 and 644 nmol NH3/mg protein/h). Ammonia output was unchanged (47·4, 57·2, 42·7 and 44·8 mg/day). Thus each additive was dominant over the other for different activities, with monensin generally suppressing the effects of AO rather than vice-versa.

Research into the mechanisms that control the initiation and development of wool follicles has been severely hampered by the inaccessibility of fetal skin in utero. The aims of this study were to determine whether ovine fetal skin, when grafted onto the athymic nude mouse (nu/nu), could be maintained outside the uterine environment and, if so, whether it would retain its ability to initiate and develop wool follicles capable of producing wool fibres.

Skin was removed from the mid-side region of ovine fetuses on days 45, 55, 65, 75, 85 and 95 of gestation, and transferred to graft beds prepared on anaesthetized nude mice. After developing for 20 days on the recipients, the grafts were excised for histological examination. Control ovine fetal skin was also obtained at the above times and from fetuses on days 105 and 115, and similarly processed for histological examination.

Fetal skin at all ages was successfully grafted onto nude mice; 90% of all grafts were accepted and maintained by the recipients. Follicle initiation and/or development occurred in all grafts, including those formed from day 45 fetal skin, collected and grafted prior to follicle initiation in vivo. The number of follicles in grafted skin was reduced compared to that in control fetal skin of equivalent age; however, follicle development was generally accelerated. Follicle initiation and development occurred predominantly in the peripheral zone of the grafts. Some of the follicles present in the skin at grafting were lost due to the grafting procedure, while others continued to develop and produce wool fibres. Follicle development varied considerably between grafts. All grafted skin exhibited premature loss of the periderm layer and cornification of the epidermis, probably in response to the exposure of the skin surface to the atmosphere. There was a notable absence of both arrector pili muscles and sweat glands associated with graft follicles, and a retardation of sebaceous gland development.

The grafting technique developed in this study has enabled ovine fetal skin to be maintained outside the uterine environment for extended periods of time and may provide an improved means for future investigation of wool follicle initiation and development.

Two experiments were conducted at the Massey University Deer Unit, New Zealand in 1990 and 1991 to evaluate the performance of lactating red deer hinds and their calves grazing conventional perennial ryegrass-based pastures, red clover or chicory. In both experiments, hind and calf performance was evaluated from 1 month post-parturition over a 2½ month summer period to weaning at 3½ months of age.

In Expt 1, hinds and calves were grazed on low (5·4 kg dry matter (DM)/hd/day), medium (10·8) or high (16·4) allowances of red clover, or on a medium allowance of a conventional ryegrass/white clover sward (9·9 DM/hd/day). In Expt 2, hinds and calves grazed equal DM allowances (12 kg DM/hd/day) of perennial ryegrass/white clover, chicory or red clover.

Red clover generally had higher organic matter digestibility (OMD) and higher total N than ryegrass/white clover, and when grazed at equal DM allowances, promoted higher voluntary food intake in the hinds, increased calf growth (430 v. 330 g/day) and increased hind liveweight change. Although decreasing the red clover allowance in Expt 1 slightly but non-significantly decreased hind voluntary food intake and decreased both calf and hind liveweight change, all low red clover values were still consistently higher than all ryegrass/white clover values (P < 0·05). In Expt 2, chicory was of higher OMD and ash content than red clover but N content was lower and similar to ryegrass/white clover. Chicory promoted lower levels of calf liveweight change than red clover but higher than ryegrass/white clover. Hind liveweight change on chicory was lower than on red clover and was similar to ryegrass/white clover. It was concluded that red clover offers potential as a special purpose forage for deer production and that further experimental work is needed with chicory.

A study of growth and venison production from weaner red deer grazing pure tetraploid red clover (RC) or conventional perennial ryegrass/white clover (PRG) pasture was conducted in 1990, with the objective of attaining a minimum slaughter liveweight of 92 kg (50 kg carcass) by 12 months of age in the stags. Ten weaner red deer stags and ten weaner red deer hinds were randomly selected and rotationally grazed on either RC or PRG forage. In autumn and spring, forage allowances were 7 kgDM/hd/day and 8 kgDM/hd/day respectively. In winter, the animals from both groups were grazed together on PRG pasture, at a residual dry matter (DM) of 1100 kgDM/ha. Total nitrogen (N) concentration was higher in RC on offer than in PRG on offer (autumn 3·4 v. 3·1 %DM; spring 3·8 v. 3·1 %DM), whilst organic matter digestibility (OMD; autumn 80·5 v. 76·5%; spring 82·1 v. 80·3%) was also higher for RC on offer. Diet selected showed similar differences in total N concentration, but there were negligible differences between forages in OMD.

Liveweight gains of RC and PRG stags were respectively 263 v. 192 g/day, 101 v. 106 g/day and 354 v. 341 g/day during autumn, winter and spring, with the corresponding values for hinds being 198. 173 g/day, 52 v. 53 g/day and 242 v. 218 g/day. At one year of age, stags grazing RC were 7 kg heavier and hinds 3 kg heavier than animals grazing PRG pasture. Animals grazing RC forage had higher voluntary feed intake (VFI) in both autumn (P < 0·10) and spring (P < 0·001), than animals grazing PRG pasture.

All stags grazing RC forage reached the minimum slaughter liveweight by one year of age, compared to 75% of those grazing PRG pasture. At slaughter, stags that had grazed RC produced heavier carcass weights (59·9 v. 54·5 kg, P < 0·01), had higher carcass dressing percentage (55·3 v. 53·2 %; P < 0·01), and tended to have slightly greater carcass subcutaneous fat depth than stags grazing PRG pasture, but this effect disappeared when the data were corrected to equal carcass weight. All stags grazing RC produced velvet antler, relative to 75% of those grazing PRG; in stags producing harvestable velvet antler, there was no difference in antler weight between those grazing RC and PRG. It is concluded that RC offers potential as a special-purpose forage for the growth of weaner red deer.

Sambar deer (n = 8) and red deer (n = 8) fawns were successfully artificially reared to 70 days of age, using ewe milk replacer, at Flock House Agricultural Centre, New Zealand, during 1991. Sambar deer fawns had a lower overall milk consumption than red deer fawns (312 v. 359 g DM/day; P < 0·05), and showed an earlier peak in milk consumption, a faster rate of decline and earlier self weaning. Birth weight as a proportion of dam liveweight was lower for sambar than for red deer, but liveweight gains to weaning (347 v. 330 g/day) and 70 day weaning weights (300 v. 30·4 kg) were similar. The age at which deer commenced a range of activities, including eating forage and ruminating, was similar for both species, except that jumping activities commenced 5 days later in sambar than in red deer (P < 0·01). Following milk feeding, sambar fawns were less active than red deer fawns. It was concluded that sambar deer fawns can be successfully artificially reared using ewe milk replacer, but that extra precautions are needed to avoid scouring and abomasal bloat, which were more prevalent in sambar than in red deer fawns.

This group, which is concerned with the applications of mathematics to agricultural science, is sponsored by the Agricultural and Food Research Council. It was formed in 1970, and has since met at approximately yearly intervals in London for one-day meetings. The twenty-third meeting of the group, chaired by P. H. Nye of the Department of Plant Sciences at the University of Oxford, was held in the Wellcome Meeting Room at the Royal Society, 6 Carlton House Terrace, London on Friday, 7 May 1993, when the following papers were read.