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A scheme was developed to describe how the rate at which apical primordia are initiated in cereals depends on temperature and photoperiod. It was used to analyse initiation in field crops of winter wheat (Triticum aestivum L., cv. Maris Huntsman).For the initiation of leaves and spikelets, a straightforward linear regression of initiation rate R against mean air temperature (T a ) gave base temperatures (T 6 ) close to 0 °C for leaves and for spikelets. However, initiation rate was also correlated with mean photoperiod (P) and base photoperiods (P t ) of -0-5 h and 4h were established for leaf and spikelet initiation. When R was correlated with the index (T -T t ) (P -P b ), r 2 was 0-62 for both leaves and spikelets.

Main-stem apical development and its inter-relations with other aspects of plant morphology were studied in seven field crops of winter wheat (Triticum aestivum L., cv. Maris Huntsman).Apical primordia were produced slowly in autumn and winter, then faster in spring as the weather became warmer. Spikelets were initiated about three times as fast as leaves. Leaf initiation ended at around the time that tiller bud initiation stopped. Apical dome length and diameter both increased during leaf initiation, reached a maximum at double ridges and then decreased. Double ridges appeared when very nearly 50 % (range 49-52 %) of the final number of spikelets was present. Spikelet initiation ended a few days after the first florets were initiated and when stem extension began, but usually before the stem apex rose above the soil. Floret initiation finished at about the time that the flag leaf appeared. Throughout tillering there was a strong association between tiller emergence and leaf appearance.There were considerable differences between seasons in the timing of developmental events on the apices of main stems, but a difference in sowing date of 5 months was reduced to little more than 1 month in the timing of anthesis and the maximum numbers of organs varied only slightly with sowing date.

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An experiment to measure the variation in the phenological and apical development of winter wheat (cv. Avalon) in England and Scotland is described. Ten sites which ranged from Aberdeen (57-2° N), the most northerly, to Newton Abbot (50-6° N), the most southerly, were included in the survey, and at each site seed was hand-sown in midSeptember, October and November 1983. Developmental stages and sampling procedures were precisely defined to ensure uniformity in scoring by the observers at each site. Temperatures during the growing season were in line with the long-term means, though spring was cooler at all sites and summer warmer at most. The range of monthly-mean temperatures between sites was about the same as the difference between consecutive months. The method of analysis of development rates and durations was in terms of thermal time, modified by sensitivity to photoperiod and a vernalization requirement that slowed early development until a number of days of low temperatures had been experienced.In general, crops at northern sites developed more slowly than those in the south and particularly the south-west of England. There was less variation in the timing of apical stages for later sowings. Developmental rates responded linearly to temperature and photoperiod, with the base temperature increasing for later phases of development. The effect of photoperiod in modifying the rate of development was apparent for all developmental phases from emergence to anthesis, longer days accelerating development, but there was no effect on the duration of the grain-filling period. Vernalization exerted its effect solely within the phase from emergence to double ridge, and had a major influence on the variation between sites only for the first sowing.