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The first signs of the transition from the vegetative to the reproductive phase are that the stem apex elongates to 0.3 mm with the appearance of primordia as single ridges. At this stage, the stem apex is still close to ground level. The buds in the axils of the apex ridges are spikelet primordia and, with their leaf initials form double ridges as the developing spike elongates to between 0.8 and 1 mm (Figure 1.7). In broad terms, double ridges occur as the plants take on a more upright appearance as leaf sheaths elongate and pass through the pseudostem erect growth stage (Figure 1.11) (Barber et al. 2015).

The structures of the ear develop as it is simultaneously elevated through the leaf sheaths of the canopy by the lengthening stem. Stem elongation is evidenced by the detection of nodes behind the rising apex (Figure 1.11). As the spike continues to elongate, the central spikelets swell while additional double ridges are formed acropetally (upwards) until the terminal spikelet is formed at the apex. By this time, the developing ear is usually 10–30 mm above the base (or crown) of the plant, and between one or two nodes are detectable (DGS31–32; Figure 1.7; Barber et al. 2015). The spikelets continue to differentiate and develop scaly tissues (glume, lemma, palea) and potential sites for grain formation (florets) (Figure 1.12). Floret initiation continues until about the time that the last leaf (the flag leaf) emerges and unfolds (DGS37–39; Figure 1.11). Between 7 and 11 florets may be initiated in each spikelet but, as with tillers, many more florets are initiated than develop to maturation with a grain.

Figure 1.11 Stem extension, booting, ear emergence, and anthesis of wheat. Boxed numerals are the decimal growth stage scores from Table 1.4.