L. and Gossypium barbadense
Section 3 Morphology 3.1 Plant morphology
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- 3.2 Reproductive morphology
Section 3 Morphology3.1 Plant morphologyIn nature, G. hirsutum is a perennial shrub that grows to approximately 1.5–2 m in height, while G. barbadense grows to approximately 3 m in height. Commercially, however, both species are cultivated as approximately 1–1.5 m tall annuals, with destruction of plants after harvesting the fruit for seed and fibre. Cotton plants have an indeterminate growth habit, meaning the plant can develop leaves, stems, flowers, fruit (bolls) and seed all at the same time. The branches on the cotton plant can be classified as either vegetative or fruiting branches. Vegetative branches have only one meristem and so grow long and straight, whereas fruiting branches have multiple meristems, each starting after the previous fruiting bud and as such exhibit a zig-zag growth habit. The first five main stem nodes support primarily vegetative growth and fruiting branches commence thereafter, with branches showing a ⅜ alternate phyllotaxy as shown in Figure 8 (Oosterhuis & Jernstedt 1999; Ritchie et al. 2007). 
Figure 8: Cotton plant morphology (a) A defoliated cotton plant shows the 3/8 alternate phyllotaxy of branches. Each branch is 3/8 of a turn around the stem from the branch below it. The branches form from the axils of main stem leaves. (b) A diagram of the general timing of flower emergence from buds on the fruiting branches by fruiting position (used with permission from Ritchie et al. 2007) G. hirsutum is heliotropic, its leaves are generally flat and track the sun to maximise light adsorption throughout the day, whereas G. barbadense leaves are stationary and are cupped to maximise capture morning and afternoon sunlight, but provide shading in the middle of the day to reduce photobleaching and transpiration (Sassenrath-Cole 1995; Wise et al. 2000). G. barbadense also have more stomata than G. hirsutum, but these stomata are smaller so the stomatal area per leaf is less than G. hirsutum (Lu et al. 1997; Wise et al. 2000). Generally leaves on the main stem axis have seven lobes in G. barbadense or five for G. hirsutum, whereas those on the fruiting branches have three lobes in either species (Gore 1935). Further comparisons between the vegetative morphology of G. hirsutum and G. barbadense are outlined in Table 4. Table 4 Comparative cotton plant morphology (Fryxell 1984).
3.2 Reproductive morphologyCotton flowers are large (5–9 cm), perfect (that is contain both male and female structures) and pentamerous (parts arranged in fives). They have both floral and extra-floral nectaries (Moffett 1983). The style is 2–5 cm long and terminates in the 0.5 1 cm-long stigma. The ovary contains 5–10 ovules in each of 3–5 sections, or locules. The stamina sheath, which encloses most of the style, bears numerous stamens 0.5–1 cm long, each terminating in an anther that normally produces an abundance of viable self-fertile pollen (McGregor 1976). There are approximately 20,000 pollen grains per flower (Ter-Avanesian 1978). The flowers of G. hirsutum and G. barbadense differ in appearance and in their presentation of pollinator foraging cues (see Figure 9). G. hirsutum flowers are cream in colour, with cream pollen and secrete a low volume of nectar, whereas G. barbadense flowers are yellow, with a maroon nectar guide, orange pollen and produce more nectar with a lower sugar concentration than G. hirsutum (McGregor 1976; Moffett 1983). Furthermore, the G. barbadense stigma extends well above the anthers, unlike G. hirsutum (McGregor 1976), and this may affect the likelihood of cross pollination occurring. It has not been determined whether or not these differences make G. barbadense flowers more attractive to native Australian insect pollinators than G. hirsutum. Further comparisons between the reproductive morphology of G. hirsutum and G. barbadense are outlined in Table 5. Figure 9: Cotton flowers (a) Annotated G. hirsutum flower (used with permission from Ritchie et al. 2007); (b) G. barbadense flower (photo OGTR 2007). Table 5 Comparative cotton reproductive morphology (Fryxell 1984).
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