development of a cotton plant 9

SEED GERMINATION AND EMERGENCE and a poorly developed epicotyl (embryonic A seed with all fibers removed is an ovoid, stem). The cotyledons, or seed leaves, form somewhat pointed, dark brown structure. The the first green leaves after emergence. Initially seed consists of a seed coat surrounding an cotyledons contain stored food that supplies the embryo with two well-developed cotyledons energy for germination and early development. (Figure 9). The embryo axis consists of a radicle There are usually about 3,500-4,000 delinted seed (embryonic root), a hypocotyl, two cotyledons, per pound.

A B

Figure 9. A diagram (A) and photo-micrograph (B) of a longitudinal section through a mature seed showing the seed coat and young embryo with folded cotyledons. (A, D. Oosterhuis; B, R.W. Seagull) 10 cotton fiber development

Germination begins within the first few grow rapidly, arching near the cotyledons. With hours of the entry of moisture into the seed. continued elongation of the hypocotyl, the This results in increased oxygen uptake and the cotyledons and embryonic shoot are pulled up utilization of stored energy reserves to build through the soil surface (Figure 10). This is called new cells and tissues during embryonic growth. emergence. Typically, the seed coat is shed and The seed/embr­yo swells as water is absorbed remains in the soil. Soil crusting due to surface causing the seed coat to split. Under favor­ compaction or high clay content may hinder the able conditions for germination, the radicle emergence of the cotyle­dons and embryonic emerges through the pointed micropylar end of shoot. When the cotyledons are free of the soil, the seed in two to three days (Figure 10). The they unfold and expand. After emer­gence and radicle becomes the primary root that grows exposure to the ligh­t, the cotyledons develop downward into the soil. The tissues between chlorophyll and are capable of synthesizing food the radicle and cotyledons (i.e. the hypocotyl) via photosynthesis.

Figure 10. Diagram of stages of seed germination and seedling emergence. (D. Oosterhuis)