The emergence of the plumule above the growing medium is usually the first occasion that the seedling is subjected to light. This stimulus inhibits the extension growth of the stem so that it becomes thicker and stronger, but the seedling is still very susceptible to attack from pests and damping-off diseases. The leaves unfold and become green in response to light, which enables the seedling to photosynthesize and so support its development. The first leaves to develop, the cotyledons, derive from the seed and may emerge from the testa while still in the soil, as in peach and broad bean (hypogeal germination), or be carried with the testa into the air, where the cotyledons then expand (epigeal germination), e.g. in tomatoes and cherry.
Phototropism occurs so the shoot grows towards a light source that provides the energy for photosynthesis. A bend takes place in the stem just below the tip as cells in the stem away from the light grow larger than those near to the light source. A greater concentration of auxin in the shaded part of the stem causes the extended growth (see Figure 11.2). Roots display a negative phototropic response, growing away from light when exposed at the surface of the growing medium, e.g. on a steep bank. The growth away from light may supersede the root's geotropic response, and will cause the roots to grow back into the growing medium.
Hydrotropism is the growing of roots towards a source of water. The explanation of this tropism has not been found, but it can be shown to occur. The cotyledons that emerge from the testa contribute to the growth of the seedling in photosynthesis, but the true leaves of the plant, which often have a different appearance to the cotyledon, very quickly unfold.
Hypogeal germination occurs when the cotyledons develop above ground outside the seed.
Epigeal germination occurs when the testa merges above ground initially enclosing the cotyledons.
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