THE LIVING WORLD
Unit Seven. Plant Life
What happens to a seed when it encounters conditions suitable for its germination? First, it absorbs water. Seed tissues are so dry at the start of germination that the seed takes up water with great force, after which metabolism resumes. Initially, the metabolism may be anaerobic, but when the seed coat ruptures, aerobic metabolism takes over. At this point, oxygen must be available to the developing embryo because plants, which drown for the same reason people do, require oxygen for active growth (see chapter 7). Few plants produce seeds that germinate successfully underwater, although some, such as rice, have evolved a tolerance of anaerobic conditions and can initially respire anaerobically. Figure 34.8 shows the development of a dicot (on the left) and monocot (on the right) from germination through early stages. The first stage in both cases is the emergence of the roots. Following that, in dicots, the cotyledons emerge from underground along with the stem. The cotyledons eventually wither and the first leaves begin the process of photosynthesis. In monocots, the cotyledon doesn’t emerge from underground; instead a structure called the cole- optile (a sheath wrapped around the emerging shoot) pushes through to the surface where the first leaves emerge and begin photosynthesis.
Figure 34.8. Development of angiosperms.
Dicot development in a soybean. The first structure to emerge is the embryonic root followed by the two cotyledons of the dicot. The cotyledons are pulled up through the soil along with the hypocotyl (the stem below the cotyledons). The cotyledons are the seed leaves that provide nutrients to the growing plant. As other leaves develop, they provide nutrients through photosynthesis, and the cotyledons shrivel and fall off the stem. Flowers develop in buds at the nodes. Monocot development in corn. The first structure to emerge is the radicle or primary root. Monocots have one cotyledon, which does not emerge from underground. The coleoptile is a tubular sheath; it encloses and protects the shoot and leaves as they push their way up through the soil.
Key Learning Outcome 34.6. Germination is the resumption of a seed's growth and reproduction, triggered by water.