填空题 .Plant Pollination
    Plants reproduce when pollen from the anther—the male part—spreads to the stigma—the female part—and fertilizes the plant, enabling it to produce the seeds necessary for reproduction. Because the anther and stigma are far apart in most plants—and not even present in the same plant at times—the pollen must spread from the anther to the stigma by external means. Pollination occurs in most plants by one of two methods: The pollen can be spread by the actions of insects and birds, or it can be carried by the wind. Insects and birds spread pollen in most flowering plants while the wind carries pollen in a few flowering plants and most nonflowering plants, including members of the conifer family, grasses, and cereal crops such as wheat.
    The majority of plants pollinated by the wind have evolved without colorful, scented flowers that contain the nectar which attracts insects and birds, so they cannot rely on them for pollination. In addition, while many nonflowering plants have both male and female parts, nature has made them unable to inbreed, so a plant cannot use the pollen from its anther to pollinate its own stigma. ❶Therefore, these plants must spread their pollen to the female parts of other plants, which could be growing nearby or might be hundreds of meters away. ❷These plants must therefore rely on the wind to carry their pollen to other plants of the same species, ❸Some nonflowering plants have developed so that they contain only male parts or only female parts, making it impossible for them to inbreed, so they must be pollinated by another plant.❹
    Nonflowering plants have, however, evolved in a manner that has bestowed them with several characteristics making it simpler for them to be pollinated by the wind. For example, pollen is produced in the anther in far greater quantities in nonflowering plants than in flowering plants to increase the success rate of pollination. The majority of this pollen never arrives at its intended destination, but some does, so having more pollen increases the chances of successful interaction with the stigma of another plant. Furthermore, the anther in nonflowering plants is very loosely attached and dangles more outside the plant compared to the anther in flowering plants. This allows the wind to strike it and then carry pollen away. Plants that receive pollen also possess enhanced abilities to catch it. The stigma hangs outside these plants so that it can lie in the path of windblown pollen, and many nonflowering plants have a feathery or netlike stigma, permitting them more easily to capture pollen.
    Unlike flowering plants, which have sticky pollen that can cling to insects and birds, nonflowering plants have loose pollen that the wind can pick up and distribute. The pollen is also light, ensuring that it can travel great distances. The pollen is small and not particularly nutritious, so it is unattractive to insects and birds. These characteristics all combine to increase the likelihood that, of the entire mass of pollen released by a plant's anther, at least some will survive the journey to the stigma of another plant and permit reproduction to take place.
    *pollen: the fertilizing part of a plant, which often appears in powder form
    *conifer: a type of tree that produces pollen in cones and includes evergreen trees
    *inbreed: to breed with closely related units or oneself36.  Vocabulary
    ______=having a strong smell, usually pleasant in nature