Reading Passage 1


Plants have evolved a wide variety of methods to reproduce themselves. Some plants reproduce asexually by splitting off new roots or bulbs (e.g., garlic, lilies) or even branches, stems, or leaves (e.g., mangroves, spider plants). Plants that reproduce asexually are essentially reproducing clones of themselves. This is a simple and direct method of reproduction, producing new plants more quickly and with less energy than plants using sexual reproduction. The majority of plants, however, reproduce sexually. The advantages from an evolutionary perspective include more genetic variety and better dispersal than the colonies of clones formed by asexual reproduction. In flowering plants, pollen (male) grains are moved from the anther to the stigma, where the pollen fertilizes the ovaries (female), resulting in seeds.

A few flowering plants such as peas, beans, and tomatoes pollinate themselves, but more commonly, pollination occurs between separate plants, either through pollen being borne by the wind (most conifers and many grasses) or by pollinators, animal species that plants rely on to help move the pollen from one plant to the ovaries of another. Most pollinators are insects, but some species of bird and bats also play an important role.

Plants have evolved a variety of methods to entice pollinators to do their work. Many produce nectar, a sugary substance that pollinators use as-food. A well-known example is the honeybee, which collects nectar as well as pollen for food. When a bee enters one flower, it brushes against the anther, and pollen grains are picked up by the surface of its body. When the bee enters a second flower and brushes against the stigma, some of that pollen comes in contact with the ovaries of the second plant, thus fertilizing it, resulting in seeds that contain genetic material from the male gametes of the first plant combined with the female reproductive organs of the second plant. Most bees, butterflies, and moths, as well as certain species of bats and birds, are attracted to nectar-producing flowers.

Flowering plants have evolved a variety of methods for signaling their usefulness to pollinators or for otherwise making their work easier. Butterflies are attracted to flowers that are open during the day, are bright—typically red, yellow, or orange—and have a “landing platform.” In contrast, many moths are active at night and thus are attracted to flowers that are pale or white, have a strong fragrance, but also have broad areas to land on. Both butterflies and moths have long tongues and have coevolved with plants that have developed deep sources of nectar that are available only to certain species. Hummingbirds are also attracted by color especially by bright reds, and flowers that attract these tiny birds also have strong stems and are designed for pollen to be brushed on the hummingbirds’ heads as they sip nectar.

Bees do not see red; thus, flowers that attract bees tend to be blue, yellow, purple, or other colors. Many bee attractors also have nectar guides, which are spots near the center’ of each flower that reflect ultraviolet light, making it easier for the bees to find the nectar. Bees are also attracted to flowers with a mint like or sweet smell. Snapdragons not only attract bees visually, they are adapted to appeal to certain bee species: snapdragons have a landing platform that, if the bee is the correct weight, opens—allowing access to the nectar and pollen.

Pollinators play a major role in agriculture. While many staple crops such as rice, corn, canola, and wheat are self-pollinating or pollinated by the wind, farmers are dependent on pollinator species for many fruit, vegetable, nut, and seed crops. Over 30 percent of the world’s crops require the work of pollinator species. Bees are the most common agricultural pollinators, with crops including fruit trees such as apples and cherries; vegetables such as squash, beans, tomatoes, and eggplant; flowering shrubs and annual and perennial flowers; forage crops such as clover and alfalfa; and fiber’ crops such as cotton. Other pollinators include midges (cocoa), wasps (figs), moths (yucca, papaya), butterflies (asters, daisies, marigolds), and even a few species of bats (agave, palms, durians) and humming-birds (fuchsia).

Recent declines in honeybees and in other pollinator species around the world have raised concerns about future food production, and many scientists have called for increased study of the role of pollinators, the agricultural and environ-mental changes involved in the declines, as well as the economic and environ-mental effects and ways to prevent further declines.