Entry Six: Flowering Plants, Pollination, and Coevolution

In recent evolutionary times, some flowering plants have formed relationships with an animal that not only disperses their seeds but also provides the plants with water and mineral nutrients and vigorously protects them from encroaching competitors, pathogens, and predators. In return for these favors, the animal gets to eat a fraction of the plants’ seeds and fruits. Take a guess at what the animal is. Come on, guess. After reading five entries of words and information and facts and more words, it’s okay. You were probably too bored to use your brain, so I will tell you that the plants involved in these mutually beneficial interactions are called crops and the animals are humans. Since the origins of crop domestication over ten thousand years ago, plant breeders have genetically manipulated the traits of a few hundred angiosperm species by artificial selection, transforming them into the crops we grow today. In this entry, we will be talking about angiosperm reproduction, or angiosperm sex.

The life cycles of plants are characterized by an alternation of generations, in which multicellular haploid (n) and diploid (2n) generations take turns producing each other. The diploid plant, the sporophyte, produces haploid spores by meiosis. These spores divide by mitosis, giving rise to the multi cellular gametophytes, the male and female haploid plants that produce gametes (sperms and eggs).  Fertilization, the fusion of gametes, results in diploid zygotes, which divides by mitosis and form new sporophytes.

Flowers, the reproductive shoots of angiosperm sporophytes, are typically composed of four whorls of modified leaves called floral organs. Flowers are determinate shoots; they cease growing after flower and fruit are formed. Floral organs – sepals, petals, stamens, and carpels – are attached to a part of the stem called the receptacle. Stamens and carpels are reproductive organs, whereas sepals and petals are sterile. Sepals, which enclose and protect unopened floral buds, are usually look more leaf-like in appearance than the other floral organs. Petals are typically more brightly colored and advertise the flower to insects and other pollinators. A stamen consists of a stalk called the filament and a terminal structure called the anther; within the anther are chambers called microsporangia (pollen sacs) that produce pollen. A carpel has an ovary at its base and a long, slender neck called the style.  At the top of the style is a generally sticky structure called the stigma that captures pollen. Within the ovary are one or more ovules. Complete flowers have all four basic floral organs. Some species have incomplete flowers, lacking sepals, petals, stamens, or carpels. Flowers also vary in size, shape, color, odor, organ arrangement, and time of opening.

In angiosperms, pollination is the transfer of pollen from an anther to a stigma. It is accomplished by wind, water, or animals. In wind-pollinated species, the release of enormous quantities of smaller-sized pollen compensates for the randomness of dispersal by the wind. Some species of aquatic plants rely on water to disperse pollen. The majority of angiosperm species, however, depend on insects, birds, or other animal pollinators to transfer pollen directly from one flower to another. If pollination is successful, a pollen grain produces a pollen tube, which then grows down into the ovary via the style for fertilization.

The joint evolution of two interacting species each in response to selection imposed by the other is called coevolution. Many species of flowering plants have coevolved with specific pollinators. Natural selection favors individual plants or insects having slight deviations of structure that enhance the flower-pollination mutualism. An example of coevolution would be how the long floral tube of the Madagascar orchid has coevolved with the twenty eight centimeters long proboscis, a straw-like mouthpart, of the hawkmoth, its pollinator. Another example of coevolution would be how plant toxins with caterpillars. Caterpillars evolved to be able to eat more poisonous plants. As they eat more of these plants, they build better resistance to these toxins and even predators and are able to pass their traits toward their offspring. The plants are also evolving in which they would produce more toxins to ensure their survival.