Cloning isn’t new. Gardeners have been doing it for centuries. And before that, plants were doing it for millennia—without any help from humans. Cloning is just a more modern term to describe the asexual reproductive process by which one organism gives rise to another, genetically identical individual. As we’ll see in this chapter, for the vast majority of plant species, sexual reproduction is the predominant mode of reproduction, but nearly all plant species have also retained the ability to reproduce without any participation from another individual.

Most of the trees or shrubs we see in gardens are, in fact, the result of asexual reproduction. So, too, are nearly all houseplants purchased from nurseries. How does asexual reproduction in plants occur? The process is simple. New individual plants grow directly from the tissue of established plants through mitosis. There is no meiosis to generate gametes, and there is no fertilization of one gamete by another. Asexual reproduction is also called vegetative reproduction, because it starts with a vegetative structure of the plant (such as a stem or root or leaf), not involved in sexual reproduction, which becomes separated from the parent plant and continues to grow to form a new individual.

Raspberry plants, for example, can be propagated by cutting sprouts from their roots (FIGURE 18-3). And potato plants can be generated from their underground storage stems (that is, from the potato itself). Thus, a new potato plant can be generated just by planting a potato and waiting for it to sprout into a genetically identical individual. We consider the new plant and the “parent” plant distinct individuals because they have become separated and can grow and function independent of one another. In a group of plant species called Bryophyllum small “plantlets” grow right along the edges of the leaves, eventually dropping to the soil, taking root, and growing as independent plants—a reproductive pattern responsible for the plants’ common name: “mother of millions.”

Figure 18-3Reproduction without a partner. Most plants have the option of asexual reproduction. No partner is required, so it is quick and efficient, but no genetic variability is present among the cloned offspring.

The critical feature in every case of asexual reproduction is that the bit of the plant that is used to start a new individual must have some undifferentiated cells. Such cells—like stem cells in humans—are totipotent, meaning that they have the developmental potential to become any type of cell and tissue. Later in the chapter, in our discussion of plant growth, we talk more about what totipotent cells are and where they are located.

The ability to reproduce asexually is an evolutionary adaptation that helps plants overcome the fact that they are rooted in the ground and cannot easily seek out sexual partners. By removing the need to locate a suitable partner, asexual reproduction confers important benefits. It is not a perfect solution, however. In most situations, there is a critical disadvantage to asexual reproduction. The advantages and disadvantage can be summarized quite simply.

Advantages of Asexual Reproduction There are three main advantages to reproducing asexually (FIGURE 18-4).

Figure 18-4Asexual reproduction. It is fast and efficient but produces no genetic variation among offspring.

• 1. It is energetically efficient. No energy is wasted in producing flowers, pollen, and fruits—plant tissues used exclusively for reproduction. Instead, the plant’s energy can be invested in producing roots, support and nutrient-transporting tissues, leaves, and other vegetative matter for growth and increased photosynthetic capacity.
• 2. It is faster. Participation of another individual is not required, nor is a means of bringing together the male and female gametes. As a result, new individuals can be produced at a much faster rate, which enhances an individual’s reproductive success and fitness.
  
• 3. It preserves winning allele combinations. If a plant is well adapted to a particular environment, all the individuals it produces by asexual reproduction will carry the same well-adapted set of alleles and are likely to flourish just as the parent plant did.

Disadvantage of Asexual Reproduction Although it has some advantages, asexual reproduction has one huge disadvantage relative to sexual reproduction: it leads to no new genetic variation among the offspring. Asexually reproduced individuals are identical to their parents (with the rare exception of any mutations that might occur). If the environment is changing—perhaps the climate is changing, or a new variety of pest species is on the increase—the new individuals are less likely to be adapted to the changed environment. Moreover, as more and more of the individuals in an area are produced by asexual reproduction, if those individuals then reproduce together sexually, there will be less genetic variation among their offspring than if the parents were not genetically identical. Each parent plant will simply draw from the same set of alleles when producing gametes. This reduced variety in genetic material can leave plant populations in a vulnerable state, with a reduced capacity to evolve.

We turn next to how plants produce genetically varied offspring through sexual reproduction.