12.1: Plants are just one branch of the eukarya.

The land plant Psilotum nudum, growing in Hawaii Volcanoes National Park.

An animal can move from place to place to get food or water, to avoid being eaten, or to find a mate. It’s harder for a plant. It must do all the things an animal does—obtain food and water, protect itself from predators, and reproduce—but can’t move a centimeter in the process. Wherever a seed puts down roots, that’s where the plant stays anchored for its entire life.

So, plants are organisms that are fixed in place, but that’s only one aspect of being a plant. A plant is a multicellular eukaryote that produces its own food by carrying out photosynthesis, using energy from sunlight to convert carbon dioxide and water to sugar—and has an embryo that develops within the protected environment of the female parent. Plants occur almost exclusively on land, and they vary in size from less than 0.04 inch (1 millimeter) to 380 feet (116 meters) tall (FIGURE 12-1). (Note that although there are a small number of exceptions to our definition of a plant, it applies to the vast majority of the more than 280,000 species of plants on earth today.)

Figure 12.1: The defining characteristics of a plant.

There are other multicellular and photosynthetic eukaryotes on earth, including many species of algae, such as seaweed, as well as aquatic species that are the closest relatives of land plants. They differ from plants, however, in that they live only in water or on very moist land surfaces. This is in sharp contrast to most plants, which can live even in deserts.

As we have said, most plants make their own food by the process of photosynthesis. However, a plant can’t live on carbohydrates—the product of photosynthesis—alone. A plant needs nitrogen to build proteins, phosphorus to make ATP, and salts to create concentration gradients between the inside and outside of cells. Plants use roots, the part of a plant below ground, to obtain these needed substances from the soil. Above ground, plants have a shoot that consists of a stem and leaves. The stem is the structure that supports the main photosynthetic organ of a plant: its leaves.

When we think of plants, we often think of them as “chlorophyll-containing,” but some plants have no chlorophyll—their ancestors had chlorophyll (which is why they are classified as plants), but they have lost almost all of it over evolutionary time—and they can’t carry out photosynthesis. Instead, they live as parasites that steal nutrients from other plants. Dodder is an example of a parasitic plant that almost completely lacks chlorophyll and gets its sugar from the host plant it grows on (FIGURE 12-2). You can probably find dodder growing on plants in a nearby vacant lot or at the roadside.

Figure 12.2: Dodder: a parasitic, non-photosynthetic plant.

Plants that carry out photosynthesis need sunlight, and that creates a challenge for some plants. Think about plants growing on the floor of a forest. It’s mostly shady under the tree canopy, with just a few sunny spots where light penetrates the tree branches overhead (FIGURE 12-3). If a seed sprouts (germinates) in the shade, the young plant needs to somehow get to the nearest sunny spot. Plants can do that, of course, but not by moving. Instead, they grow toward the light. You may have seen a plant called a “lucky bamboo” (it’s not really bamboo), which can grow in spirals or loops. The growers have produced these shapes by moving the light source every few days, forcing the plant to bend to follow the light. If you buy lucky bamboo, you must continue rotating the light source or it will start to grow straight, like any other plant.

Figure 12.3: Plants must overcome the constraints of being immobile.

Sex can also be a challenge for an organism that is anchored in place by its roots. Not surprisingly, plants have a sex life very different from that of animals. Consider humans as a typical animal. From the moment an egg is fertilized by a sperm, a human has the diploid number of chromosomes—that is, two sets of chromosomes, one set from each parent. Only the egg and sperm are haploid—an egg or a sperm has only one set of chromosomes. In plants, the haploid stage allows the male and female to reproduce, even though the plants will never meet because they can’t move. Some plants enlist the help of animals to carry the male gamete to the female gamete.

Resisting predators is another challenge for organisms that cannot move. Running away is how most animals react to a predator, but plants have found other ways to defend themselves. Thorns, for example, are an anatomical defense that plants use to avoid predation (see Figure 12-3). Plants also use chemicals to deter predators, as many people learn when they develop an itchy rash from poison ivy.

The earliest land plants were the first multicellular organisms to live on land. They were non-vascular, meaning that they had no tube-like vessels to transport water and nutrients (FIGURE 12-4). The subsequent evolution of land plants was a series of radiations of forms with characteristics that made them increasingly independent of water. The evolutionary tree of plants shows these stages clearly: first the development of vessels to conduct water from the soil through the plant (the vascular plants), then seeds that provide nutrients to get the next generation off to a good start (the gymnosperms), and finally flowers that allow plants to entice or trick insects and birds into spreading the plant’s male gametes (the angiosperms). We’ll look at each of these plant adaptations in this chapter. We also explore the ecologically important kingdom of fungi. Although fungi are not plants, they are very closely associated with plants.

Figure 12.4: Phylogeny of the plants.