Monocots or monocotyledons make up nearly one-quarter of all angiosperms. Monocots come in all shapes and sizes and are found in virtually every terrestrial habitat on Earth (Fig. 33.21). Coconut palms, hanging Spanish moss (a relative of pineapple), and tiny floating duckweeds, less than 2 mm across, are all monocots, as are the sea grasses of tropical lagoons and the lilies and daffodils in the garden. Some monocots grow in arid regions—including agave, which is used to make tequila. Others, such as orchids, grow primarily as epiphytes in tropical forests. A few monocots form vines—for example, the rattan palms, whose stems are used to produce rattan furniture. Monocots are the most important group of plants in terms of what we eat. Grasses, for example corn, rice, wheat, and sugarcane, are staples in the diets of most people. Other monocots that make their way to our dinner table include banana, yams, ginger, asparagus, pineapple, and vanilla.

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Monocots take their name from the fact that they have one embryonic seed leaf, or cotyledon, whereas all other angiosperms have two. However, monocots are distinct in form in so many other ways that one rarely has to count the number of cotyledons to identify a member of this large, monophyletic group. Monocots represent a major evolutionary departure in the way seed plants build their bodies, in that a vascular cambium is never formed. How then has this group been so successful?

In monocot leaves, the major veins are typically parallel and the base of the leaf surrounds the stem, forming a continuous sheath. This type of leaf base means that only one leaf can be attached at any node, consistent with the formation of a single cotyledon. Although monocots do not form a vascular cambium, they can still produce stems that are quite large. For example, both corn and coconut palms are monocots. In monocots, all of the increase in stem diameter occurs in a narrow zone immediately below the apical meristem. As a result of this lateral expansion, the vascular bundles become distributed throughout the stem instead of being arranged in a ring as in all other seed plants. Monocots that have slender stems—for example, some species of climbing yams—have their vascular bundles in a ring.

The lack of a vascular cambium has a profound impact on the way monocots form roots. Because individual roots cannot increase their vascular capacity, monocots continuously initiate new roots from their stems. Thus, the root systems of monocots are more similar to those found in ferns and lycophytes than in other seed plants. Finally, monocot flowers typically produce organs in multiples of 3 (for example, having 3, 6, or 9 stamens), whereas eudicot flowers typically produce organs in multiples of 4 or 5.

Monocots have a relatively poor fossil record, in part because they do not produce wood. What factors might have led to such a radical revision in how they build their bodies? This question requires that we look at where and how living monocots grow. One hypothesis is that monocots evolved from ancestors that produced creeping, horizontal stems as they grew along the shores of lakes and other wetlands. Many monocots today, including the earliest diverging groups of monocots, grow in such habitats, and many of the features of the monocot body plan are well adapted to environments with loose substrates, flowing water, and fluctuating water levels. For example, their leaf base provides a firm attachment that prevents leaves from being pulled off by flowing water. Furthermore, many monocots produce strap-shaped leaves that elongate from a persistent zone of cell division and expansion located at the base of the leaf blade. By continually elongating from the base, monocot leaves can extend above fluctuating water levels. Finally, it is easier to imagine evolutionary changes that affected the vascular system occurring within an environment that makes only modest demands for water transport.

We may never know for sure in what environment the monocots first evolved. We thus turn our attention to one of the most diverse groups within the monocots: the grasses. Many grasses produce stems that grow horizontally and branch, allowing them to cover large areas. Grasses have linear leaves that elongate from the base, allowing them to survive grazing as well as fire and drought. Many grasses have evolved the ability to tolerate dry environments by producing roots that extend deep into the soil. In addition, C₄ photosynthesis has evolved within the grasses multiple times. As described in Chapter 29, C₄ photosynthesis allows plants to avoid photorespiration and thus photosynthesize with greater efficiency. Grasses are among the most successful group of plants, becoming widespread within the past 20 million years as climates changed (Fig. 33.22). Today, nearly 30% of terrestrial environments are grasslands.

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Quick Check 4 What are some of the distinctive features of monocots?