Introduction

Grass morphology. (Source: Arizona-Sonora Desert Museum)

The grass family Poaceae (also known as Gramineae) has some 10,000 species and 650 to 900 genera. Only the sunflower (Asteraceae, or Compositae), legume (Fabaceae, or Leguminosae), and orchid (Orchidaceae) families are larger. The grass family has more individual plants and a wider environmental range than does any other family, reaching the limits of vegetation in polar regions and on mountaintops, enduring extremes of cold, heat, and drought, and dominating various landscapes worldwide. Grasses are the most successful monocots (seed-bearing plants with single seed leaves), and the most beneficial plants for humankind, providing highly nutritional grains and livestock forage, and preventing soil erosion. Of the 5 crops that provide almost two-thirds of the food we eat, including corn, barley, potatoes, rice, and wheat, only potatoes are not a grain.

Morphology

Grasses are highly specialized monocots distinguished by certain characteristics of the stems, leaves, and inflorescences. The jointed stems (culms) are round or flattened (never triangular); they are usually hollow except at the nodes (points on the stem from which leaves arise), where they are solid. The leaves generally consist of 2 main parts: a tubular sheath around the plant's stem and a blade (the broader, expanded part). Leaves are usually single at each node, but are 2-ranked, that is, spaced alternatively on opposite sides of the stems. Very small, individual, simple flowers (florets) are grouped in inflorescences called spikelets, which are subtended by small leaf-like bracts, the glumes. The florets are enclosed by other specialized bracts (the lemma and palea).

Photosynthetic Pathways

Grass spiklet. (Source: Arizona-Sonoran Desert Museum)

There are grasses that use both C3 and C4 photosynthetic pathways to fix carbon into tissue. Primitive grasses such as the bamboos and modern grasses that live at higher latitudes or elevations where temperatures in the growing season are relatively cool, mostly have the more typical C3 photosynthesis predominating. However, in the intense sunlight of warmer habitats at lower latitudes and elevations, grasses with the more specialized C4 metabolism are better able to fix carbon. C4 grasses are important in tropical forests, semiarid grasslands, and warm deserts.

Evolutionary History

The earliest grasses lived in shady tropical forests. The evolution and spread of grasses undoubtedly resulted from their ability to adapt to seasonally dry habitats created as tropical-deciduous forests developed in the Eocene (58 to 34 mya, million years ago). Considering their importance and taxonomic diversity, grasses have a relatively poor fossil record. Whilethe earliest potential fossil grass pollen was described from late Cretaceous sediments, the oldest reliable megafossil grass fossils were spikelets and inflorescences from the latest Paleocene (about 58 mya). These were primitive proto-bamboos with broad leaves, quite unlike the narrow-leaf modern grasses of desert grasslands and deserts. Although the early fossils could not be assigned to living grasses, their morphological details reflect the early evolution of wind pollination in a seasonally dry tropical environment. In the late Oligocene (about 30 to 24 mya) fossils of central North America, more diverse grass fossils were found, including both archaic forms and quite a few living genera. By the Miocene (24 mya), many more modern genera appeared in the fossil record. A great deal of the history of grasses was clearly not captured in the fossil record, notably the evolutionary radiation from primitive proto-bamboos to modern grasses in the Eocene and Oligocene. By the early Miocene, however, grasses in all our modern subfamilies were present, indicating that our modern taxonomic and physiologic diversity had been well established by that time.

Further Reading

• Poacea – University of Wisconsin
• Phillips, S.J. and P.W. Comus (eds.) 2000. A Natural History of the Sonoran Desert. Arizona-Sonora Desert Museum Press, Tucson, and University of California Press, Berkeley.