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The individual flowering grass tiller consists of extremely simple structures. A phenomenal amount of diversity, however, occurs within these structures, making up the characteristics that differentiate over 700 genera and 11,000 species of grasses. Roots At the time of germination, the grass seed (caryopsis), extends along 2 axes. The radicle, protected by the coleorhiza, produces the primary root system that begins to stabilize the seedling and absorbs water and nutrients from the soil. At the other end of the embryo the plumule, protected by the coleoptile, elongates upward to become the primary shoot system (fig. 2.1). This aboveground portion is responsible for photosynthesis, supplying the plant with energy to grow and reproduce. The primary root system functions for only a few short weeks or months and soon withers and dies. There is no taproot in grasses as is typically found in herbaceous forbs or woody plants. For the remainder of the life of the grass, belowground function is performed by a series of adventitious roots that compose the secondary root system. These adventitious roots are fibrous and arise from the lowermost nodes of the grass culm (stem). Occasionally, some grasses (e.g., Zea, Sorghum, Rottboellia) develop stout “prop” roots from the lower aboveground culm nodes (fig. 2.2). Prop roots are necessary for mechanical support of the large aboveground portion of the plant. Chapter 2 : THE GRASS PLANT Figure 2.1. Germinating corn seed (University of Colorado Press). Figure 2.2 Prop roots. Grass Plant 11 Culms Grass culms are made up of swollen nodes and smooth internodes. Internodes are typically round or elliptical in cross section. Nodes are always solid, but the internodes may be solid, semisolid, or hollow. Grass leaves, branches, and adventitious roots arise from meristematic tissue near the nodes. It is convenient to think of the aboveground portion of a grass plant as composed of a series of “building blocks,” which are called phytomeres. A phytomere, or basic unit of the grass shoot, is composed of a node, internode, meristematic tissue, and leaf (fig. 2.3). Usually , the internodes are very short during the major portion of the life cycle, particularly in grasses with a cespitose growth form. This low growth form also maintains the apical meristem (growing point) of the plant near the soil surface, protecting it from grazing animals and wildfires. In this stage the grass plant appears to be nothing more than a clump of blades. However, when flowering occurs, the internodes elongate and push the reproductive portion of the plant upward. Meristematic tissue (vegetative bud) located just above the basal node of the phytomere in the axil of the leaf can develop into several structures. In most cases it produces a tiller or lateral branch; less frequently, a horizontal culm develops. If the horizontal culm is underground, it is called a rhizome (fig. 2.4a–c), but if the horizontal culm is located aboveground, it is referred to as a stolon (fig. 2.5a–d). Some grasses may have both stolons and rhizomes (Cynodon). Leaves Grass leaves are 2-ranked and alternate. Thus, at each node a leaf is oriented 180º from the previous one. The grass leaf is differentiated Figure 2.3. Characteristics of the grass plant (used with permission of Stubbendieck et al. 2003). 12 Chapter 2 2.5c 2.5a 2.4c 2.5d 2.4a 2.4b 2.5b 2.5b 2.4b Figure 2.4a. Stout, woody rhizome. b. Long, slender rhizome. c. Scaly rhizome. Figure 2.5a. Stolon with long internodes. b. Runnerlike stolon. c, d. Stolons. (Photographs by Robert B. Shaw) Grass Plant 13 2.6 d 2.6 c 2.6 b 2.6 a into a basal portion (sheath), which encloses the culm, and an upper portion (blade or lamina). The sheath typically has free margins , but in some genera the sheath margins are connate, forming a tubular-like structure (Bromus, Glyceria, and Melica) (fig. 2.6a). The blade is characteristically flat and elongated. In arid environments the blade may be terete and very narrow or have variously rolled margins , whereas in forested areas the blades may become broad and ovate. Generally there is a ligule at the inside (adaxial) junction of the leaf sheath and the leaf blade. The ligule may be a ring of hairs (fig. 2.6b), membranous (fig. 2.6c), or a ciliated membrane (fig. 2.6d). Rarely is it absent (in most species of Echinochloa ). The ligule is the most often used and reliable vegetative character...


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