Chapter 16. Magnoliophyta (II)
The Class Liliopsida constitutes the monocotyledonous angiosperms and includes some of the largest plant families such as the orchids with some 20,000 species and the grasses with perhaps 15,000 species. In the previous chapter we learned how to separate the two major flowering plant groups: the dicotyledons and the monocotyledons, and covered a number of dicot families. In this chapter, we shall do the same by considering representative monocot families.
The liliopsids are considered to form a monophyletic group evolved from an early dicot. The oldest fossils presumed to be monocot remains date from the Early Cretaceous (Herendeen & Crane, 1995). Features that are generally common to monocots include vascular bundles that are irregularly distributed (in cross-section of the stem), leaves with parallel venation, and flower parts in multiples of three. Although true secondary growth is absent, most growth habits are found in the group including floating and submerged aquatics, lianas, trees, epiphytes, and forbs of all sizes (Hahn, 2005).
The second largest (to Orchidaceae) and one of the most successful of families of monocotyledons is the grass family, classified as the Family Poaceae (or Gramineae) and comprising nearly 10,000 species distributed more widely than any other angiosperm family. This family is also the most important economically, providing species that are the world's staple food supply. The grasses have reduced floral structures compared with most angiosperms for the reason that grasses are almost exclusively pollinated by wind. Therefore, these plants have had no cause to evolve floral structures that are attractive to insect (or other animal) pollinators. The grasses also have a fairly specific body plan that is immediately recognizable and very successful for colonizing seasonally dry landscapes, yet modifiable to suit a wide range of ecological conditions. Bor (1960) stated:
- The amazing diversity of shapes, sizes, texture and so on in the various parts that make up the vegetative and the reproductive shoot is alike the admiration and exasperation of the taxonomist. It can be said... that in no other family of monocotyledons, or dicotyledons for that matter, has a relatively simple structure been so altered in the process of evolution as to produce the amazing wealth of forms known to us. It is as if Evolution, in an excess of exuberance, had added this, subtracted that, enlarged this, suppressed that, in fact tried out every possible combination and permutation of characters to produce a family as complicated and as difficult [taxonomically] as, if not more difficult than, any other.
- Read Poaceae (Links need not be pursued at this time)
Grasses have fibrous roots and three kinds of stems: culms, rhizomes, and stolons. The culm is the main aerial shoot to which leaves and flower head are attached. The culm is a rounded or slightly flattened stem with one or more solid joints known as nodes. The leaves are attached at the nodes and if the stem is not simple but branched, branches arise only at nodes. Roots may also develop from a node where the node comes into contact with the ground (as in decumbent and prostrate stems). The portion of the stem between the nodes is called the internode, and is usually hollow in temperate zone grasses and solid in tropical grasses (Rotar, 1968). All or a portion of an internode may be surrounded by the basal part of the leaf known as the sheath.
A rhizome is a modified stem that grows underground. Rhizomes are jointed (thus distinguishable from roots) with bladeless leaves (scales) arising from the joints. Rhizomes enable the grass plant to spread horizontally as new culms develop vertically from the joints. Thus, grasses with extensive rhizome development will form a turf rather than distinct tufts or bunches.
A stolon is a stem that creeps across the surface of the ground, and is really a basal branch of the culm that will develop roots and shoots from some or all of its nodes. Like a rhizome, a stolon results in a spreading or turf forming grass plant.
Grasses display two types of leaves: (1) green leaves consisting of a sheath and blade, and (2) reduced leaves consisting of only a sheath. With but a few exceptions, the green leaves arise at nodes alternately up the culm. Leaves that are concentrated near the base where the internodes are very short are termed basal leaves; leaves arising at nodes along an elongated culm are cauline leaves. These vegetative leaves typically surround the culm as a sheath, then diverge outward (at the "collar") as a long narrow blade with longitudinal parallel venation. If the veins are conspicuous, the leaf is striate; if the veins are raised, the leaf is ribbed.
The sheath of the leaf surrounds and protects the shoot. In some species, the sheath extends beyond the next node, so that consecutive leaf sheaths overlap, hiding the nodes. The longitudinal edges of the sheath may overlap, completely surrounding the culm, or the sheath may be tubular (the margins connate). The upper end of the sheath, known as the sheath mouth is the collar on the lower (outer) surface that may be produced into short appendages called auricles. On the inner, upper surface of the leaf, between the sheath and the blade, is an outgrowth called a ligule. This may be a flap of membranous tissue or simply a fringe of hairs, an inconspicuous rim, or even absent all together, marked only by dark tissue.
Although there is variation in leaf blade shape, most grasses have linear-shaped leaves that are many times longer than wide, with margins that are parallel then taper to a point at the apex. Grasses that grow in shady places may have lanceolate or even ovate leaf blades.
Flowers of grasses are borne in an inflorescence or flower head which terminates the culm and other branches of the stem. Smaller units of the inflorescence are called spikelets and these are arranged on one or more branches in a wide variety of different ways to which the standard terminology for inflorescences can be applied, but using the spikelet instead of the individual flower. Thus:
- In a panicle, spikelets are carried on branches of the axis.
- The panicle can be either open or dense and crowded, with the branches so short that they are concealed by the spikelets, forming a false spike.
- In a spike, the spikelets are carried, unstalked, directly on an unbranched axis, called a rachis.
- In a raceme some or all of the spikelets are attached to the rachis by stalks called pedicels.
Chapter 16 ~ ReferencesEdit
- Bor, N. L. 1960. The Grasses of Burma, Ceylon, India and Pakistan. Pergamon Press, London. 767 pp.
- Hahn, William J. 2005. Monocotyledons in: Tree of LIfe web project at Monocotyledons.
- Herendeen, P. S. and P. R. Crane. 1995. The fossil history of the monocotyledons. pp. 1-21 in: P. J. Rudall, P. J. Cribb, D. F. Cutler, & C. J. Humphries (eds.). Monocotyledons: systematics and evolution. Royal Botanic Garden, Kew.
- Rotar, Peter R. 1968. Grasses of Hawaii. Univ. of Hawaii Press, Honolulu. 355 pp.
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