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| Monocotyledones - coconut |
The Monocotyledones, or monocots, are a large and very distinctive class of angiosperms or flowering plants, phylum Anthophyta, consisting of some 133 families, 3,000 genera, and 65,000 species. Monocotyledones form one of the two major subdivisions of angiosperms, the other being the Eudicotyledones (eudicots), with about 165,000 species.
Typical monocots have a single cotyledon (seedling leaf), stems with scattered vascular bundles, root systems composed entirely of adventitious roots (arising directly from stem tissues), leaves with parallel venation and sheathing bases, and flower parts in threes. Monocots lack the ability to produce secondary growth (wood).
In most monocots, stems remain at or below ground level and take the form of rhizomes (horizontal stems), bulbs (short, vertical stems covered with modified, fleshy leaves), corms (short, wide stems), or tubers (wood), which produce new adventitious roots continually or seasonally.
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The growing tips (apical meristems) or buds of these plants remain below ground, except when they rise to produce flowers, and thus are protected against seasonal cold, drought, fire, and grazing animals. The growing tips are also surrounded and protected by the sheathing bases of the leaves.
Monocot leaves are typically long and strap-shaped with numerous parallel veins that connect individually to the stem. Leaves grow primarily from their bases, increasing in length and pushing the earlier formed tips upward but not increasing much in width.
Mostmonocot flower parts occur in threes, such as three sepals, three petals, six stamens, and three carpels. Some members of the largely aquatic subclass Alismatidae have other patterns and show the primitive condition of apocarpy (carpels remain free from one another).
In most other monocots, carpels are completely fused together into a three-chambered pistil. (Exceptions occur also in some palms). Monocots thus appear to have evolved from primitive anthophytes and have had a long, separate history.
Ecology and Variation
Because of their predominantly underground stem structures and basally regenerating leaves, monocots predominate in open habitats with strong seasonal contrasts or unpredictable droughts, such as grasslands.
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| Egyptian papyrus plants |
Grasses and similar monocots are particularly well adapted to survive drought, fire, and overgrazing, the three primary threats in grasslands. Bulbs, corms, and tubers are other forms of monocot stems that allow for underground survival, both in grasslands and in other regions that experience long winters or dry seasons.
The numerous monocots that inhabit marshes and aquatic habitats often have elongate petioles that grow from the base to lift their expanded leaf blades above the water level (such as Sagittaria and Aponogeton).
The spectacular Egyptian papyrus plants have highly specialized upright stems that consist of a single internode (stem segment between nodes that elongates from the base), lifting a tuft of leaves and reproductive branches above fluctuating water levels. Other submerged aquatic monocots may remain rooted, producing conventional strap-shaped leaves (such as sea grasses) or may drift rootless, with short, whorled leaves produced along the stem.
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Many monocots have adapted to live as epiphytes (plants that live on top of other plants), particularly on the upper branches of tropical rain-forest trees, a habitat that is often quite dry.
The orchid, bromeliad, and aroid (Philodendron, Anthurium) families in particular have evolved largely in this habitat. Epiphytic orchids have succulent leaves or roots that store water for use during the dry periods between rains.
A bromeliad grows as a rosette of tightly overlapping leaves radiating from the central growing point, forming a water-holding cup, or tank, in the center. Aroids can be found climbing up tree trunks or perched on upper branches and are unusual both for their flower structure and their leaves.
Flowers are tiny and crowded on the clublike spadix, which in turn is enveloped by a large, often colored, leaflike structure called a spathe, while the leaves are broad, often dissected or divided into separate leaflets, and net-veined—quite unlike the typical monocot leaf.
Lack of the ability to form conventional tree trunks has led to some other novel growth forms in monocots. One of these is the pseudostem (false stem). In banana plants, and to a less obvious extent in gingers, heliconias, and other giant tropical herbs, an apparent stem forms from the elongate, tubular, concentric leaf sheaths, and gets taller as each new leaf grows up through the center.
Monocot Trees
Some monocots have become arborescent (treelike) by growing upward and developing thick, fibrous trunks without conventional secondary growth. In most arborescent monocots (palms, screw pines), the thickening growth of the stems occurs at the growing tip, in what is known as the primary thickening meristem.
In these specialized meristems, the dividing cells expand laterally more than vertically. Stems reach their full thickness in this initial growth at the tip of the plant and expand very little after that.
In members of the Dracena family, however, a new way of thickening stems has evolved. Extensions from the apical-meristem remain active along the sides of the stem and produce new parenchyma tissue and whole vascular bundles (not layers of wood). This allows the plants continually to increase the thickness of the stem and the amount of vascular tissue.
Branching is limited or absent in arborescent monocots, and leaves therefore tend to be large. Screw pines and dracenas have very long, narrow leaves, while palm leaves are broad and dissected into folded linear leaflets in either a pinnate (featherlike) or a palmate (fanlike) arrangement.
Palm leaves are the largest in the world, reaching more than 60 feet (18meters) in length in some species, and are highly fibrous. Development of these compound leaves is complex, involving multiple meristematic areas.
