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Vegetative Terminology (Part 1)
| Economic Plant Illustrations #30 |
Botany 115 Vegetative Terminology
Modified Roots, Stems and Leaves (Part 1)
Types Of Subterranean Roots
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A. Fibrous root system of rip-gut grass (Bromus diandrus). B. Tap root of a carrot (Daucus carota). C. Fascicled (clustered) storage roots of sweet potato (Ipomoea batatas). Note: The sweet potato storage root is not a modified stem as in the potato tuber; however, it is often referred to as a tuberous root.
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Fascicled storage roots of smilax asparagus (Asparagus asparagoides). The roots are also referred to as "tuberous" by some botanical authorities.
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Adventitious Roots Arising From Stems
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Left: Red mangrove (Rhizophora mangle) growing in seawater along the shoreline of St. John (U.S. Virgin Islands). The adventitious prop roots help to support and securely anchor this shrub in the mud and loose sand of tidal waters. Right: Close-up view of prop roots showing numerous pores called lenticels which provide gas exchange and an additional source of oxygen for the submersed roots.
M angroves survive in seawater with a salinity that would be lethal to most trees and shrubs. Like celery or
carrot sticks placed in saltwater, the roots of most plants rapidly lose
water if they are suddenly emersed in seawater. Halophytes (salt-loving
plants) such as mangroves generally have a lower concentration of water
molecules (lower water potential) in their root cells so they can take in
water. They maintain lower water potentials in their roots by having
higher internal salt concentrations than seawater and by losing water at
the leaf surface. Since high internal salt concentrations can be lethal to
plant cells, some species such as the black mangrove and white mangrove
(Laguncularia racemosa) can excrete excess salt through special
glands in their leaf blades and petioles. Red mangroves have root cell membranes which prevent the absorption of
excess salt.
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Left: The creeping fig (Ficus pumila), a remarkable Asian vine that is commonly cultivated in southern California. There are two distinct types of stem growth: Young, juvenile branches and older, mature branches. The juvenile branches (with smaller leaves) produce aerial roots that adhere to concrete, stucco, masonry and even glass windows. Without pruning, a single plant can envelop a four-story building. The aerial roots secrete a clear, gummy latex that works like rubber cement. This remarkable adhesive was first described in detail by Charles Darwin in his book The Movements and Habits of Climbing Plants (1876). Older, mature branches with larger leaves produce fleshy, flower-bearing syconia. In fact, this species has been crossed with the edible fig (F. carica) to produce a hybrid vine (F. x pumila-carica) with edible syconia. Right: Close-up view of the aerial roots (red arrow) that develop at the nodes on juvenile branches. The roots secrete a gummy adhesive that adheres to concrete, masonry and glass. This species is commonly planted in southern California to cover the monotanous concrete walls of buildings and freeways.
Grass Jelly From Creeping Fig & Mesona (Lamiaceae)
In China, the syconia of creeping figs are picked ripe and placed in a porous bag to squeeze out the juice. The juice is cooked and then cooled into a gelatinous consistency called "pai-liang-fen." This jellylike material is cubed, mixed with water, syrup and flavorings and consumed as a refreshing drink. It is canned and sold in Asian markets as "grass jelly" of "ai-yu jelly." According to Cornucopia II by S. Facciola (Kampong Publications, 1998), boiled plants of Mesona chinensis in the mint family (Lamiaceae) are also made into cubes of grass jelly. In fact, cans of grass jelly often list this species on the labels.
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Aerial roots of the strangler fig (Ficus citrifolia) wrapped around the trunk and limbs of a swamp tree called "wing-nut" (Pterocarpus officinalis) on the island of Dominica. The Caribbean island of Barbados is supposedly named for this species of strangler fig whose abundant aerial roots give it a "bearded" appearance.
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Pneumatophores: Aerial Extensions From Roots
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Black mangrove (Avicennia germinans) along the eastern shore of the Yucatan Peninsula, Mexico. The slender, erect outgrowths from the roots are called pneumatophores. The porous pneumatophores enable the water-logged roots to carry on gas exchange with the atmosphere. Unlike positively geotrophic roots, the pneumatphores (called "aerial roots") are negatively geotrophic and grow upward out of the mud and away from gravity. Black mangrove belongs to the verbena family (Verbenaceae).
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Tea mangrove (Pelliciera rhizophorae) growing along the Pacific coast of Costa Rica. The buttressed trunk contains numerous lenticels (red circle) which fascilitate gas exchange with the atmosphere while the roots are submerged in water-logged mud. The
unusual, top-shaped fruit of tea mangrove (below red circle) is the size of a onion and contains one of the largest seeds in the world (excluding palms). It floats with the elongate, embryonic root pointing downward, and readily becomes implanted in soft mud. This mangrove is a member of the tea family (Theaceae).
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A swamp of bald cypress (Taxodium distichum) in the Florida Everglades. The erect, woody extensions from the roots are called "cypress knees" or pneumatophores. Some botanists maintain that pores in the "knees" fascilitate gas exchange between the water-logged roots and the atmosphere, although other botanists disagree with this theory. When photographing cypress knees it is wise to be aware of nearby alligators basking along the water's edge.
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Lignotubers Of Chaparral Shrubs
Many species of dominant shrubs in fire-climax plant communities, such as the chaparral and coastal sage scrub, develop from enlarged, woody, subterranean stems called basal burls or lignotubers. Adventitious buds embedded in these woody stems resprout following fire, thus enabling the shrub to survive periodic brush fires. The presence or absence of a distinct lignotuber is one of the major keys to the identification of species of manzanita (Arcotostaphylos) in the southern California chaparral and pine forests. For example, Mexican manzanita (A. pungens), Cuyamaca manzanita (A. pringlei ssp. drupacea), Otay manzanita (A. otayensis), and bigberry manzanita (A. glauca) do not stump sprout from a lignotuber. Peninsular manzanita (A. peninsularis) and subspecies of A. glandulosa, including Eastwood manzanita (ssp. glandulosa), Del Mar manzanita (ssp. crassifolia) and Laguna manzanita (ssp. adamsii), all resprout from well-developed basal burls.
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Left: Lignotuber of chamise (Adenostoma fasciculatum), a common shrub in the chaparral of southern California. Right: Charred branches of chamise with bright green resprouts from the subterranean lignotuber. The intense heat of the fire caused the blackened granodiorite boulders to exfoliate into thin flakes.
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Types Of Subterranean Stems
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Two examples of scaly, underground stems called rhizomes. A. Rhizome of achira (Canna edulis). B. Rhizome of the common sword fern (Nephrolepis exaltata).
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A. Bulb of an onion (Allium cepa). B. Corm of gladiolus (Gladiolus hybrid). The bulb is an upright subterranean stem surrounded by overlapping, fleshy scales. The corm is an underground stem without overlapping scales or with only a few scales.
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The root system and tubers of a potato plant (Solanum tuberosum). The potato tuber is technically a modified underground stem produced at the tip of a rhizome. The "eyes" of the potato are actually buds.
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Stolon: A Creeping, Aboveground Stem
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A stolon or runner of Bermuda grass (Cynodon dactylon). The stolon is an above-ground, trailing stem that typically produces roots at the nodes where leaves and stems arise. This very invasive, perennial grass also produces creeping, underground stems called rhizomes. Another plant that spreads by means of stolons is the strawberry (Fragaria x ananassa).
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Gross Stem Anatomy & Leaf Arrangement
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Left: Five years of growth on the stem of a deciduous flowering tree. The vascular bundle scar (trace) is within the leaf scar (place where leaf was attached). The lateral bud is located just above the attachment scar of the leaf. Right: Three different leaf arrangements: Alternate (one leaf per node), opposite (two leaves per node) and whorled (three or more leaves per node). A node is the place where one or more leaves are attached along the stem. The area between nodes is called the internode.
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Caudiciform Plants With An Enlarged Caudex
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In his beautifully illustrated book Caudiciform and Pachycaul Succulents (1987), Gordon Rowley summarizes three main types of enlarged stems, including succulent, caudiciform and pachycaul stems. In succulent stems, the stems are typically photosynthetic and covered with protective spines. This adaptation is widespread in the cactus family (Cactaceae) and euphorbia family (Euphorbiaceae); two classic examples of convergent evolution where unrelated plants from distant continents have developed very similar adaptations for living in arid deserts. Caudiciform plants generally have an enlarged basal caudex or stem axis from which the stems and roots arise. The caudex may extend below the ground, is typically non-photosynthetic and often gives rise to slender, twining stems. It generally does not include plants with bulbs, corms, rhizomes or tubers. This ingenious adaptation is well developed in the gourd family (Cucurbitaceae). In fact, the wild cucumber vine (Marah macrocarpus) that climbs over shrubs in the chaparral and coastal sage scrub of California develops from a tuberous, subterranean caudex that may weigh up to 100 pounds. One of the reasons the wild fig of Baja California (Ficus palmeri) is able to establish itself on boulders and vertical canyon walls is its remarkable caudiciform seedling stage. With a food and water reserve in the bulbous stem base, the roots can grow considerable distances in search of moisture. The effectiveness of this strategy is readily apparent when you gaze at the remarkable locations of these figs trees.
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With the onset of spring, Jatropha berlandieri sprouts from a large, globose caudex. This is a caudiciform plant adapted to prolonged periods of drought. It belongs to the diverse euphorbia family (Euphorbiaceae). Caudiciform plants can store water and survive many months without rain in hot, arid desert regions. They include some of the most remarkable and truly bizarre plants on earth.
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A third category called pachycaul plants includes massive, non-photosynthetic, spineless trunks tapering upward into stout branches. This category is very evident in desert regions, especially the elephant tree (Bursera microphylla) and the remarkable Cyphostemma juttae, a poisonous member of the grape family (Vitaceae) native to Africa.
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Alula (Brighamia insignis), a rare member of the lobelia family (Campanulaceae) endemic to steep sea cliffs on the island of Kauai. As of the year 2000, fewer than 100 of these remarkable pachycaul plants grew in the wild. Like the California condor, this unique species has been brought back from the brink of extinction through breeding programs at botanical gardens. Alulu is perfectly adapted for living on vertical volcanic cliffs. A single rosette of leaves arise from the top of a thick, succulent stem, like a cabbage head on a baseball bat. The rosette varies in size relative to the stem, depending on the availability of moisture. Roots penetrate the cliffs horizontally, and the base of the plant is rounded, permitting the plant to rock slightly in the wind. Water stored in the stem enables the plant to survive periods of drought which may last days or weeks. Another rare species with white flowers (B. rockii) grows on sea cliffs along the windward coast of Molokai. Like Hawaii's endemic silver sword alliance that evolved from an ancestral tarweed (Asteraceae), the alulu is another example of adaptive radiation in the lobeliads. Unlike the silver swords, the lobeliads evolved from several ancestral introductions rather than a single ancestral colonization. For more information please refer to (Hawaii: A Natural History by Sherwin Carlquist (Pacific Tropical Botanical Garden, 1980).
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