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Morphology of Cacti - Trunks/Stems

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All cacti have succulentus stems. The cacti have single trunks and several branches. Further, the branched cacti may be candlesticks or shrubby type.

Unbranched (Single) trunks: The cacti like Cephalocereus columna-trajani, C. senilis, and Echinocactus platyacanthus have single trunk.

Candlesticks: The distinct trunks and several branches making them resemble branched candlesticks, as in Cereus lamprospermus, Myrtillocactus geometrizans, Pachycereus pringlei, and P. weberi.

Shrubby: The shrubby cacti have several stems arise at or near ground level, as in Echinopsis cephalomacrostibas, Haageocereus acranthus, and Stenocereus thurberi. Such cacti are also usually smaller than the treelike cacti.

The shape of stem: Cacti may be cylindrical or columnar, segmented or non-segmented or small or long. 

Cylinderical - usually erect and shaped like a cylinder elongated and rounded in cross section.

Columnar - Branched (e.g. Cereus jamacaru) or unbranched (e.g. Cephalocereus senilis).

Short/long (Small/big) - Cacti show astounding characteristics in size, from the tiniest Blossfeldia liliputana, about nine millimeters in diameter, to candelabra-like giants such as Pachycereus weberi, to 16 meters, and Cereus lamprospermus subsp. colosseus, to more than 20 meters high.

The tropical climbing cacti even exceed these lengths but are almost impossible to measure accurately as they ascend out of sight into the dense forest canopy. A plant of Hylocereus sp. had been reported to be 100 meters in length (Cullmann et. al. 1986). The genus Pereskia contains species such as P. sacharosa that are broad-leaved trees that may reach heights of 15 meters.

Shape of individual stem: The individual stems may vary from globose to cylindrical. Stems may also be flattened, as in Opuntia engelmannii var. linguiformis and O. martiniana. Such stems may be called cladodes, and they are usually distinctly segmented or jointed.The stem of cacti may be spherical, round or solitary in shape

Spherical or Globular or Globose e.g. Mammillaria barbata.

Rounded with a flattened top (e.g. Echinocactus platyacanthus and Gymnocalycium hossei).

Growth of stem: They may grow solitary or in clusters.

Solitary (Ferocactus fordii and F. latispinus).

Clustering (caespitose) i.e. many stemmed from a common base, often forming low mounds or cushions. The mounds or cushions may be closed or open.

Closed compacted or mounds seen in Copiapoa conglomerata, Gymnocalycium leeanum, Mammillaria brachytrichion, and M. compressa.

Open mounds seen in Echinocereus cinerascens, E. stramineus, Echinopsis lamprochlora, and Mammilaria columbiana.

Stem surface: In some cacti the stem surface may be smooth as seen in Opuntia sp. but more often surfaces are covered with tubercles (podaria - enlarged leaf bases, arranged around the stem in crisscrossing spirals, an arrangement called alternate helical phyllotaxy).

Tubercles are nipple like: Seen in Mammillaria longimamma, M. polythele, and M. uncinata.

Glandular and leaf like: Seen in Ariocarpus species like as A. bravoanus and A. retusus, and in Obregonia denegrii.

Elongated: Very elongate resembling a small stem with spines at the tip as in Leuchtenbergia principis.

Arrangement of tubercles: They are spirally arranged on the stem. In many cases, they merge into vertical ribs as can be seen on most columnar cacti, including Carnegiea gigantea, as well as on many of the globular forms such as Astrophytum myriostigma, Echinocactus grusonii, and Ferocactus macrodiscus. Depending on the water content within the plant, the ribbed or fluted stem show an accordion like expansion. Up to 90% of the fresh weight of a cactus may be due to water (Gibson and Nobel 1986).

  

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