Growth Forms

Cacti live for many years, in a variety of habitats, and have a wide range of shapes from simple to branched, from globose to columnar, and some are essentially subterranean. Considerable effort has been made by botanists and horticulturists to describe the variety of forms. Innes and Glass (1991,8-11) described the habits of cacti simply as globular, climbing, pendent, leaflike, columnar, or clustering. Hunt (1989a, 204) listed 13 different cactus growth forms: (1) with felted areoles in the axils of persistent leaves, (2) stems cylindrical, un-segmented; leaves deciduous, terete, (3) stems cylindrical, unsegmented; leaves small (fallen); spines absent, (4) stems flattened, segmented; spines absent, (5) stems two-winged, unsegmented; areoles confined to margins, (6) stems flat, segmented, (7) stems slender, cylindrical, (8) stems long, cylindrical, (9) stem spherical with caplike cephalium, a specialized flower-bearing area at the top of a cactus stem with very short internodes (the portions of stem between adjacent leaves) and usually densely woolly or hairy, (10) stems spherical to shortly cylindrical,clustering, (11) stem flattened spherical, five-ribbed, (12) stem tuber-cled, resembling a rosette of leaves, and (13) stems with teatlike tubercles.

It is difficult to create an inclusive, relatively simple group of categories for the many different growth forms or habits of cacti. The following categories and subcategories are used in The Cactus Family.

Cacti may be treelike or arborescent, usually meaning they have single trunks and several branches. One notable form is candelabra-like in which cacti have distinct trunks and several branches, making them resemble branched candlesticks, as in Cereus lamprospermus, Myrtillo-cactus geometrizans, Pachycereus pringlei, and P. weberi. Sometimes cacti may be quite large but unbranched, as in Cephalocereus columna-trajani,

C. senilis, and Echinocactus platyacanthus. The Candelabra-like Pachycereus other, contrasting growth habit is shrubby, in weberi, valley of Tehuacan,

Hidalgo, Mexico

Haageocereus Acranthus

Shrubby Stenocereus thurberi subsp. thurberi in the Desert Botanical Garden, Phoenix, Arizona which 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.

Cacti can also be described as cylindrical or columnar, meaning the plant is usually erect and shaped like a cylinder (elongated and rounded in cross section). Columnar cacti may be branched (as in Cereus jamacaru) or unbranched (as in the simple columnar Cephalocereus senilis), segmented or unsegmented, short or long. Other cacti are best described as globose or globular, meaning they are spherical in shape, that is, rounded (as in Mammillaria barbata) or rounded with a flattened top (as in Echinocactus platyacanthus and

Gymnocalycium hossei). Cacti maybe either solitary (as in Ferocactus fordii and F. latispinus) or clustering or caespitose, that is, many stemmed from a common base, often forming low mounds or cushions. The mounds or cushions maybe either compacted or closed (as in Copiapoa conglomerate G. leeanum, Mammillaria brachytrichion, and M. compressa) or open (as in Echinocereus cin-erascens, E. stramineus, Echinopsis lamprochlora, and M. columbiana). The individual stems may vary from globose to cylindrical. Stems may also be flattened, as in Opuntia engelmannii var. lin-guiformis and O. martiniana. Such stems may be called cladodes, and they are usually distinctly segmented or jointed.

Ferocactus Barranca Toliman

Unbranched treelike or columnar Cephalocereus senilis, Barranca de Meztitlän, Hidalgo, Mexico

Unbranched treelike or columnar Cephalocereus senilis, Barranca de Meztitlän, Hidalgo, Mexico

Silver Bell Arizona
Unbranched columnar Ferocactus cylindraceus subsp. cylindraceus near Silver Bell, Arizona
Biznaga Tehuacan Puebla
Left: Rounded and flattened Echinocactus platyacanthus in Nuñez, San Luis Potosí, Mexico

Right: Flattened stem segments of Opuntia engelmanniivar. linguiformis, Desert Botanical Garden, Phoenix, Arizona

Barranca Toliman Cactus Flora
Caespitose, and compacted or closed, Mammillaria brachytrichion in the collection of W. A. Fitz Maurice

Caespitose, open Echinocereus cinerascens subsp. cinerascens, Hidalgo, Mexico

Cacti may also be described in relation to the substrate on which they grow. Epiphytic cacti grow on other plants but do not derive direct nourishment from them, that is, their roots do not penetrate into the host plant's tissues, nor are their roots in the ground. Examples include Le-pismium ianthothele, Rhipsalis baccifera, and R. cereuscula. Very similar are the lithophytic cacti, which grow on bare rock. Prostrate cacti, for ex ample, Stenocereus eruca, have stems that creep along the ground and produce aerial roots from the nodes. Scrambling cacti, on the other hand, for example, Haageocereus decumbens, clamber or spread irregularly. Some cacti are also climbing, Hylocereus costaricensis and H. undatus, for example, the plants ascending upon a firm support, often clinging with aerial roots or by twining. Geophytic cacti display one of the most interest

Prostrate Stenocereus eruca, Baja California Sur, Mexico

Prostrate Stenocereus eruca, Baja California Sur, Mexico

ing growth habits. Most or all of the plant grows at or below ground level, often with a large underground storage organ, as in Copiapoa laui.

Descriptions of the growth habits of cacti are difficult not only because of the numerous forms that the plants have but also because of intergradations. For example, there are many intermediates between a purely candelabra-like form such as that of Myrtillocactus geometrizans or Pachyce-reus pringlei and a columnar form such as that of Cephalocereus senilis. There are also intermediates, mostly in height but also in diameter, between globose and cylindrical plants.

Cacti display amazing differences in size, from the tiniest Blossfeldia liliputana, about 9 mm (0.4 in) in diameter, to candelabra-like giants such as Pachycereus weberi, to 16 m (52 ft) high, and Ce-reus lamprospermus subsp. colosseus, to more than

20 m (66 ft) high. The climbing cacti of the tropics even exceed these lengths but are almost impossible to measure accurately as they ascend out of sight into the dense forest canopy. An individual plant of Hylocereus was reported to be 100 m (330 ft) in length (Cullmann et al. 1986,11). The genus Pereskia contains species such as P. sacha-rosa that are broad-leaved trees that may reach heights of 15 m (49 ft).

Epiphytic cacti seem to lack many typical cactus characteristics, but even though they occur in tropical rain forests, water may still be a limiting factor. These plants do not have roots that tap the water reservoir in the soil; they must quickly absorb rainwater by their roots as it trickles down the trunk or branch upon which the plants grow. Some tropical regions also have lengthy dry periods. In contrast to desert cacti that have an abun

Epiphytic Rhipsalis baccifera subsp. baccifera east of Samaipata, Santa Cruz, Bolivia

Climbing Hylocereus undatus, Huntington Botanical Garden, San Marino, California

Cactus Canopy Desert

Pereskia sacharosa west of Samaipata, Santa Cruz, Bolivia dance of solar radiation, epiphytic cacti often have limited light because of the dense forest canopy in which they live. Thus many epiphytic cacti such as Lepismium houlletianum and Rhipsalis pen-taptera have an enlarged surface area with flattened, thin, or angular stems. Their spines are often reduced, not only because of fewer herbivores in the forest canopy but also because there is no need to shade the sensitive photosynthetic tissue. Finally, dispersal is mostly limited to birds in the tropical canopy, so many epiphytic cacti possess colorful, fleshy, sticky fruits.

Neoteny or paedomorphosis is another interesting phenomenon that occurs in some cacti: the plants retain juvenile characteristics throughout their entire life. Thus a juvenile individual is capable of sexual reproduction. I suggested that Tur-binicarpus represents such a condition because some species reproduce while still very young and tiny (Anderson 1986,8).

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