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The cactus stem and its associated features vary greatly, adding to the diversity of form of cacti. These differences have been used by researchers in distinguishing species and other taxa of cacti. Unfortunately for the purposes of classification, these are vegetative structures, usually more subject to environmental modification than floral structures. Nonetheless, the variation patterns of stem features, including the surface, tubercles, ribs, leaves, areoles, and spines, are important in taxonomy.

SUCCULENCE

Succulence, derived from the Latin succus, juice, refers to a plant's ability to store water. Cacti are succulent plants in which the stems are specialized to store water. Other succulents such as Aloe (Aloeaceae) and Agave (Agavaceae) have succulent leaves. Stapelia (Asclepiadaceae) and some members of the large genus Euphorbia (Euphor-biaceae), like cacti, are stem succulents.

STEM SURFACE

The surface of the cactus stem may be smooth as in some species of Opuntia, including O. martiniana, but more often they are covered with tubercles, also called podaria (singular, podarium). These structures are enlarged leaf bases, arranged around the stem in crisscrossing spirals, an arrangement called alternate helical phyllotaxy. Tu-

Nipple-like tubercles of Mammillaria longimamma bercles take a variety of forms. In Mammillaria, for example, M. longimamma, M. polythele, and M. uncinata, they are nipple-like, whereas in some Ariocarpus species such as A. bravoanus and A. retusus, and in Obregonia denegrii, they are triangular and leaflike. The tubercles of Leuchtenbergia principis are very elongate, each resembling a small stem with spines at the tip.

Tubercles may be spirally arranged on the stem or, 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. The ribbed or fluted stem is capable of an accordion-like expansion or contraction, depend ing on the amount of water within the plant. Remarkably, up to 90% of the fresh weight of a cactus maybe water (Gibson and Nobel 1986,46).

LEAVES

Most cacti lack typical flattened green leaves capable of photosynthesis. On the other hand, the subfamily Pereskioideae is characterized by having just such flattened, photosynthetic leaves (Mauseth and Landrum 1997) as in Pereskia diaz-romeroana and P. sacharosa. Subfamily Opunti-oideae also has a few members with fleshy, persistent green leaves, Pereskiopsis rotundifolia and Quiabentia verticillata, for example. Several other genera of that subfamily such as Cylindropuntia (C. versicolor, for example) and Opuntia produce ephemeral (seasonal), green, cylindrical leaves during the growing season (Boke 1944), but Aus-trocylindropuntia (A. subulata, for example) produces persistent cylindrical leaves. Subfamily Cac-toideae has no members with visible leaves, though vestigial microscopic ones are produced. Thus the majority of cactus species do not produce typical green leaves. Rather, leaves have become the highly specialized spines so characteristic of cacti. Some people mistake the large,

Triangular, leaflike tubercles of Ariocarpus retusus subsp. retusus

Elongate tubercles of Leuchtenbergia principis

Ribs of Carnegiea gigantea flattened pads of prickly pears, Opuntia quimilo, for example, for leaves. They are actually clado-des or stem segments that bear the green ephemeral leaves and spines.

AREOLES

Areoles are unique to cacti. An areole is a highly specialized axillary or lateral bud or short shoot or branch (Mauseth 1983b; Gibson and Nobel 1986,4). A short shoot possesses nodes, the points of leaf attachment, and internodes, the portions of stem between adjacent leaves. The internodes are very short, however, creating a mass of compacted nodes, and the axillary bud associated with each leaf position gives rise to the spines and reproductive structures. When looking at both the areole and tubercle, one can often see that the areole is, in fact, borne on the enlarged leaf base or tubercle.

Most areoles produce an indumentum or a covering of spines and multicellular hairs or tri-chomes, the latter often giving the areole a woolly appearance, as in Mammillaria hahniana and Rebutía marsoneri. In general, areoles are bilateral in their symmetry, with the spines usually arising from the edges. This can be seen clearly if the developing areole is examined microscopically. Whereas the growing point of the shoot, the apical meristem, of most flowering plants produces growing points for leaves, the leaf primordia, all around the meristem area, spine primordia in the short shoot or young areole are aligned in rows on each side of the meristem (Mauseth 1983b, 273). In some cacti such as Rebutía atenacea the areole is distinctly elongate but in other cacti it tends to be circular, for example, Gymnocalycium ragonesei. Some cacti have the spine- and flower-producing portions of the areole spatially separated from one another. In Coryphantha, Escobaría, and Neolloydia the distal floral portion is connected to the more basal spine-producing portion by a groove (Boke 1952,1961). Mammillaria, some species of Ariocarpus, and Pelecyphora still have the separated basal and distal portions

Inner perianth part Stigma

Glochids

Opuntia flowers and stem joint, drawing by Lucretia Breazeale Hamilton

Central spine

Flowering stem of Echinocereus with detail of an areole, drawing by Lucretia Breazeale Hamilton

Inner perianth part Stigma

Outer perianth Pericarpel

Stem joint (cladode)

Radial

Glochids

Opuntia flowers and stem joint, drawing by Lucretia Breazeale Hamilton

Central spine

Flowering stem of Echinocereus with detail of an areole, drawing by Lucretia Breazeale Hamilton

Areole

Radial

Areole but lack the groove (Boke 1953). Such areoles are said to be dimorphic.

The majority of cacti have areoles that produce spines or flowers for only a year or two, but a few genera such as Neoraimondia, Opuntia, and Per-eskia may produce spines from individual areoles for many years (Mauseth and Kiesling 1997). Thus there is an increase in spine number at each areole over a period of years. This continued growth of an areole is particularly evident in the shootlike structures of cacti such as Neoraimondia arequipensis, illustrated on page 30.

SPINES

Among the most distinctive features of most cacti are the spines, which arise from the areoles. Most agree that they are spines, which are modified leaves, rather than thorns, which are derived from branches (Benson 1979,1982; Eggli 1993). Spines are the only evidence of leaves in most cacti. Some cacti such as Ariocarpus, living rock, and Lopho-phora, peyote, usually have spines only as seedlings (Boke and Anderson 1970) but most cacti have spines in one form or another as adults, too. Like the true leaves of other plants, however, spines vary considerably in number, size, shape, and color. Spines may also vary in number, size, and color with age. In general, spines are some of the most variable structures on cacti. Some cacti such as Cephalocereus and Espostoa (E. melanos-tele, for example) have thin, hairlike spines. Others such as Maihueniopsis glomerata and Scle-rocactus papyracanthus have flattened, papery spines. Several species of Mammillaria and Fero-cactus (F. gracilis and F. hamatacanthus, for example) bear hooked spines. The majority of cacti, however, bear more or less straight or slightly curved, needle-like, awl-like, or bristly spines.

Spines apparently fulfill several functions. The most obvious is mechanical protection from herbivores. The spines of some cacti such as Sclero-cactuspapyracanthus camouflage the plants, providing additional protection. Another function of spines is to reflect light, shading the stem and reducing water loss by evaporation. Spines also form a boundary layer around the stem, reducing air movement and further reducing evaporation (Gibson and Nobel 1986,110). Cacti growing in areas of fog, as in the Atacama Desert of Chile, use spines to condense the fog into liquid that drops to the ground around the base of the plant. Finally, some cacti such as the chollas are dispersed by spines sticking to the skin or fur of animals.

The subfamily Opuntioideae is characterized by having short, sharp, barbed, deciduous spines called glochids. These make both the chollas, Cy-lindropuntia, and prickly pears, Opuntia, some of the most unpleasant cacti to encounter, because glochids readily penetrate skin and are difficult to remove.

Some cacti have areoles that produce spines that are all alike, but more often the outer or radial spines may be distinguished from the inner or central spines, as in Echinocactus texensis and Fe-rocactus latispinus. The number of spines per areole and their arrangement are some of the most frequendy used morphological characters to describe cacti, just as leaf characters are often used for other plants.

Areolar grooves of Coryphantha; see also the illustrations of C. elephantidens and C. maiz-tablasensis

Glochids, the small, detachable spines of Opuntia aciculata

ANATOMICAL FEATURES The organization of the internal structure of the cactus stem is like that of a typical dicotyledonous or broad-leaved plant stem, being composed of epidermis, cortex, vascular system, and pith. A cactus stem usually has a single layer of thin-

Thin Walled Cell Stem

Outer cell layers of Cleistocactus parviflorus, showing epidermis, sunken stomata, thick-walled hypodermis, and cortex; photomicrograph by James Mauseth

Electron Microscopes

Scanning electron micrograph of the epidermis of Strombocactus disciformis, showing the guard cells of the stomata arranged parallel to their subsidiary cells, a paracytic arrangement, X300

walled cells that make up the epidermis, which is heavily cutinized (long-chain fatty acids polymerized) on the outside and in many cases impregnated with heavy waxy layers as well. This wax is responsible for the glaucous, whitish or even bluish coloration in some species. Extremely heavy cutin-wax layers are present in Ariocarpus and Copiapoa (C. solaris, for example). In some instances the outer portion of the epidermis is papillate or bumpy. The stomata, specialized pores whose opening and closing is controlled by guard cells, involved in gas exchange are typically paracytic, meaning that the subsidiary cells are aligned parallel with the guard cells (Eggli 1984b).

Directly beneath the epidermis lies the hypodermis, which arises differently from the epidermis and may consist of several layers of cells with tough, thick cell walls. These cells provide much of the stem's mechanical protection. The cortex is the massive part of the cactus stem, consisting mostly of unspecialized cells that carry out photosynthesis, the chlorenchyma, and that store water, the parenchyma (Sajeva and Mauseth 1991). Often, hypodermal cells have calcium oxalate crystals. Latex ducts of certain species of Mammillaria are also found within the cortex (Mauseth 1978a,b; Wittier and Mauseth 1984). Inside the cortex are the fluid-conductive cells, the vascular bundles, composed of cluster of xylem and phloem cells arranged side-by-side (Mauseth and Sajeva 1992), and in the center of the stem is a relatively small pith, which also has vascular bundles in many species (Mauseth 1993a).

Stem cross section of Echinopsis hammerschmidii, showing the thick cortex and vascular cylinder (note that chlorophyll is only in the outer layers of cortical cells, the chlorenchyma); photomicrograph by James Mauseth

Cacti have a specialized wood anatomy (Mauseth 1993a,b; Mauseth and Plemons-Rodriguez 1997). It is best described as consisting of xylem or water-conducting elements composed of short, narrow vessels with a single perforation plate and multiseriate pitting. The fibers, which remain alive at maturity, are narrow and resemble phloem fibers in shape. Generally, the vascular cambium has large regions that produce parenchyma, resulting in wedges of wood separated by large rays. Growth rings are rarely formed from one season to the next. Some cacti have wide-band tracheids in addition to true vessels; the wideband tracheids have no perforations at all (Mauseth 1984a, 135; Mauseth et al. 1995). Tracheids are water-conducting xylem cells that lack perforations at their ends. They are considered more primitive than vessels, which have perforations at their ends, connecting vessel cells to each other.

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