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Data are from Gibson (1973, 1977a,b, 1978), Loza-Cornejo and Terrazas (1996), Mauseth and Plemons-Rodriguez (1998), Bobich and Nobel (2001), and Herrera et al. (2001).

with stored water, which can help prevent embolisms (or help refill cavitated ones; Mauseth 1993a).

Wide-band tracheid wood appears to be highly adaptive for plants in extremely xeric habitats. It typically contains vessels, but in some species vessels exist only as a component of earlywood. Such vessels have essentially the same diameter as for vessels in parenchymatous wood for Cactioideae, but are much less frequent (Table 2.2). Cactus wood of any type virtually never has annual growth rings.

The most clear-cut examples of growth rings are restricted to wide-band tracheid wood of North American small cacti (Ferocactus, Echinocactus, and relatives), but not to the South American ones (Gymnocalycium, Echinopsis, and others). In wood that has annual rings, the presence of a vessel-rich earlywood combined with a latewood of such wide-band tracheids makes growth rings visible to the naked eye. In South American taxa with wide-band tracheid wood, vessels and apotracheal parenchyma are mixed in with late-

wood; consequently, earlywood and latewood are difficult to distinguish, and no growth rings can be detected. Such wood of North American taxa also differs from that of South American taxa with respect to rays. Rows of wideband tracheids occur next to well defined rows of ray parenchyma cells for North American taxa, whereas in South American taxa, a series of parenchyma cells that appears to be a ray will be interrupted by a single wide-band tracheid.

Existing studies on wide-band tracheids are mainly on Cactoideae and Maihuenia in the Pereskioideae/ Maihuenioideae. In Opuntioideae, wide-band tracheids occur mostly in the rays, and the wood consists of narrow axial masses. Also for the Opuntioideae, wide-band tra-cheids, which can be found in either parenchymatous or fibrous wood, are usually in the ground tissue next to the protoxylem and occur as both axial and ray cells in the wood, being larger and more numerous in ray cells that are adjacent to the axial regions than they are in the axial regions (Gibson 1973, 1978). In platyopuntias, wide-band tracheids are often arranged in radial files of three or more, and the annularly thickened secondary cell walls of adjacent cells often alternate, giving them an interlocking appearance (Bobich and Nobel 2001).

Several cactus species have dimorphic or trimorphic wood, meaning that when young, they produce one type of wood, but they produce a distinctly different wood type as they mature (Mauseth and Plemons 1995; Mauseth and Plemons-Rodriguez 1998). A common dimorphism is one in which a plant produces wide-band tracheid wood when the plant is young and small enough that turgor pressure will support the shoot. As the plant becomes taller and heavier, a stronger wood is needed, and fibrous wood is produced. When viewing a transverse section of this wood dimorphism, wide-band tracheid wood will occur near the center of the stem and fibrous wood will occur near the vascular cambium. A second, rather common type is one in which vine-like or hemiepiphytic cacti with elongate shoots clamber through the branches of trees, allowing the cactus shoots to rest on the tree's branches. These will often have fibrous wood in the center and parenchymatous wood near the vascular cambium, reflecting the growth of these cacti, which initially have upright self-supporting branches that eventually lean on tree branches for support.

A third example of dimorphic wood involves Melo-cactus, a genus in which adult plants produce a cephalium (a shoot terminus producing flowers and densely covered by spines or trichomes). While growing as a juvenile, non-flowering plant, the shoot makes fibrous wood, but once it becomes old enough to flower, its morphology changes and switches to making parenchymatous wood. Thus, the lower regions of a Melocactus has both types of wood, whereas the cephalium has only the adult-phase, paren-chymatous wood. The stimulus that triggers the conversion affects the entire plant simultaneously. For other species, the conversion appears to be related to the age of the cambium at each particular area. For example, the basal part of a plant may have converted to the second type of wood, whereas the upper part of the stem is too young, so the stem is producing two types of wood at the same time.

A particularly intriguing aspect of dimorphic wood is that, when wide-band tracheid wood is present, it is always the first type of wood formed. Dimorphic wide-band tra-cheid-to-fibrous wood has originated several times, with the wide-band tracheid wood being a juvenile feature produced for only a few years. Small cacti with short life spans generally have the greatest fractions of wide-band tracheid wood in their stems when compared with taller, longer-lived forms. In fact, some of the smallest cacti never produce fibrous wood, having either wide-band tracheid wood or parenchymatous wood throughout their lifespan. For species that normally have small adults with wide-band tracheid wood, exceptionally large individuals may produce fibrous wood once they become old. In taxa such as Epithelantha or Frailea, plants always remain too small, but individuals in taxa such as Echinopsis, Soehrensia, Ferocactus, Echinocactus, and Echinocereus may actually produce fibrous wood.

Morphology

Fundamentally, the morphology of cacti is like that of other seed plants: the shoot consists of internodes, nodes where leaves are attached (although leaves are little more than leaf primordia in all Cactoideae), and axillary buds (the spine-producing areoles). The bud scales and leaves of axillary buds have become the signature spines of cacti. An axillary bud produces spines as soon as it is initiated by the shoot apical meristem; afterward it may produce a flower and/or a vegetative branch. Cactus stems exhibit short shoot-long shoot architecture. The areoles with their spine-leaves are short shoots, whereas the body of a cactus is a long shoot with highly reduced green leaves (the long-shoot leaves are not reduced in Pereskioideae and Opuntioideae).

Cactus morphology varies from ordinary trees (Pereskia, Fig. 2.4A) to large arborescent Opuntioids (Opuntia, Fig. 2.4B) and columnar cacti (Carnegiea, Fig. 2.4C; Pachy-cereus) to scrambling succulents (Acanthocereus; Harrisia; Rathbunia, Fig. 2.4D) to short-columnar and sparsely branched shrubs (Echinocereus, Haageocereus) to un-branched globose forms (Eriosyce; Mammillaria, Fig. 2.4E)

Figure 2.4. Morphology of various cactus species: (A) Pereskia grandifolia; (B) Opuntia echios var. gigantea; (C) Carnegiea gigantea; (D) Rath-bunia alamosensis; (E) Mammillaria magnimamma; and (F) Pterocactus tuberosus. Photographs are courtesy of Edward G. Bobich (A, C—F) and Park S. Nobel (B).

to geophytes (Pterocactus, Fig. 2.4F). Reduction in the number of branches with increased succulence is noteworthy and perhaps is related to the increased weight due to succulence—a cactus with branches as thick as those of a Pachycereus or a Trichocereus simply cannot have as many branches as does a Pereskia in which the branches are slender and relatively light. A correlated change accompanying reduced branching is the reduced number of shoot apical meristems—an unbranched Cephalocereus, Ferocactus, or Soehrensia has just one single apical meristem with which to construct its entire body. Certainly this has significant consequences. Branching within a taxon is often affected by environmental factors; eg., greater annual precipitation is associated with more infrequent branching in the tribe Pachycereeae (Cornejo and Simpson 1997).

A significant change in morphology with age involves juvenile/adult heteromorphy. For many cacti, mature flowering adults resemble sexually immature plants. However, for genera with cephalia, such as those in Backe-bergia (Fig. 2.5A), Discocactus, or Melocactus (Fig. 2.5B), older reproductive plants and younger plants do not resemble each other. The juvenile body looks like an ordinary globose cactus with prominent ribs, a green body, widely spaced axillary buds, and larger spines. The cephal-ium—which is a continuation of the stem, produced by the same apical meristem (Niklas and Mauseth 1981)—lacks ribs, has very closely spaced axillary buds, and produces an abundance of trichomes and short spines that hide the stem surface. The cephalium epidermis converts to cork cambium, so the surface of a cephalium is bark-covered

Figure 2.5. Cephalia: (A) apical cephalia for Backebergia militaris; (B) apical cephalium for Melocactus peruvianus; and (C) lateral cephalium for Espostoa melanostele. Photographs are courtesy of Arthur C. Gibson.

and brown (Mauseth 1989). In most cacti, each axillary bud (areole) can produce only a single flower. For cephalia, axillary buds are close together and the stems are narrow, which reduces the expense of producing each new bud (and thus each new flower). Also, because flower production is the only apparent role for cephalia, the buds can be protected with the tight mass of spines and trichomes—a mass so dense that not only are insects prevented from reaching the buds, but so is light (and hence no photosynthesis). Apical cephalia prevent the plant from producing any new photosynthetic tissues, so the photosynthetic capacity decreases with age because the existing photosyn-thetic tissues become less efficient over time.

Plants with lateral cephalia (Fig. 2.5C) are able to produce both new chlorophyllous tissue and closely spaced, well-protected axillary buds and thus flowers. These plants grow as juveniles for several years; when old enough, several ribs on one side of the body undergo a transition to adult morphology and their axillary buds are able to produce flowers. In addition, they also produce copious tri-chomes and spines, so the flowers are well protected, and the phyllotaxy of the affected ribs differs from that of the rest of the stem, with buds much closer together. The plants continue to grow like this, with one side being adult, and the other juvenile. Lateral cephalia occur for Espostoa (Fig. 2.5C), Espostoopsis, Micranthocereus, Thrixan-thocereus, Vatricania, and others (Mauseth 1999b). In Cephalocereus, the cephalium is initially lateral, but as the plants continue to grow, occasionally adjacent ribs are recruited to the cephalium so the cephalium eventually becomes terminal.

For Neoraimondia and Neocardenasia, axillary buds can produce several flowers simultaneously and are active year after year (Mauseth and Kiesling 1997). With each flowering, a short shoot stem is produced, but after many years the areoles develop into shoots up to 10 cm long and occasionally even branch. In effect, the short shoots of these genera are a type of cephalium. Members of the genus Pilosocereus produce a pseudocephalium. Again, only a few ribs produce flowers as well as copious, long trichomes, but the phyllotaxy is not altered nor is the interior anatomy changed. Once the trichomes break off, the axillary buds that have flowered are more or less indistinguishable from those that never flowered.

Biomechanics

The biomechanics of plant organs are affected by both anatomy and morphology. For all cacti, both wood composition and accumulation affect stem strength (Molina-Freaner et al. 1998; Niklas 2000; Bobich and Nobel 2001). For instance, Carnegiea gigantea (saguaro) stems appear to become stiffer over time—possibly due to increases in lignified tissue in the wood—without having appreciable increases in stem diameter, thus allowing plants to become disproportionately slender as height increases (Niklas and Buchman 1994). The amount of lignification of wood, rather than the fiber length or cell wall thickness, appears to be responsible the for increases in strength with age for the wood of Pachycereus pringlei (Niklas et al. 2000). Interestingly, increases in wood strength from the apex to the base for a P pringlei stem is non-linear (Niklas et al. 1999) as a result of the stem wood having less lignin at the base than for regions more than 1 m above the ground (Niklas et al. 2000). This apparently lessens shear stress at the cellular level near the base and also allows for the dissipation of tensile and compressive stresses, thereby decreasing the probability of stem failure at the base. Increases in the frequencies of cells with secondary cell walls, especially libriform fibers, also appears to have a positive correlation with resistance to bending stresses for the junctions between two stem segments (cladodes) for arborescent and frutescent platyopuntias (Bobich and Nobel 2001).

Increases in strength for the junctions of cladodes also correlates with the section modulus of the stem, which increases with wood accumulation (Bobich and Nobel 2001). The same correlation of stem strength with stem diameter is observed in columnar cacti (Molina-Fraener et al. 1998). In fact, slender columnar cacti like Stenocereus gummosus may often exceed the critical height allowed by their stem diameter, thus leading to the mechanical failure of the stems. For a threefold increase in length for the cladodes of the arborescent platyopuntia, Opuntiaficus-indica, the angular deflection under their own weight approximately doubles, reflecting flexure of both the cladode and the cladode-cladode junction (Nobel and Meyer 1991).

Conclusions and Future Prospects

Because of adaptations to xeric conditions and the presence of less derived woods in Pereskioideae, the Cactaceae is one of the most interesting families both anatomically and morphologically. Increases in water-storage tissue, especially in the cortex and wood, thickened cuticles, and the presence of a hypodermis are all well-known xeromorphic adaptations. However, there is much more to be gained by further anatomical and morphological research. For instance, what causes the presence of wide-band tracheids in wood is relatively unknown. Furthermore, the role of these cells is not fully understood. Also, the "annual growth rings" seen for some cacti need further study; most such anatomical features probably reflect extremely wet versus extremely dry conditions seasonally. A careful study is necessary of the vascular cambium to distinguish between ray and fusiform initials. Finally, the relationship between anatomy and morphology, especially in structural terms, has yet to be investigated for a variety of growth forms, e.g., epiphytes, vine cacti, and shrubby forms. There is indeed much more to be known about the anatomy and morphology that lead to the various forms of cacti.

Acknowledgment

The authors gratefully thank Edward Bobich and Park Nobel for substantial contributions to the writing of this chapter.

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