Chlorenchyma In Cactus

Stomatal frequency for photosynthetic tissue in cacti

Subfamily Frequency (number per mm2)

Pereskioideae leaf, 17-99 (51); stem, 2-20 (11)

Opuntioideae leaf, 7-16 (12) (Pereskiopsis spp.); stem, 9-115 (80) (Opuntia spp.)

Cactoideae 18-60 (31)

Data indicate the range, with the mean in parentheses, and are from Mauseth and Sajeva (1991), Pimienta-Barrios et al. (1993), Nobel (1994), Arias (1996), and Nobel and De la Barrera (2000).

unlignified and more flexible than those of the palisade cells, readily allowing for volume changes of the cell. Five to ten layers of such cells can occur in the innermost part of the inner cortex, but not in the pith or medullary rays; such cells occur even in regions of shoot tips that are less than one year old, which have never experienced water stress. When drought occurs and the plant's rate of water intake falls below its rate of water loss, the flexible walls permit the collapsible parenchyma cells to release water while the less flexible walls of the palisade cortex cells retain water (Goldstein et al. 1991). Consequently, water from the collapsible water-storage tissue replaces water lost from the photosynthetic tissue.

Most of the water in the stems of cacti is in the inner cortex. The cells have large vacuoles and can lose four times more water than is lost from the smaller cells of the chlorenchyma (Nobel 1994). For the barrel cactus Ferocactus acanthodes, solutes decrease in the inner cortex and pith (either by polymerization or transport out of the cells), lowering the osmotic pressure and thereby favoring the redistribution of water to the chlorenchyma (Barcikowski and Nobel 1984). That is, water diffuses from this storage region into regions of higher osmotic pressure in the chlorenchyma. Similar processes occur for the platyopuntia Opuntia basilaris, except that for this species most water storage occurs in the pith (Gibson and Nobel 1986). By maintaining higher water content in the chlorenchyma, nocturnal opening of stomata and net CO2 uptake are allowed to continue for a longer period than would be the case if all the tissue were to dry at the same rate (Chapter 4).

Pith

One difference of many Cactaceae in relation to other dicotyledons is the presence of a radially thick pith in the center of the stem (Bailey 1962, i963a,b; Gibson and Horak 1978; Mauseth 1989). This pith occurs within the stele and generally occupies a small fraction of the stem volume

(Mauseth 1993a), except for platyopuntias (Nobel 1988). The cells are generally thin walled, isodiametric, and living; they act as a water reservoir, often contain starch grains, and may store a variety of allelochemicals. In large-stemmed Cactoideae, the pith may also contain medullary bundles, which facilitate radial water movement (Mauseth 1993a). The short cylindrical, globose, or disc-shaped cacti of the Cacteae, Echinocereae, and Notocacteae tribes lack medullary bundles in their relatively small piths.

Mucilage Cells, Laticifers, and Sclereids

The stem tissues of cacti often contain large quantities of the complex carbohydrate mucilage, which is hydrophilic and affects water relations (Gibson and Nobel 1986; Goldstein and Nobel 1991; Nobel et al. 1992). Mucilage cells (idioblasts that produce mucilage; Fig. 2.2A), which lack chloroplasts and starch grains, were first described by Lauterbach in 1889 (Mauseth 1980). Lloyd (1919) pointed out that although hydrated intracellular mucilage crowds the protoplast, the cell mucilage can aid in cell growth because it imbibes water. The mucilage content and composition in a mucilage cell vary with time of year and species. For older stems or during extensive drought, the mucilage cells may contain crystals (Fig. 2.2B). Both Pereskia and Maihuenia have mucilage cells, but they are more abundant in Maihuenia, for which the very large mucilage cells often compose over half of the leaf volume (Mauseth 1999). Mucilage cells are also abundant in the Cactoideae and Opuntioideae, generally occurring in the inner cortex and pith. Sometimes mucilage cells can occur just below the hypodermis within the palisade parenchyma, as for Echinocereus sciurus (Fig. 2.2A). Mucilage cells vary from about 40 pm to over 1.0 mm in diameter and often resemble cavities in the inner cortex, as for Stenocereus thurberi and S. martinezii (Gibson 1990; Terrazas and Loza-Cornejo 2002). Other mucilage-containing cavities are present in the pith of several species of Ariocarpus

Figure 2.2. Mucilage cells and mineral inclusions: (A) Echinocereus sciurus, abundant mucilage cells; (B) Wilcoxia poselgeri, crystal in a mucilage cell of the cortical region; (C) Escontria chiotilla, sphaerocrystal in the cortical region; and (D) Stenocereus gummosus, silica grains in the epidermal cells. Scale bars: A = 1 mm, B = 100 pm, C-D = 20 pm.

Figure 2.2. Mucilage cells and mineral inclusions: (A) Echinocereus sciurus, abundant mucilage cells; (B) Wilcoxia poselgeri, crystal in a mucilage cell of the cortical region; (C) Escontria chiotilla, sphaerocrystal in the cortical region; and (D) Stenocereus gummosus, silica grains in the epidermal cells. Scale bars: A = 1 mm, B = 100 pm, C-D = 20 pm.

(Anderson 1960, 1961), and a unique network of anastomosed canals containing cells essentially filled with mucilage occur in Nopalea spp. (Mauseth 1980).

In 1889 Lauterbach reported laticifers (idioblasts that produce latex) for Coryphantha, Leuchtenbergia, and Mammillaria, but they were not extensively described until 1978 (Mauseth i978a,b). The composition and abundance of latex varies among species. In the Mammillaria section, laticifers are abundant and referred to as "milky," those in section Subhydrochylus are referred to as "semi-milky," whereas those in section Hydrochylus lack laticifers altogether (Mauseth 1978b). Laticifers in Mammillaria differ from those of other plant families and are unique among cacti (Mauseth 1978a). In section Mammillaria, laticifers occur in the pith, throughout the cortex, the basal half of the tubercles (modified leaf bases), and even the entire tubercle, where they laterally branch to the hypodermis. Laticifers in section Subhydrochylus form only in the outermost cortex and the bases of the tubercles. For plants in sections Mammillaria and Subhydrochylus, laticifers develop by rapid cell division of wide groups of parenchyma cells. At maturity both have a central lumen lined by an epithelium one to several cell layers thick (Wittler and Mauseth 1984). However, laticifers in section Subhydro-chylus resemble the ancestral condition because they are more irregular in shape, lumen development, and epithelium form than those in section Mammillaria.

Two distinct forms of idioblastic sclereids (dead, lignified cells) occur in the stems of certain columnar cacti, such as Eulychnia spp., Pachycereus pringlei, and Stetsonia coryne (Gibson and Nobel 1986; Nyffeler et al. 1997). One type of sclerenchyma cell is slender and distinctly elongated and occurs in the outer cortex. The other form is globular or subglobular and occurs in the inner cortex and the pith. Sclereids provide mechanical strength due to their thickened, lignified cell walls and aid in lessening collapse of the cortex during drought. Other columnar cacti do not possess idioblastic sclereids, but instead have a cortex with many large mucilage cells, indicating different strategies for adaptation to arid environments.

Mineral Inclusions

Cacti can accumulate enormous quantities of calcium oxalate. For example, up to 85% of the dry weight of Cephalocereus senilis can be Ca oxalate (Cheavin 1938). As a result, most cacti have Ca oxalate crystals in their stems, which may be prismatic (sharp angles), druses (star-like), and, rarely, acicular (needles). Crystals are formed in the central vacuole via a complicated precipitation process, which may be an end-product of metabolism and/or may serve as a means of removing excess Ca from the cells (Franceschi and Horner 1980). Plants grown using solutions high in Ca often form more crystals than control plants. In addition to the insoluble Ca salts, many plants contain high concentrations of soluble oxalate, which can affect osmotic pressure (and thus turgor and volume regulation) in the cells. A major function attributed to Ca oxalate crystals is that of protection against foraging animals. The irritation and burning sensations of the mouth caused by eating crystal-containing plants is well known, and large quantities of oxalate can be fatal.

Different forms of Ca oxalate and other chemicals are involved in crystal formation. Using X-ray diffraction, Rivera (1973) found druses with the monohydrate form of Ca oxalate in Opuntia imbricata and the dihydrate form in Echinocactus horizonthalonius, E. intertextus, and Escobaría tuberculosa. The dihydrate form also occurs in prismatic crystals. Leaves of Pereskiopsis contain both Ca oxalate and Ca malate crystals (Bailey 1966). Members of Cactoideae may contain sphaerocrystals (spherical; Fig. 2.2C), the composition of which is unknown, and their form differs from other crystal types (Metcalfe and Chalk 1950; Loza-Cornejo and Terrazas 1996). Some species contain only one crystal type, whereas others may have two or more types, even in adjacent cells (Gibson 1973). Crystals are common in secondary xylem and may be deposited in axial or radial parenchyma (Gibson 1973; Mauseth 1996, 1999; Terrazas and Loza-Cornejo 2001).

The occurrence of crystals in the epidermal cells often has taxonomic value (Chapter 1), but their occurrence in the cortex and pith is more variable and therefore has low taxonomic value. For example, Cephalocereus and Neobux-baumia are the only members of tribe Pachycereeae with prismatic crystals in their epidermal cells (Gibson and Horak 1978; Terrazas and Loza-Cornejo 2001). Members of tribe Hylocereeae contain acicular crystals in their epidermal cells (Gibson and Nobel 1986; Mauseth et al. 1998; Loza-Cornejo and Terrazas 2001), while other species have distinctive druses in their hypodermal cells (Pimienta-Barrios et al. 1993; Mauseth 1996; Loza-Cornejo and Terrazas 2001).

Silica bodies are also prominent in the epidermal and hypodermal cells of certain cacti and are valuable taxo-nomically (Fig. 2.2D). Their occurrence is diagnostic for all members of Stenocereus and Rathbunia (Gibson and Horak 1978). Silica grains also occur in the epidermal cells of other Cactoideae members (Loza-Cornejo and Terrazas 2001) and in the ray cells of Pachycereus weberi (Terrazas and Loza-Cornejo 2002), but they have not been observed in the Pereskioideae or Opuntioideae (Gibson and Nobel 1986).

Vascular Tissue

Vascular tissue, which is involved in movement of substances in plants, is highly specialized in cacti. The main and largest vascular bundles occur in the stele, which lies between the inner cortex and the pith. The two tissue types are the xylem, which serves to move water as well as dissolved nutrients, and the phloem, which distributes pho-tosynthetic products and other organic molecules. Primary xylem and phloem develop during the initial stages of growth, and, periodically, secondary tissues subsequently develop. Vascular tissue also occurs in the cortex (cortical bundles) and the pith (medullary bundles).

Cortical and Medullary Bundles

Cortical bundles, which are absent in Pereskioideae and Opuntioideae, generally occur throughout the cortex but do not extend to the hypodermis in members of the Cactoideae (Mauseth 1995a, 1999a). They occur in all directions and change direction frequently. Cortical bundles are collateral and contain primary and secondary xylem and phloem. Secondary phloem accumulates at a higher rate than secondary xylem, which may or may not increase with stem age (Mauseth and Sajeva 1992). For example, for Mammillariaparkinsonii and Pediocactus simpsonii, older bundles have much more xylem than younger ones. In some species, cortical bundles contain phloem fibers that differentiate adjacent to the conducting cells of the phloem, such as for species of Acanthocereus (Mauseth et al. 1998), Bergerocactus emoryi (Terrazas and Loza-Cornejo 2002), and Selenicereus inermis (Mauseth and Sajeva 1992). Xylary fibers in cortical bundles are rare but occur in Pilosocereus mortensenii (Mauseth and Sajeva 1992).

Cortical bundles appear to be involved in three processes (Mauseth and Sajeva 1992): (1) transporting photosyn-thate from the outer, chlorophyllous palisade cortex to the stele; (2) transporting sugars to and from storage cells in the inner, nonphotosynthetic cortex; and (3) transporting water throughout the cortex. Phloem in cortical bundles is probably involved in sugar transfer when the cortex acts as a starch storage tissue. Cortical bundles accumulate phloem as they age, indicating the continued production of phloem and presumably greater translocation of sugars, which probably cannot be transported from the outer cortex to the stele rapidly enough by diffusion alone (Nobel 1999). Cortical bundles resemble leaf veins in spacing, structure, presence of narrow conducting cells, and solute distribution. Mauseth and Sajeva (1992) conclude that cortical bundles, whose life span in cacti is long compared to the leaf veins in most dicotyledons, have arisen independently in the Cactaceae.

Medullary bundles, which are similar in size to cortical bundles, are initiated close to the apical meristem, may have secondary xylem and phloem, and occur only in subfamily Cactiodeae (Boke 1954; Bailey 1962; Gibson and Horak 1978; Mauseth and Ross 1988; Mauseth 1993a, 1999). Medullary bundles are closely spaced when initiated near the shoot tip, but as the shoots continue growing, the pith expands and medullary bundles are pushed to a wider spacing, with very low densities in the older trunks (Mauseth 1993a). A few species (e.g., Brachycereus nesioti-cus, Jasminocereus thouarsii, Monvillea marítima) are distinctive because primary phloem fibers differentiate adjacent to the medullary bundle phloem. Xylary fibers in medullary bundles are rare but present in Jasminocereus thouarsii. Medullary bundles are interconnected with stele bundles, and, in several cases, they completely transverse the broad primary rays and are interconnected with the cortical bundles. Although medullary bundles appear to be relictually absent in the family, they may have originated during the early stages of the evolution of Cactoideae. In fact, a secondary loss of medullary bundles may have occurred in several species of Cactoideae that have a narrow pith and a relatively broad cortex (tribe Cacteae, some Notocacteae, and some Echinocereae; Boke 1956, 1957; Gibson and Horak 1978; Mauseth 1993a; Loza-Cornejo and Terrazas 1996; Mauseth et al. 1998).

Medullary bundles should permit a cactus to translocate water and starch to and from a broad pith. Because they continue to produce phloem throughout the life of the plant and starch is abundant in many pith sections, transport of carbohydrates is an important role for medullary bundles. Water transport throughout the pith may also be important, but may proceed slowly.

Xylem

The water-conducting conduits of the shoots of cacti are vessels occurring either solitary or in small clusters of 2 to 10 vessels (Gibson 1973, 1978). Vessels are narrow, usually ranging from 10 to 60 pm in diameter, are dead at maturity, and consist of tubes of primary and secondary wall (Nobel 1999). The widest vessels occur in the primitive genus Pereskia and the narrowest in species of the epiphytic genus Rhipsalis (Gibson and Nobel 1986). The vessel elements have simple perforation plates, a highly derived trait that facilitates fluid movement along a vessel (Nobel 1999). Also present are libriform fibers (phloem-like fibers; Fig. 2.3A) and wide-band tracheids (often referred to as vascular tracheids; Fig. 2.3B). Wide-band tracheids are im-perforate, broadly fusiform cells with either helical or annular thickenings; the thickenings project deeply into the

Figure 2.3. Secondary xylem and phloem: (A) Peniocereus striatus, distinctive short living fibers; (B) Echinocereus schmollii, abundant wide-band tracheids; (C) Pachycereus pringlei, sclereid in the collapsed phloem and dilatated rays; and (D) Wilcoxia poselgeri, phloem parenchyma cells with tannins between noncollapsed phloem and cortical cells. Scale bars: A, B, D = 20 pm, C = 1 mm.

Figure 2.3. Secondary xylem and phloem: (A) Peniocereus striatus, distinctive short living fibers; (B) Echinocereus schmollii, abundant wide-band tracheids; (C) Pachycereus pringlei, sclereid in the collapsed phloem and dilatated rays; and (D) Wilcoxia poselgeri, phloem parenchyma cells with tannins between noncollapsed phloem and cortical cells. Scale bars: A, B, D = 20 pm, C = 1 mm.

lumen (Gibson 1973, 1977, 1978; Mauseth et al. 1995; Mauseth and Plemons 1995). Because wide-band tracheids lack perforations, they play little role in rapid water conduction between adjacent cells. Rays in cacti are extremely variable in width and length but are generally wider than in typical dicotyledons, an adaptation that facilitates water storage (Gibson and Nobel 1986). Rays also function in the lateral movement of water, as evidenced by the perforations in the ray cell walls for some cacti (Terrazas 2000). In addition to wide rays, other special features of cactus xylem for water storage are that the fiber cell walls are thin, which increases lumen volume, and that vessels and paratracheal parenchyma constitute a large fraction of the volume of the axial system (Mauseth 1993b).

Phloem

Primary and secondary phloem in Cactaceae varies among species but is generally composed of sieve tube members, companion cells, and axial and radial parenchyma. In most cacti with elongate stems, the young vascular bundles of the stele may have a cap of primary phloem fibers, which are thick-walled, septate, and nucleated. The size of the phloem fiber cap varies among species but does not appear to be an indicator of phylogenetic relatedness (Mauseth 1996). As secondary phloem accumulates, the older, more peripheral phloem collapses and dies, while the younger non-collapsed phloem near vascular cambium consists of living, functioning cells. The older sieve tube members and companion cells collapse into dark-staining masses that may be oriented either tangentially or radially to the stele.

For some species, parenchyma cells may redifferentiate into sclereids (Fig. 2.3C), as for Pereskia and some members of the tribes Browningieae, Cereeae, Echinocereeae, and Pachycereeae (Mauseth 1996; Mauseth et al. 1998; Terrazas and Loza-Cornejo 2002). Other species—usually those with short globose shoots — develop neither primary phloem fibers nor sclereids associated with the collapsed phloem (Gibson and Nobel 1986; Loza-Cornejo and Terrazas 1996; Mauseth et al. 1998; Mauseth 1999a). Species of Wilcoxia have specialized parenchyma cells that contain abundant tannins (Fig. 2.3D; Loza-Cornejo and Terrazas 1996). The lack of a phloem fiber cap generally is associated with the occurrence of wide-band tracheids in the secondary xylem (Mauseth et al. 1998). Sclereids or fibers do not form associated to phloem in cephalia of Leptocereus, Melocactus, and Neoabbottia (Mauseth 1989; Mauseth et al. 1998). Phloem rays expand as secondary growth occurs. Phloem rays have living thin-walled parenchyma cells and may contain abundant mucilage cells or druses. They connect with the primary or secondary xylem rays and may contribute to the distribution of photosynthetic products and other organic molecules.

Wood

Various types of wood occur in the Cactaceae (Gibson and Nobel 1986; Mauseth and Plemons 1995; Mauseth and Plemons-Rodriguez 1998; Arnold and Mauseth z999). They can be classified according to the type of matrix they have (fibrous, parenchymatous, or wide-band tracheids) or according to the uniformity of wood development (monomorphic or polymorphic). The axial portion of fibrous wood, i.e., the portion produced by fusiform initials and whose cells are elongate parallel to the axis of the shoot, consists of vessels embedded in a matrix of living libriform fibers. Typically, the fibers constitute over half of the wood volume, giving it strength, flexibility, and resistance to breaking. Fibrous wood is found in all cacti that are too large to hold themselves up by turgor pressure—all members of Pereskia, aborescent Opuntiodeae, Armatocereus, Pachycereus, and other large cacti (Gibson 1973; Mauseth 1992; Mauseth and Landrum 1997; Terrazes 2001). Considerable variation occurs in the anatomical details of the fibrous wood. Most species have fibers that lack septae, whereas in other species all or most of the fibers are septate, with a single septum of primary cell wall. Vessels in fibrous wood are generally wider and less frequent than in other types of wood (Table 2.2). The added strength of fibers may allow for larger vessels, thus increasing water movement per vessel because volume flux is dependent on the vessel radius raised to the fourth power (Nobel 1999E!

Parenchymatous wood has a matrix of unlignified parenchyma cells with thin primary cell walls. Paren-chymatous wood is too soft to provide significant support to the shoot and is found in cacti whose shoots are procumbent, globose, or immersed in the soil. Within the parenchymatous matrix, vessels are smaller in diameter than for vessels in fibrous wood of taller or older plants (Tables 2.2 and 2.3). The vessels are either solitary or occur in clusters, as in fibrous wood, but they have a tendency to form large clusters of up to 100 vessels and thus are more frequent than for the fibrous wood of larger growth forms (Table 2.2 and 2.3). Rays in parenchymatous wood tend to be large and purely parenchymatous and the cells lack secondary walls. Parenchymatous wood tends to be formed in small amounts, with only a few millimeters from pith to vascular cambium. Even though some shoots can have massive amounts of cortex or pith parenchyma, they never have massive amounts of parenchymatous wood. Vessels within parenchymatous wood are in intimate contact

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