Shoot Anatomy And Morphology

Teresa Terrazas Salgado and James D. Mauseth


Epidermis and Hypodermis Fundamental Tissue Chlorenchyma Inner Cortex Pith

Mucilage Cells, Laticifers, and Sclereids Mineral Inclusions Vascular Tissue

Cortical and Medullary Bundles Xylem Phloem Wood

Morphology Biomechanics

Conclusions and Future Prospects Literature Cited


Basic anatomical features of Cactaceae have been studied since the 16th century (Metcalfe and Chalk 1950; Conde 1975). More recently, other features have been observed for cultivated plants, such as variations in cuticle thickness, number of hypodermal cell layers, and hypodermal wall thickness (Nyffeler and Eglii 1997). Boosfeld (1920) was one of the first to emphasize the correlation of internal anatomy with external form, noting that taxa that have very different external forms can also have very different internal structure. Among the modifications accompanying the evolution of cacti from leafy ancestors that employ C3 photosynthesis to stem-pho-tosynthetic Crassulacean acid metabolism (CAM) succulents (Chapter 1) are stems with an increased stomatal frequency, a palisade cortex, a large internal surface area due to extensive intercellular spaces, cortical and medullary vascular bundles, wood modifications, and atypical pith features. The wood not only contains the water-conducting tissue (vessels in the xylem) but can also function in support and affect the shape of cacti. In turn, the shape helps dictate the biomechanical properties of the shoot.

Epidermis and Hypodermis

The epidermis is the outermost layer of cells through which all exchanges with the environment occur; it also provides important taxonomic characters to help distinguish between closely related genera, e.g., Encephalocarpus and Pelecyphora (Boke 1959) or species, e.g., Neoevansia striata and N. zopilotensis (Herrera-Cardenas et al. 2000). A typical cactus stem generally has a uniseriate (one cell layer thick) epidermis with square or rectangular cells in transverse section. Subsequent epidermal cell divisions parallel to the periclinal (external) walls produce a distinctive multiseriate epidermis in some species of certain genera, including Astrophytum, Eriosyce, Eulychnia, and Pachycereus. In other taxa, epidermal cell divisions lack a definitive orientation parallel to the periclinal walls, occur in various angles, and may have divisions only in patches rather than for all epidermal cells (Mauseth 1996; Nyffeler and Eggli 1997).

Most cactus species possess thin-walled epidermal cells; however, for a few taxa, such as species of Armatocereus, Cereus, Jasminocereus, and Mammillaria, the periclinal (external) wall is thicker than the internal and radial walls (Mauseth 1996; Loza-Cornejo and Terrazas 2001). The periclinal epidermal cell wall may be flat or convex. Convex projections are recognized in several species of Ariocarpus, Ferocactus, Lophophora, Opuntia, Peniocereus, Thelocactus, and Turbinicarpus. For other genera, the convex outer surface is caused by a cell that divides repeatedly in different planes to produce a cluster of epidermal cells (Fig. 2.1A). This type of rough epidermis occurs in several members of the Cactoideae, e.g., Eriosyce (Nyffeler and Eggli 1997), Polaskia (Gibson and Horak 1978), and Browningia (Mauseth 1996). Modifications in the hypodermis of Uebelmannia (Mauseth 1984a) also lead to a rough epidermis. Convex projections in the form of papillae arising from a single epidermal cell or as a series of cells can affect transpiration by influencing the boundary layer of air adjacent to a stem surface (Fahn 1986; Nobel 1999).

The hydrophobic cuticle that forms on the external wall of epidermal cells (and often on the internal wall) contains cutin, a mixture of fatty acids that polymerize on exposure to oxygen. Typically, the fatty acids are produced in the protoplasm and then migrate through the plasma membrane and the cell wall. The cuticle commonly is smooth, but in some cacti it is rough and thick, as in Ariocarpus fissuratus (Fig. 2.1B). Young epidermal cells near the stem apex are covered by a thin cuticle, but older epidermal cells usually have a thick cuticle when compared with typical dicotyledons. Cuticle thickness varies from

1 pm to more than 200 pm in species of Cactoideae (Loza-Cornejo and Terrazas 2001) and from 8 to 58 pm for species of Opuntia (Pimienta-Barrios et al. 1993). Variations in cuticular thickness may be related to the water conserving ability of a species, although a relationship between cuticle thickness and water-stress resistance has not been observed for opuntias (Pimienta-Barrios et al. 1993). A thick cuticle may also increase the reflection of radiation, which will reduce stem temperatures (Nobel 1999).

As indicated, a cuticle can occur on the inner side of epidermal cells, as for Homalocephala texenis and Uebelmannia gummosa (Mauseth 1984a). Also, the cuticle can penetrate deeply into the anticlinal (radial) walls, as for Armatocereus, Bergerocactus, Echinocereus, Escontria, Myr-tillocactus, Nopalea, Oreocereus, and Pereskia (Gibson and Horak 1978; Mauseth 1984a, 1996; Loza-Cornejo and Terrazas 2001). Another way epidermal cells provide extra protection is for the protoplasm to produce long-chain fatty acids, which polymerize into wax. These also migrate to the outer surface of the external wall and are deposited on the existing cuticle. This epicuticular wax layer can be smooth or consist of particles of diverse sizes and shapes, such as aggregated beads, flakes, or threads (Mauseth 1984a) and is responsible for the grayish or bluish color of certain cactus stems (Gibson and Nobel 1986).

The only physical openings in the epidermis for the exchange of gases with the surrounding air are the stomata; the aperture of each stoma is controlled by two guard cells. Frequently, the stomata and guard cells are at the same level as the other epidermal cells, but sometimes they are located at the bottom of a pit or depression (Mauseth 1984a). In some species, the cuticle on mature tissue is greatly thickened and causes an increase in the distance of the stomata from the turbulent air, which makes the sto-mata appear sunken. When stomata are at the bottom of pits or surrounded by thick cuticle, the resistance to water vapor loss is increased slightly (Nobel 1999). Species of Maihuenia, Pereskia, Pereskiopsis, and Quiabentia possess stomata mainly in their leaves (or near the areoles; Mauseth 1999a), whereas most Opuntioideae and Cactoideae have stomata mainly in their stem epidermis. A few species of Cactoideae have stomata restricted to certain regions of the stem, as in the valleys between the ribs or on the edges of the tubercles (Gibson and Nobel 1986; Porembski 1996; Loza-Cornejo and Terrazas 1996; Herrera-Cardenas et al. 2001), and stomata are absent in the epidermis of certain cephalium shoots (Mauseth 1989; Mauseth and Kiesling 1997). Genes that control stomatal development for a leaf epidermis are postulated to be active for the stem epidermis of cacti. This displaced developmental activity has


Leaf Anatomical Characteristics

Figure 2.1. Dermal and cortical anatomical characteristics: (A) Polaskia chende, epidermal cells with irregular cell divisions and a two layer hypodermis; (B) Ariocarpus fissuratus, thick cuticle, papillose epidermal cells and palisade parenchyma cells of the outer cortex; (C) Cephalocereus columna-trajani, epidermis with crystals and thick-walled hypodermis; and (D) Myrtillocactus schenckii, rough cuticle, thick-walled hypodermis and mucilage cells in the outer cortex. Scale bars: A = 25 pm, B-D = 1 mm.

Figure 2.1. Dermal and cortical anatomical characteristics: (A) Polaskia chende, epidermal cells with irregular cell divisions and a two layer hypodermis; (B) Ariocarpus fissuratus, thick cuticle, papillose epidermal cells and palisade parenchyma cells of the outer cortex; (C) Cephalocereus columna-trajani, epidermis with crystals and thick-walled hypodermis; and (D) Myrtillocactus schenckii, rough cuticle, thick-walled hypodermis and mucilage cells in the outer cortex. Scale bars: A = 25 pm, B-D = 1 mm.

been called "homeosis" (Sattler 1988; Mauseth 1995a) and may explain other aspects of cactus evolution.

Stomatal frequencies for cacti are low, 20 to 80 per mm2, compared with leaves of C3 and C4 species, where 100 to 300 stomata per mm2 are common (Nobel 1994, 1999; Nobel and De la Barrera 2000). Within the Cactaceae, stomatal frequencies are highly variable (Table 2.1). Some species of Opuntioideae and Cactoideae have frequencies that are as high as those for the lower leaf epidermis of species of Pereskia. The stomatal pore opening for cacti tends to be large compared to other dicotyledons. For instance, for Opuntia amyclaea, O. ficus-indica, O. jo-conostle, O. megacantha, O. robusta, and O. streptacantha, the major axis of the pore varies from 33 to 62 pm (Conde 1975; Pimienta-Barrios et al. 1993), whereas pore major axes are typically around 20 pm for non-cacti (Nobel 1999). The pore length is oriented along the longitudinal axis of the stem in Pereskioideae and Opuntioideae, but exhibits a random orientation in most Cactoideae (Eggli 1984; Butterfass 1987). In any case, the area of the open stomatal pores for cacti tends to be less than for leaves of C3 and C4 species, reflecting the water-conserving use of CAM by cacti (Nobel 1994, 1999; Chapter 4).

A hypodermis generally occurs under the epidermis and usually consists of more than one cell layer in the stem succulents of the Cactoideae and the Opuntioideae, but is absent in Pereskioideae (Mauseth and Landrum 1997; Mauseth 1999). The number of layers of the hypo-dermis and the cell wall thicknesses may be related to the rigidity and xeromorphy of the stems. The cell walls of the hypodermis are often thickened with an accumulation of pectin substances, and no hypodermis is lignified (Gibson and Nobel 1986). For Cephalocereus columna-tra-jani (Fig. 2.1C) and Myrtillocactus schenckii (Fig. 2.1D), the hypodermis is thick and consists of multiple layers. For Opuntia spp. the hypodermis consists of a single layer of cells, many of which contain solitary druses and a mul-tilayered band of strong collenchymatous cells (Conde 1975; Pimienta-Barrios et al. 1993). Because of the druses and its thickness, the hypodermis can affect the penetration of solar radiation to the underlying chlorenchyma and represents a path through which gases must diffuse (Parkhurst 1986; Darling 1989; Pimienta-Barrios et al. 1993).

Fundamental Tissue

The fundamental tissue, cortex and pith, carries out at least two important functions related to xeric adaptations— photosynthesis and water storage. For nearly all cacti, the cortex is the most prominent region of the fundamental tis sue and is comprised of long-lived, thin-walled parenchyma cells; even when the epidermis is replaced by periderm (outer bark), the cortex is retained. In both Opuntioideae and Cactoideae the pith tends to maintain its size with the age of the stem and remains alive, which differs from many other dicotyledonous species. The fundamental tissue also includes specialized cells involved with secretion, such as mucilage cells and laticifers. Also, cells in this tissue can produce the alkaloids, hormones, and other chemicals that contribute to metabolism (Mauseth 1984b; Nobel 1988, 1994).


The outer cortex just below the hypodermis is commonly characterized by multiple layers of cells arranged perpendicular to the stem epidermis and is called a palisade cortex, which is made up of parenchyma cells (Figs. 2.1.A-D). The palisade cortex is green and photosynthetic. The cells are radially elongated—generally two to eight times as long as wide. About 13% of Pereskia stem tissue is intercellular air space, which is approximately the same as for the palisade parenchyma of its leaves (Sajeva and Mauseth 1991). In most species of the Cactoideae, a layer of parenchyma with large intercellular air spaces, one or two cells thick, occurs between the hypodermis and the palisade cortex. In most Cactaceae the photosynthetic tissue is in the stem, but in Pereskia it occurs in leaf palisade and spongy parenchyma as well as the stem cortex, which is narrow with small isodia-metric cells (Sajeva and Mauseth 1991). The formation of the palisade cortex in the stems of cacti is similar to that of the palisade parenchyma in dicotyledonous leaves and may similarly involve the breakdown or tearing of the middle lamella accompanied by nonrandom separation of cells, another process of homeosis (Mauseth 1995a).

Inner Cortex

The inner cortex stores water that can be drawn upon during prolonged drought. The outermost layers of this region contain some chlorophyll and presumably carry out some photosynthesis, but the chlorophyll content is progressively lower and becomes absent for the innermost layers. In the Cactoideae, about 9% of the volume of the inner cortex is intercellular air space (Sajeva and Mauseth 1991), but how easily water moves as liquid or vapor is not known. Indeed, succulents undergo successive cycles of filling and emptying their water-storage tissues. Collapsible cortex, a special type of tissue that has flexible and apparently elastic walls, is found in Bolivicereus, Borzicactus, Cleistocactus, Espostoa, Gymnocalycium, Haageocereus, Jas-minocereus, Loxanthocereus, and many other taxa (Mauseth 1995b; Mauseth et al. 1998). These walls are thin and

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