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Figure 5.2. Longitudinal section of a Mammillaria flower: (1) peduncular zone; (2) receptacular tissue; (3) locular cavity; (4) ovules in a parietal position; (5) receptacular tube; (6) stamen; (7) style; (8) stig-matic lobule; and (9) perianth segment. Modified from Bravo-Hollis (1978).

Figure 5.2. Longitudinal section of a Mammillaria flower: (1) peduncular zone; (2) receptacular tissue; (3) locular cavity; (4) ovules in a parietal position; (5) receptacular tube; (6) stamen; (7) style; (8) stig-matic lobule; and (9) perianth segment. Modified from Bravo-Hollis (1978).

when touched (Rosa and Pimienta 1986). The thig-motropic movements can facilitate pollen collection by insects during floral visits (Grant and Hurd 1979) and also promote self-pollination (Rosas and Pimienta 1986). The number of pollen grains produced per flower varies from 160,000 for O. rastrera (Mandujano et al. 1996) to 330,000 for O. robusta male plants (del Castillo 1986a). The number of ovules per flower for opuntias varies from 150 to 400 (Rosas and Pimienta 1986; Nerd and Mizrahi 1997), so the number of pollen grains per ovule varies from 400 to 800. The ratio is low compared with that for anemophilous (wind pollinated) flowers producing 500,000 to 3,000,000 pollen grains per ovule, and from 5,000 to 100,000 pollen grains per ovule for entomophilous (insect pollinated) plants (Linskens 1983). Similarly low pollen grains-to-ovule ratios occur for five species of Venezuelan columnar cacti: Pilosocereus moritzians, P. lanuginosus, Stenocereus griseus, Subpilocereus horispinus, and S. repandus—300 to 1,050 (Nassar et al. 1997). Low investment of energy in the pro duction of male gametophytes may be a strategy to save energy in stressful environments.

Flower Types, Pollinators, and Pollination

Studies on cactus pollination span more than a century (Toumey 1895; Mandujano et al. 1996), although many genera of cacti have not been studied at all. The pollination vectors for cacti are animals that exhibit specificity but not exclusivity. Based on the main pollinators attracted, cactus flowers can be roughly classified as bee flowers, moth flowers, hummingbird flowers, and bat flowers (Porsch 1938, 1939; Grant and Grant 1979c).

The typical bee flower is bowl- or cup-shaped (Fig. 5.1) and has many perianth segments, a brightly color perianth, diurnal periodicity, numerous stamens, a single style, and a lobed stigma (Grant and Hurd 1979; del Castillo and González-Espinosa 1988; del Castillo 1994, 1999). The perianth is usually yellow, but may be pinkish, orange, magenta, red, or violet. The color may turn darker after the flowers are pollinated, as for some opuntias. Red or other colored stripes are present in some species of Ferocactus, Mammillaria, and Stenocactus. Ultraviolet floral patterns may increase pollinator visits and efficiency, as observed for Echinocereus spp. (Leuck and Miller 1982).

Although butterflies, diptera, beetles, and hummingbirds are also flower visitors, bees are the most frequent and the most likely pollinators. Nearly 20 genera of native bees as well as the common honey bee, Apis mellifera, are flower visitors, but not all of them are equally efficient for pollination. Various factors apparently determine the effectiveness for bee pollination of cacti: (1) flying activity, (2) flower size, (3) constancy, and (4) the adherence of pollen grains to the body. Large bees, such as bumblebees (Bombus spp., 11-19 mm in body length), may be important pollinators because, although they are generalists, they perform long-distance flights that favor outcrossing between distant individuals. Medium-sized bees, such as Diadasia spp. (10-14 mm in body length), are the major pollinators of several species whose flowers are 50 to 70 mm in perianth diameter, such as many Opuntia spp. (Fig. 5.1; Grant and Hurd 1979; del Castillo and González-Espinosa 1988) and Echinocereus spp. (Grant and Grant 1979a; Leuck and Miller 1982). They have good pollen adherence to their bodies, are very active flying among the flowers, and are the most common floral visitors for these species. They usually land on the stigma, where the pollen is deposited, and then submerge into mass of stamens to visit the nectary and collect nectar.

Bee pollinators select whether or not to visit a cactus flower based on floral size. For bee-pollinated species with

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