Flowers

Cactus flowers arise from areoles. Usually a single flower is produced from each areole, though several may arise from a single areole of Myrtillocactus geometrizans. Pereskia (P. grandifolia, for example) is the only genus that produces a true inflorescence of individual flowers arranged on a stem. The areoles of Neoraimondia may produce flowers from an individual areole for an indefinite period, thus producing a distinctive short-shoot structure. Flowers may arise on quite different parts of the stem, but few cacti produce true terminal flowers from the shoot apex. Most commonly, flowers are produced from the areole on the tip of the tubercle (recall that the areole is actually a short shoot), but some species have di morphic or two-parted areoles, a spine-producing portion and a flower-producing part. In Mam-millaria, for example, spines are located at the tip of the tubercle, with flowers at the base (Boke 1953). In Coryphantha and some species of Ario-carpus the two portions of the areole are connected by a furrow (Boke 1952,1955,1961).Flowers of some species of Echinocereus burst through the epidermis near the areole. Usually, flowers arise from areoles near the apex of the normal-appearing stem though this varies. Often, areoles of flower-producing age bear bristly spines or tri-chomes.

Most cacti do not show a major difference in structure between juvenile and mature parts of the plant. However, some cacti show a distinct transition into adulthood. Pachycereus schottii has a clear transition in the type of spination as the stem matures and prepares to produce flowers. Other cacti are even more obvious in this change because they produce cephalia, the specialized flower-bearing areas at the tops of stems, which represent the adult stage of the plant (Mauseth 1984b, 1989). There is change in spination, wood structure, nature of the trichomes, and the epidermis from the juvenile condition to the adult

Flowers of Myrtillocactus geometrizans, a single areole producing a cluster

Distinctive short shoots of Neoraimondia arequipensis subsp. arequipensis

Distinctive short shoots of Neoraimondia arequipensis subsp. arequipensis

Lateral cephalium oí Espostoa melanostele, east of Sayán, Lima, Peru

Terminal cephalium oí Melocactus intortus subsp. intortus, St. Croix, U.S. Virgin Islands cephalium stage. Lateral cephalia are formed on one to several of the ribs of columnar cacti such as Coleocephalocereuspluricostatus, Espostoa lanian-uligera, and E. melanostele. Terminal cephalia are present in Discocactus (D. horstii, for example) and Melocactus (M. intortus, M. azureus, and M. conoideus, for example). Vegetative growth stops once a terminal cephalium is produced. Cephalo-cereus apicephalium is an example of a cactus having collarlike cephalia. Browningia candelaris of Peru and northern Chile is one of the most remarkable cacti. In contrast to many cephalium-bearing cacti that produce more densely spined mature portions, B. candelaris is less spiny in the

Stems with juvenile spines (lower on stems) and mature spines (toward stem tips), Pachyce-reus schottii, Baja California Peninsula, Mexico

Lateral cephalium oí Espostoa melanostele, east of Sayán, Lima, Peru

Terminal cephalium oí Melocactus intortus subsp. intortus, St. Croix, U.S. Virgin Islands adult stage and only that portion is capable of branching.

The cactus flower is one of the most distinguishing features of the family, possessing a number of important characters of use to taxonomists. A flower, borne from an areole, is a specialized short shoot or branch. The flower consists of sets of specialized leaves that perform specific functions in the process of sexual reproduction. The receptacle is the actual shoot, containing the apical meristem or growing point, which in contrast to most stems is determinate in growth; it does not keep growing indefinitely. The outermost set of specialized leaves constitutes the sepals, which in most flowers are green and leaflike. Their main function is to protect the delicate flower parts in the developing bud. The next leaflike structures are the petals, which are often showy in order to attract pollinators. Dicotyledonous flowers usually produce sepals and petals, collectively called the perianth, in multiples of four or five (versus the monocotyledonous perianth in multiples of three). The next set of specialized leaves hardly look leaflike for they are the stamens or androe-cium. Stamens consist of filaments and anthers, the latter producing pollen grains in which the male gametes develop. The final set of specialized leaves is located in the very center of the flower and is called the pistil or gynoecium. It is actually composed of fertile leaves called carpels that have

Flower, in longitudinal section, and fruit of Echinocereus, drawing by Lucretia Brcazeale Hamilton

Stigma

Areole

Outer perianth part

Stamens Style

Floral tube

Persistent perianth

Areole

Stigma

Floral tube

Inner paria nth part

Inner paria nth part

Pericarpel Areole fused to form a single unit consisting of ovary, style, and stigma. Within the ovary are one or more ovules that contain the eggs to be fertilized by the male gametes from the pollen grains. The process of pollination involves the transfer of pollen from one flower to the stigma of another, usually by wind or animal; a pollen tube containing the male gamete then grows down the style to the ovule where fertilization occurs. The result of this process is the creation of a new generation in the form of an embryo. The ovule, containing the embryo, develops into a seed, and the ovary into the protective fruit.

The cactus flower has essentially all these structures, performing the same functions, but there are some significant differences from the typical flower (Boke 1964). The actual flower parts of a cactus are partly enclosed by shoot or stem tissue, which is called the pericarpel. This is sometimes referred to as the floral tube though it is composed of more than just perianth parts. It is as if the lower portion of the true flower has been pushed down into the receptacular tissue below it. Nectar, the solution provided as a reward for the pollinating animal, is usually produced in the lower portion of the floral tube by the stamen bases. Often, the exterior of this pericarpel is covered by rudimentary scale-like leaves or bracts called pericarpel scales, with reduced areoles bearing some woolly hairs (trichomes) and a few spines or bristles in their axils. Some genera of cacti such as Ar-iocarpus and Mammillaria have naked flowers without any scales, hairs, or external structures.

A gradual transition from the scale-like leaves or bracts occurs toward the apex of the floral tube, with bracts gradually becoming sepal-like perianth segments. Closer toward the apex they gradually become petal-like perianth segments. Usually, there are many perianth segments in this transition series.

Cactus flowers typically have many stamens, usually inserted evenly over the inner part of the floral tube. The stamens of Echinopsis are arranged in two distinct series, however, and those of Per-eskia and Rhipsalis are in a single series. Each anther has two locules or pollen-producing sacs, are attached dorsally, and dehisce longitudinally. A few cacti produce staminodes, which are modified, infertile stamens.

The ovary is embedded in the pericarpel or re-ceptacular tissue, making it an inferior ovary because the ovary appears to be located below the other flower parts. From the ovary there is a single style that bears one or more stigma lobes. The ovary is multicarpellate or derived from several ovule-producing fertile leaves, and ovules are arranged with parietal placentation. The ovules or egg-bearing structures that become seeds following fertilization are numerous, campylotropous (each curved upon itself so that the micropyle lies next to the funiculus), and crassinucellate (the embryo sac, containing the egg within the ovule, lies deep within nucellar tissue).

The cactus flower usually is sessile or without a stalk, solitary, bisexual, and regular or radially symmetrical, though there are several instances of bilaterally symmetrical flowers in bird-pollinated cacti such as Schlumbergera and some species of Cleistocactus (C. sepium, for example) and Cochemiea (C. poselgeri, for example). An important aspect of cactus flowers is the presence of different syndromes or sets of characters facilitating pollination by bats, birds, or insects. Bee pollination is the most common and is probably the primitive condition in the family. Some genera such as Rebutia (R. spinosissima, for example) are pollinated by butterflies and have salverform flowers. Hummingbird pollination occurs in many genera, the flowers of which are characterized by red color, no odor, copious nectar, bilateral symmetry, and well-developed floral tubes (Rowley 1980). This syndrome has arisen independently in the subfamilies Opuntioideae, Pereskioideae, and several tribes of the Cactoideae. Some predominantly bird-pollinated genera are Cleistocactus, Disocactus, Eriosyce, Matucana, Oroya, and Schlumbergera. Pilosocereus is pollinated primarily by bats.

Most cacti are obligate outcrossers, meaning that pollen must come from flowers of other individuals to effect fertilization, but autogamy or self-pollination occurs in some groups. Most cacti have flowers with both female and male organs, that is, that are structurally monoecious, but in a few instances functional dioecy occurs (individual plants have flowers that are either functionally male or female). For example, Mammillaria dioica has flowers in which only one of the sexes is fertile even though both stamens and gynoecium are present. A few cactus flowers never open to permit cross-pollination to occur and are thus cleistogamous, as in Frailea. Barriers to reproductive isolation are poorly developed in cacti with the result that both interspecific and inter-generic hybrids are possible, both in nature and in cultivation (Hawkes 1982-1983; Rowley 1982, 1994).

Flower size in cacti varies considerably, with some of the smallest borne by Rhipsalis (R. floc-cosa, for example); some are barely 5 mm (0.2 in) in diameter. Some of the largest of all flowers also occur in the family, with flowers of Hylocereus and Selenicereus (S. rubineus, for example) to 40 cm (16 in) in diameter and with very long floral tubes.

Cacti produce a variety of flower colors with the most common being yellow to magenta. Red is frequent in regions where bird pollination is common, and night-flowering cacti, which are pollinated by hawk moths and bats, produce white flowers. Flower color is clearly associated with pollination by animals (Rowley 1980).

POLLEN

Like other parts of the cactus reproductive system, pollen grain characteristics vary considerably within the family and provide valuable information on relationships. The most significant study of cactus pollen is that by Leuenberger (1976) and it continues to be an important source of comparative data. In general, cactus pollen is produced in monads (singly), are medium to large in size (35-125 |dm J, spheroidal to subpro-late (stretched out), and with 3-12 slitlike aper-

Scanning electron micrographs of pollen grains of Strombocactus disciformis with three slitlike apertures, X1250

Scanning electron micrograph of a polyporate pollen grain of Cylindro-puntia spinosior, xl 000

tures. The hard outer cell wall (exine) has the outermost sexine clearly separated into pillar-like columellae supporting the outer, rooflike tectum. The tectum is perforate to reticulate and often spinulose (Barthlott and Hunt 1993, 167; Eggli 1993,104-105). Pollen of the subfamily Opunti-oideae is unique within the family, being polyporate.

CHROMOSOMES

The basic chromosome number x of cacti is 11. The majority of cactus species are diploid (2 n = 2x = 22) with fewer than 20% polyploid (2 n > 22). Most polyploids occur in the subfamily Opunti-oideae (Pinkava 2001) but also in some species of Mammillaria and in Rhipsalis baccifera. Interestingly, R. baccifera, the only species of cactus to occur naturally outside the Western Hemisphere, is diploid in South America but tetraploid (2n = Ax = 44) in Central America, the Caribbean, Sri Lanka, and throughout Africa, and octoploid (2 n — 8x - 88) in Madagascar (Barthlott and Taylor 1995,63-64).

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