Integrative Biology 335:
Systematics of Plants

Families of the Caryophyllales:
Cactaceae

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Readings:

Judd et al., 3rd ed., Plant Systematics: A Phylogenetic Approach (pp. 330 – 332)

Other Resources:

Digital Flowers: Cactaceae

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Cactaceae (Cactus Family)

Habit:

• Stem succulents, perennials, some tree-like or epiphytic

  

• With betalain pigments
• Confined to the New World
• Spines [modified leaves] produced from an AREOLE [a pad-like structure representing a rudimentary shoot], which sometimes contains numerous GLOCHIDS [minute barbed bristles]

  

  

Leaves:

• Reduced or absent except in Pereskia

Flowers:

• Perfect, often large and showy
• Actinomorphic to somewhat zygomorphic
• Gradual transition from sepals to petals, both numerous [tepals]
• Carpels many
• Ovary inferior, sunken into stem tissue, 1-locular with many ovules
• Placentation parietal, sometimes a hypanthium present

Inflorescence:

• Solitary flowers emerging from areoles

Fruits:

• Berries surrounded by stem tissue
• Seeds with embryo coiled around perisperm

Cactaceae Class Notes

Cactaceae Illustrations from W. Zomlefer, Guide to Flowering Plant Families, page 59

Fig. 9.51 from Judd et al. text

Examples:

Carnegia gigantea (saguaro)

Cereus (night-blooming cereus)

Echinocereus (barrel cactus)

Epiphyllum (orchid cactus)

Lophophora williamsii (peyote)

Mammillaria (pincushion cactus)

Opuntia (prickly pear, cholla)

Rhipsalis (wickerware cactus, mistletoe cactus, coral cactus, chain cactus, popcorn cactus)

Schlumbergera (Christmas cactus, Thanksgiving cactus)

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Xeromorphic features: (1) Thick cuticle; (2) large volume to surface ratio; (3) widespread and shallow root system; (4) spines condense dew and protect plant; and (5) Crassulacean Acid Metabolism (CAM). CAM permits CO2 to be absorbed at night for use in photosynthesis during the day. When stomates open, water vapour is lost.

Varying numbers of genera and species: In the family Cactaceae, why 20-2000 genera? Why up to thousands of species? (1) Lack of adequate material (and very difficult to make good herbarium specimens!); (2) Different parts of the same species may be treated differently; (3) Lack of characters; (4) Variation in interpretation of taxonomic boundaries (splitters versus lumpers); and (5) Amateur botanists.

In general, confusion in classification exists because of: (1) Choice of characters; (2) Interpretation of characters; (3) Taxonomic opinion; (4) Tradition; (5) Method of classification; (6) Convergent evolution of similar attributes; and (7) Genetic/environmental variation.

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