Plant 166

Cereus uruguayanus R.Kiesling (Cactaceae)


Peru apple-cactus

Cereus uruguayanus is a columnar cactus up to six metres tall, forming a striking multi-stemmed, candelabra. The physical appearance and adaptations to their environments shared by arborescent cacti and species of tree euphorbias are classic examples of convergent evolution. Evolving on separate continents but in similar environmental conditions, these two unrelated groups of plants have developed similar strategies to survive under extreme conditions of intense heat and light with periods of drought.

A characteristic that distinguishes cacti from other succulent plants are organs called areoles; they are important for the identification of cacti genera and species. Growing on nodules called tubercles along the ribs of the stems, the areoles are highly condensed lateral branches from which spines and flowers arise. The stems of Cereus uruguayanus are strongly ribbed and jointed, with spines radiating from areoles at intervals along these ribs. Most cacti do not bear leaves. They have spines instead which perform no photosynthetic function but offer protection and help with water capture. Dew condenses on the spines and falls or trickles onto the shallow roots just below the soil surface.

The surface of the stems is a bluish-grey colour when young to reflect heat and light, and waxy to help reduce water loss. The stem tissue is able to photosynthesise, which is an adaptation to hostile environments where water conservation is paramount, Cereus uruguayanus uses a form of photosynthesis called Crassulacean acid metabolism. Stomata on the stem surface remain closed during the day, conserving valuable water, and open at night so that carbon dioxide may be captured and stored. During the day the carbon dioxide is released inside the plant, completing the photosynthetic cycle. With age the surface of the stems becomes corky and photosynthesis is unable to take place.

Like many other species in the genus, Cereus uruguayanus is a night-blooming species that is pollinated by bats in search of nectar. The white flowers are 20 centimetres long, showy and fragrant. Each bloom lasts just one night, fading in the early dawn as pollinators retire and temperatures increase.

The large fruit is technically a berry and is purplish-red when ripe. These are edible and either eaten straight from the cactus or made into jams and syrups for local consumption.

All trade in cacti is regulated by the Convention on International Trade in Endangered Species, an agreement that helps to ensure species' survival is not threatened by illegal trade.

Further reading

Fleming TH et al. 2009. The evolution of bat pollination: a phylogenetic perspective. Annals of Botany 104: 1017-1043.

Forster PI and Schmeider M 2000. Cereus uruguayanus (Cactaceae) and its naturalised occurrence in Queensland, Australia. Austrobaileya 5: 671-677.

Kate Pritchard