Defense or Sunscreen? The Mystery of Cactus Spines

For a long time, cactus spines have been regarded as a classic example of plant adaptation to desert environments. These spines are often thought to help reduce water loss by limiting transpiration. Yet this raises an intriguing question: are cactus spines truly an adaptation to aridity? And how did cacti evolve into the familiar leafless plants with thick, swollen stems? A group of plants from the mountainous regions of Central and South America may offer clues to these questions.

Figure 1. Flowers and leaves of Pereskia（Image source：Kauderwelsch， CC BY-SA 3.0 ）

These plants belong to the genus Pereskia, a group of tropical plants with large leaves that occur mainly in regions such as Brazil and Mexico. Most species grow as shrubs, although some develop a climbing habit. At the base of the leaves of Pereskia there is typically a ring of spines. This structure is known as an areole, a specialized organ that develops from modified leaf tissues. In addition to the sharp spines around the leaf base, the fruits of Pereskia may also bear short spines. Their function is straightforward: they serve as a defense against animals that might attempt to eat them.

At this point, attentive readers may already notice something important—Pereskia is in fact a cactus. Unlike the familiar desert cacti, however, it represents one of the most ancient lineages within the cactus family. These plants retain large leaves and slender woody branches, giving them an appearance very different from the compact, leafless cacti that most people recognize. This raises another question: what caused other cacti to diverge so dramatically from the Pereskia form? Botanists have suggested that the uplift of the Andes Mountains may have played a key role.

Figure 2. Areoles of Pereskia（Image source：Purves, M， CC BY-SA 3.0 ）

Around 25 to 20 million years ago, the Andes began to rise. This massive mountain-building event blocked warm, moist air arriving from the Pacific Ocean, gradually making large regions of South America increasingly dry. Under these drier conditions, some Pereskia populations living around the Andes began to change. Their leaves gradually evolved thicker tissues that could store more water, an adaptation that helped them cope with prolonged dryness. Over time, these plants diverged into the genus Pereskiopsis.

Figure 3. A shrubby Pereskiopsis plant, showing areoles and succulent leaves growing together along the stem（Image source：Consultaplantas， CC BY-SA 4.0 ）

At even higher elevations in the Andes, where rainfall was scarcer and environmental conditions were harsher, the evolutionary pressure became stronger. The leaves of these plants became smaller and more succulent in order to withstand intense sunlight and large daily temperature fluctuations. At the same time, their stems thickened considerably. Their areoles also developed larger and denser clusters of spines, likely as protection against hungry herbivores living in these mountain environments. This lineage eventually evolved into the genus Maihuenia.

Figure 4. Flower and plant of Maihuenia patagonica. The small green structures are extremely reduced leaves（Image source：Dick Culbert， CC BY 2.0 ）

By around 10 million years ago, some of these Andean lineages had undergone a profound transformation: their leaves disappeared entirely. Photosynthesis shifted to the thickened stems, which took over the role of capturing sunlight and producing energy. With the help of birds that dispersed their seeds, these plants gradually spread beyond mountain habitats into deserts and even tropical forests. Over time, this lineage gave rise to the cacti that are familiar today.

Although cactus species have experienced many dramatic changes over millions of years, one feature has remained constant: the areole. Nearly every cactus possesses this distinctive structure. Throughout cactus evolution, areoles have served as important defensive organs. In certain rainforest species, such as the climbing cactus Hylocereus, areoles also help the plant attach to supporting hosts and climb toward sunlight. In this way, cacti have developed the functions of the areole to remarkable versatility.

Figure 5. Illustrations of various cactus forms（Image source：G. Mütrel, Leipzig ; Berlin ; Wien : F.A. Brockhaus，CC0 1.0 ）

The evolutionary history of the cactus family reveals the extraordinary power of natural selection. From the ancestral form represented by Pereskia to the leafless, swollen stems of modern cacti, the transformation illustrates how plants can reshape their bodies in response to environmental pressures. The areole, as the defining feature of the cactus family, plays crucial roles in defense and climbing, and over tens of millions of years it has become a unifying trait linking diverse cactus species. This evolutionary story not only explains how cacti survive in extreme environments, but also provides valuable insight into the broader mechanisms by which plants adapt and diversify.

Author: Rodrigo

References:

1. Edwards, Erika J.; Nyffeler, Reto & Donoghue, Michael J. (2005), "Basal cactus phylogeny: implications of Pereskia (Cactaceae) paraphyly for the transition to the cactus life form", American Journal of Botany, 92 (7): 1177–1188, doi:10.3732/ajb.92.7.1177

2. Barthlott, Wilhelm (1983), "Biogeography and Evolution in Neo- and Paleotropical Rhipsalinae (Cactaceae)", Sonderb. Naturwiss. Vereins Hamburg, 7: 241–248.