Many people refer to the sharp, pointy growths on cacti as “thorns,” but botanically, this term is inaccurate. These specialized structures differ fundamentally from true thorns found on other plants.
Understanding Cactus Spines
What people commonly call “thorns” on cacti are actually specialized structures known as spines. These spines are modified leaves, an adaptation that allows cacti to thrive in arid environments. Spines emerge from areoles, small, cushion-like structures unique to cacti, and represent highly reduced leaf tissue. While most cacti possess spines, their appearance can vary considerably. Not all cacti have spines; for instance, the peyote cactus (Lophophora williamsii) and some Blossfeldia species lack them.
A specific type of spine found on some cacti, particularly within the Opuntioideae subfamily (which includes prickly pears and chollas), are called glochids. These tiny, hair-like, barbed spines easily detach from the plant and can cause irritation. Unlike larger spines, glochids are often difficult to remove from skin due to their barbed nature.
The Purpose of Cactus Spines
Cactus spines serve multiple functions that are crucial for the plant’s survival in harsh environments. One primary role is protection against herbivores, deterring animals seeking the cactus’s water-rich tissues.
Spines also contribute significantly to water conservation. They reduce airflow close to the stem, creating a boundary layer that minimizes water loss through transpiration. Additionally, spines provide shade to the plant’s surface, helping to protect it from intense sunlight and reduce evaporation. In some species, spines can even aid in collecting atmospheric moisture from dew or fog, which then drips to the ground and is absorbed by the plant’s roots.
Furthermore, spines assist in temperature regulation. Their dense covering can provide insulation, helping to protect the cactus from both extreme heat and cold. Spines can also play a role in seed dispersal, particularly in species like the jumping cholla (Cylindropuntia fulgida), where barbed segments easily attach to animals.
Variations in Cactus Spines
Cactus spines exhibit diversity in their form, ranging from needle-like to hair-like structures. They vary in size, shape, color, and density, influencing how a cactus appears and feels. For example, the spines of a saguaro cactus are distinct from those of a barrel cactus.
Some cacti have fine, hair-like spines that create a soft, fuzzy appearance, such as those found on the Old Man Cactus (Cephalocereus senilis) or certain Mammillaria species. While these may not seem “thorny” to the touch, they are still botanically classified as spines. Conversely, some cacti have stout, woody, or hooked spines. This wide array of spine morphologies reflects the diverse ecological needs and evolutionary adaptations across the cactus family.