From the Digest

Converging Toward a Point

By Joe Merkelbach (November 2009)

One of the fascinating features of cacti and succulents is their tendency to have striking and dramatic forms and shapes. Instead of luxuriant, leafy growth typical of most plants, they exhibit parsimony in growth habits with few or no leaves and simple blocky shapes. Their architectural forms are a response to the generally harsh living conditions that the plants experience in nature. These similar-appearing plants from various disparate locations are a fine demonstration of convergent evolution.

Leaves are the means that most plants use to capture sunlight for the process of photosynthesis; they provide lots of surface for that activity. But leaves transpire, taking in carbon dioxide gas and releasing oxygen gas and, importantly, water vapor.

Cacti and succulents cannot afford this profligacy with water, so they have either developed leaves with small surface area or dispensed with them altogether. Instead, the green branches and plant bodies contain the chloroplasts that do the sunlight capture of photosynthesis.

The generally stocky plant bodies, with minimized surface area per volume, further conserve fluids against hot, dry, windy conditions. Cacti and similar succulents exhibit these very similar adaptations in widely separated parts of the globe. As a further pointed refinement, spines, thorns and needles are also a common feature. These sharp appendages deflect away animals that would like to take the concentrated plant fluids.

Although they look the same, they arise from different sources and converge toward the same form and function. Cactus spines or needles come from the areoles, which are similar to leaf attachment points for normal plants. The sharp points of euphorbias are stipular like rose thorns, frequently dually arrayed, peduncular or derived from flower parts, or simply sharp bumps rising from the plant body. The Didieracae of the spiny forests of Madagascar have pairs of spines that arise from the leaf axils.

These similarities in body form, appearance and appendages are an informative lesson in the process of convergent evolution, wherein similar shared living conditions direct life along similarly directed paths.

Cactus and Succulent Society of America –