What is the evolution of a barrel cactus?

The barrel cactus plant [Echinocactus and Ferocactus spp] is evolved from herbaceous plants. The typical herbaceous plant has underground roots for anchorage and storage. It has above ground shoots for budding, flowering, fruiting, and photosynthesizing with sunlight for energizing starches and sugars.

The evolution of the barrel cactus diverges from the typical herbaceous, green plant. Instead, it takes on special body parts and functions, because of its extreme, harsh environment. For example, the barrel cactus needs to conserve energy and water in its bright, dry, hot environment. It therefore moves, processes, and stores all water, nutrient solutions, and energizing photosynthetic products above ground. The barrel cactus is capable of this evolutionary adaptation by changes to stems. For example, it greatly expands and thickens the typical herbaceous plant's slender stem.

Another important evolutionary adaptation draws upon changes to leaves. It does the reverse in terms of modifying the typical herbaceous plant's leaves. On the cactus, leaves are modified into pencil or hair thin thorns, spines, spikes, quills, prongs, needles, or bristles. The smaller, thinner surface controls water loss. It also directs any available moisture - be it dew, fog, or rainfall - to the ground. Once in the soil, the moisture adds to the water content or is taken up by roots.