Convergent Evolution of Cactus with other Succulents ... 3/3

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Reduced or no leaves once again reduces surface area and therefore potential exposure to drying winds, the photosynthesis operation is transferred to the stem.

Spines etc have been thought to act in a number of ways: Fend off animals who would otherwise see the plant as a good source of water for itself; act as a dew collection point and channelling the water down the body to the roots; some sort of mineral and waste deposit system for the salts the plant accumulates as it drags water out of the soil.

Water storage is essential to enable the plant to survive between infrequent/small rainfalls.

In summary totally unrelated families of plants have evolved similar morphology (body shape etc) to cope with similar environmental factors, even in completely different continents. Why reinvent the wheel?