Nature's fortress: How cacti keep water in and predators out - Lucas C. Majure
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Several of the adaptations to these dry environments include the production of highly modified leaves in the form of spines, which also protect from the sun’s rays and herbivores, succulent tissue that enables them to store lots of water, the thick waxy cuticle overlaying the skin or epidermis, rain roots that quickly develop after even moderate precipitation, and CAM photosynthesis, which allows these plants to open their stomata at night, further reducing water loss. CAM photosynthesis has evolved in numerous plants that occur in these dry, or otherwise, water-limited habitats. See here the difference between different photosynthesis types.
All of these adaptations have given cacti an advantage over many other plants, and for these reasons, they are one of the most conspicuous elements of dry habitats across the Americas. Some of the most iconic cacti, such as the saguaro, Carnegia gigantea, are known from the Sonoran Desert in the western United States and northern Mexico. The prickly pear cacti of the genus Opuntia, which are the most widespread of all cacti, are often used by people as a food source, dye source, and for their medicinal and ornamental properties. So, cacti are very valuable to people, as well as to animals that use them in their natural environments.
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