how do they reproduce?
Cactus flowers are pollinated by insects, birds and bats.
Cactus
Cactus flowers are pollinated by insects, birds and bats. 
what takes over the function of photosynthesis in opuntia In the absence of leaves, enlarged stems carry out photosynthesis. what is a cactus? It is a member of the plant family Cactaceae, a family comprising about 127 genera with some 1750 known species of the order Caryophyllales. how did they evolve? Except for a relatively recent spread of "Rhipsalis baccifera" to parts of the Old World, cacti are plants of South America and mainly southern regions of North America. This suggests the family must have evolved after the ancient continent of Gondwana split into South America and Africa, which occurred during the Early Cretaceous.
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Cactus [SEP] Reproductive ecology
Cactus flowers are pollinated by insects, birds and bats. None are known to be wind-pollinated and self-pollination occurs in only a very few species; for example the flowers of some species of "Frailea" do not open (cleistogamy). The need to attract pollinators has led to the evolution of pollination syndromes, which are defined as groups of "floral traits, including rewards, associated with the attraction and utilization of a specific group of animals as pollinators." Bees are the most common pollinators of cacti; bee-pollination is considered to have been the first to evolve. Day-flying butterflies and nocturnal moths are associated with different pollination syndromes.