Studies have also shown that some cycads at least will produce heat or odors to attract these animal vectors. One of the more novel biotic interactions among plants is an association with photosynthetic bacteria , such as Anabaena . In cycads, the cyanobacteria are sheltered in specially modified roots which have the appearance of coral, and so are called coralloid roots , as seen above at left. These roots grow up out of the soil, rather than down into it, and are thus exposed to light which the cyanobacteria need. In return for providing this stable habitat, the cycad acquires nitrogen nutrients from the bacteria.
Cycads grow in a number of tropical and subtropical climates.
Though they are often among the larger plants in their environment, cycads are no longer abundant or dominant components of the world flora. Today's cycads are found on every continent except Europe and Antarctica, but are restricted to a limited number of areas in the tropics and subtropics. Cycads grow in a number of different habitats. Species may be found as components of the forest understory in both rainforests and seasonally dry forests, or occasionally as members of the forest canopy . Other cycads grow in loose stands in grasslands, forming a kind of savanna.