Aerial Roots – Mixed Connections

By Joe Merkelbach (May 2010)

Cacti and succulents seem to belong in the desert because of their ability to conserve water, but this trait is beneficial in more than dry environments. The cacti that live in jungle environments as epiphytes use the ability to conserve moisture to thrive in dry microhabitats. Tree limbs or cliff faces do not have the capacity to hold any residual water, so even copious rainfall drains rapidly, and a rather xeric state is the usual condition.

There are many species and several genera of epiphytic cacti including Rhipsalis, Schlumbergera, Zygocactus and Epiphyllum. They are most often grown in well-drained hanging baskets in a nod to their epiphytic origins. The plants are able to withstand lack of water for quite a period of time; the stem segments shrivel but plump back up when water is provided.

Epiphytic succulents are less frequently grown, but examples include some Hoya (Asclepiads) and some Tillisandia (Bromeliads).

Many orchids are succulent epiphytes, with the pseudobulbs storing moisture for the plants. In some species, the green exposed roots carry on some or all of the photosynthesis with CAM based as in cacti. The limited growing medium for epiphytes is generally more organic in character, and the roots play an important part in holding the material in place.

Clambering cacti are another group that show aerial roots, not strictly as epiphytes, but as a means of establishing support for growth toward light.

It is interesting to realize that the ability to store water is a trait that benefits epiphytes in dry microhabitats just as well as succulents in deserts. The ability to hold moisture that can rapidly drain away is just manifested differently.

Sources:
San Gabriel Valley Cactus and Succulent Society – http://www.sgvcss.com/communique/sotm_2001_05.pdf
Wikipedia – http://en.wikipedia.org/wiki/Hoya
Epiphytic Plant Research and Information Centre – http://www.epric.org/index.php?cid=15