Friday, May 25, 2012

Life of a Cactus Part 6: Callus

Woodpecker entering a Saguaro "boot" used for nesting.
The last water conserving feature of cacti we will discuss is the abundance of mucilage all cacti produce internally.  If you were to cut open a cactus and touch the internal tissues you would notice the cut surface as being wet.  It would not be a watery wet though, it would be a slightly sticky and slimy wet.  This sliminess, known as mucilage, is a result of polysaccharides, or carbohydrates, produced by the cactus.  This mucilage helps hold on to water, slowing evaporation.  It also functions to form a callus or scab once it is exposed to the air.  This callus functions similar to how scabs function for us.  When we bleed a scab forms to prevent further loss of blood and to prevent infections from entering the body.  When a cactus is damaged it also forms a scab to prevent bleeding of water as well as to prevent infections from entering the cactus.  The scab also hinders predation of the cactus by being a barrier between the cactus flesh and potential predators.  Often the callus can become several millimeters thick.  If you are around cacti at anytime, look for damaged sections of the plant.  The callus will be a tan to black coloration on the damaged portion.
Saguaro boots like this one will often survive long after the cactus decays away.
All cacti form calluses, which may be helpful to the cactus but an annoyance to any animal that may want to feed on cactus flesh.  A lot of birds though find cactus calluses quite useful.  In large columnar cacti such as saguaros, woodpeckers will remove the spines and peck a hole.  Gila woodpeckers and flickers are the most common birds that do this in the Sonoran Desert.  These woodpeckers will excavate a hole large enough to nest in.  Of course, excavating a hole in a cactus will cause it to bleed but a callus will in short time line the hole.   Typically, this causes little harm to the plant itself in the long-run.  These callused holes are called “boots” and will remain for the entire lifespan of the cactus.  Once the boot is abandoned by the woodpecker, an assortment of other birds will also use it for nesting being it is warmer in the winter and cooler in the summer.  Birds using saguaro boots for nesting include elf owls, kestrels, ash-throated flycatchers, and purple martins.  In-fact several species of birds can be found nesting in the same large cactus if it has many different boots.  This is why saguaros have been called by some a “cactus hotel”.  Oddly, these boots are so durable that, years after a saguaro dies and most all remnants of the cactus have decayed, the boot still remains lying on the ground.
A callused hole in a Saguaro cactus.

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