The lowdown on arms

Saguaro at Sunrise
Photo by Marty Horowitz 12/29/2013

Several readers asked recently about Saguaro arms. (I try to get to all questions eventually; sooner, if they're about plants : -) So I did some reading. The bottom line is that there's tremendous variation when and where arms grow.
The "when" is related to size and age. Saguaros may grow arms after about 50 years of age, after the stem reaches maximum girth (arms grow above the maximum girth). Location is a key factor. If in a place where saguaros get more (but not too much) water, they may grow more arms. But they may not. And they may never.
Symmetry is generally important for living things. But the arms of a saguaro are clearly not, as was once thought, for balance. Again, location of the saguaro may play a role in the location of the arms. Three of the arms on the saguaro above are growing from the same side (the warmer side); there are none directly opposite.
I think it's helpful to remember that DNA tells the organism: "do X, when conditions Y and Z (and PDQ) are met." We don't always know exactly what the conditions Y, Z, and PDQ are. But we can trust that the DNA does.