I live here in the mid-size city of Tucson, AZ, just outside the saguaro cactus national monument. I have admired this amazing species of cactus my whole life, but have had one question bug me. Why do these things sprout arms? In the lifespan of a saguaro cactus, it wont to start sprouting arms untill it is advanced in it’s later years. Some explinations that I have heard are, that it does it for balance. Or it does it out of some genetic mutation that happens later in it’s life. Ive been told that no one really knows why it grows arms. Maybe some of you out there can shed the light on this mystery.
It always seemed obvious to me that it grows arms for the same reason that all other ‘trees’ grow branches, it allows the plant to capture more sunlight. Why is any other explanation required?
BTW, I just realised that this seems to be in the worng forum. The question doesn’t seem obviously related to any of Cecil’s columns.
Welcome to the Straight Dope Message Boards, warfjm, we’re glad to have you with us. You might want to take a look at the various forums and their descriptions – you’ll get better response if you post in the appropriate forum. That’s where people look for stuff that they’re interested in, so your post will attract more readers/posters if they’re filed in the right room (so to speak.)
I’m moving this to General Questions.
They’re just side shoots like you’d find in nearly every other vascular plant - they look different from the side shoots on other plants because… well… the *whole plant looks different.
They start off as little buds, like this.
Generally speaking, the dormant structures that turn into side shoots can be encouraged to do so when damage occurs to the tissue directly above them - this interferes with the flow of plant growth hormones (either stopping growth-promoting hormones from travelling up - making them accumulate below the wound, or stopping growth-suppressing hormones from travelling down, or both) and stimulates the bud to start growing.
Your answer prompts me to ask another question. Pardon my ignorance, but I am just so dang currious. What are the side shoots on other plants? Maybe I should go into bontany or something like that, if I am that currious.
I think that means “branches”. But IANABotonist so maybe someone else who’s more qualified can verify that.
It is interesting how much is not known about the saguaro. I’ve talked to the experts at the Botanical Garden and at the Extension Service and have been told that serious study has only started in recent years. Because it grows so slowly, it will take many more years before much meaningful information is developed, if at all.
I hike through the desert all the time and never tire of looking at these guys. It is astonishing how they differ. Many will develop arms close to the bottom or midway up, while others will arm near the top. You can see middle-sized cacti that have a large number of arms, then you will see a bunch of them 20-30 feet tall that have never armed.
I guess each has its own genetic code and does what it wants to when it wants. 
One poster noted that injured saguaro develop arms quickly, and that is obvious, as have seen hundreds that have been decapitated (probably by lightening) by injury, and they very soon sprout new arms right at the tip whehere it has been cut off.
If you are lucky, you will one day see a “crested” saguaro, which has a very odd top. Have only seen two or three of these in the wild.
Right now is the time to really enjoy them as they are in full bloom and that is a sight to behold. Anybody in AZ should get out and take a hike to enjoy this.
BTY, for those who don’t realize it, the saguaro grows only in the Sonoran Desert which is in Arizona and the State of Sonora in Mexico. This amazing desert is one of the most lush in the world, and those who only know about the Sahara or Mohave or similar deserts will be astonished at the amount of flora and fauna in this desert.
vascular plants typically have specific reguions of growth - nearly always at the tip (terminal) of existing growth - this allows the plant to grow into areas where new resources (such as light and food) are available.
Having more than one potential growth point is a survival advantage (if a caterpillar chomps the terminal growth structure, the plant can keep on growing simply by starting up another growth point somewhere else) - keeping most of the growth points dormant is also a survival advantage (they aren’t using up resources and diluting the plant’s growth efforts that way).
There is a gun joke in there somewhere, but I’ll be danged if I can find it.
Flowers are only borne on the ends of the growing tips (the main shoot and the arms)…so if a plant has more growing tips, it produces more seed and has a better chance of reproducing than a plant that only has a central stem.
So plants with multiple arms should be selected for over the eons. The trait for producing arms is genetic, but interactions with different external environments will cause the characteristic to be expressed somewhat differently in plants woth the same genetic makeup.
BTW, the lateral buds that produce the arms are suppressed by a hormone produced in the growing tip, so arms are not produced until the tip is a certain distance away (so that the hormone cannot exert its influence) or the tip is removed.
From my study, see Mangetout’s first reply. Most likely damage.
The Mojave can be pretty lush and is quite diverse plant wise when the rains come through. Death Valley had been putting on a pretty amazing show with the abundance of rains this past winter. It’s not as desolate as you seem to be saying.
That said, many plants are preprogrammed to flower at a certain age, or branch at a certain age. Dracaena draco will not begin to branch until it begins to flower, and this doesn’t occur untl 10 - 15 years. Carnegiea (Saguaro) are most likely genetically programmed to not begin branching from its buds until it reaches about 10 feet (the buds at that height may only then become able to branch).
A most charming thread, and here’s my contribution — pictures of crested saguaro cacti, ummm, saguaros?, saguari? Anyway, have a look:
http://images.google.com/images?q=crested+saguaro&hl=en&btnG=Search+Images