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Quixote

How does a cactus "work"?

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I have been thinking about this a lot, and haven't been able to find answers online:

How does a cactus work internally?

It produces energy by photosynthesis, but how is that energy distributed inside the plant? As far as I know, cacti have no "blood vessels" to transport fluid around its body, so if a cactus gets sun on one side, how does that help the other side? And how does the energy produced in the green part above ground get to work in the roots under the ground?

And without a signal pathway from the top to the bottom, how does the root know when to grow and when to be dormant?

Sorry if these questions seem stupid, I hope somebody can explain how it works :) Because obviously it works - somehow.

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How does a cactus work internally?

It produces energy by photosynthesis, but how is that energy distributed inside the plant?

it produces sugars from photosynthesis via the krebs cycle which in this case uses CAM respiration. Those sugars are distributed by the vascular bundle, likewise as is the case with most plants all cells are connected not only via vascular tissues like xylem and phloem but also by a network of tiny tubes connecting most every cell to every cell.

The biology of cacti is a lot like that of other plants, most notably in the same order: Caryophyllales.

As far as I know, cacti have no "blood vessels" to transport fluid around its body, so if a cactus gets sun on one side, how does that help the other side?

they do have vascular tissues that distribute sugars as well as the minerals, elements and moisture that are absorbed by the roots.

And how does the energy produced in the green part above ground get to work in the roots under the ground?

the plant sends sugars to the roots for growth, while the roots send minerals and ions to the shoot for photosynthesis and biosynthesis of relevant molecules like hormones and in some cases alkaloids.

And without a signal pathway from the top to the bottom, how does the root know when to grow and when to be dormant?

there is a signal pathway from the top, the active cell division area is known as the meristem and both this and the roots are sources of hormones that affect growth, likewise rootgrowth is triggered by specific conditions, such as moisture and darkness, or even gravity in some cases. all of this relates to hormones and genetic activation of signal pathways when specific triggers are met, to get a feel for this look into enzymatic signal pathways for secondary metabolites of plants.

Sorry if these questions seem stupid, I hope somebody can explain how it works :) Because obviously it works - somehow.

 

no problem, a bit of basic plant biology/botany would really help you in understanding this

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They do have a vascular ring and vascular tissue from the ring to the areoles so there is your pathway. Im sure you can find more!

Edit: Gunter did a little better and quicker than me

Edited by Bongchitis
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Thanks both for answering, it's very fascinating stuff indeed.

I will go to Wikipedia to read more about it, now that I have some keywords to search for.

Edited by Quixote

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good thread, I too had been wondering. After reading Gunter's post, I still am :P

But very cool, the mind boggles at questions like 'why' and 'how'. Life is excellent.

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I would love to see a cut-through diagram of a cactus with an explanation of how its system works, but haven't been able to find one online. Anyone has a link for something like that?

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What cactus? There are so many types.

Pick one and maybe someone will draw a diagram for you.

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maybe i can dumb this down a bit.

among smaller differences there is one pretty major difference in cactus which i'll get to.

about as good a diagram as any.

edit: not really. new picture. just to get an idea of the vascular ring.

dicot%20stem_labelled_web.png

dicots (eudicots, dicotyledonous plants) have stems with concentric growth rings, and usually when you look at a cactus, all you are looking at is stem. it helps to imagine that the living organism is shaped like the surface of what you see. the organism is a thin layer, consisting of a growth zone (vascular cambium) where new cells are produced, and two distinct layers of "tubes". the xylem conducts water and nutes which are gathered mainly by root hairs, up to the above-ground part of the plant, i *think* the upward movement is powered by transpiration eg it is literally sucked out of the ground and up to the top. the photosynthesising parts of the plant, usually green, produce chemical energy for the whole plant to use, this is transported in the phloem layer, you can think of this as "flow-em" because it flows down to the roots but the plant can actually use it's own energy to pump goodies around through the phloem. these vascular tissues, these little tubes, are replaced, so the process naturally moves outwards, the hollow organism you imagined earlier leaves this tissue behind and it is basically dead, if you think of the wood in a big tree. dead but not necessarily disused, the wood of a tree provides structure and i guess it's likely that cactus use it to store water.

maybe the most distinct thing about cacti is that stems can still be green despite being quite old, most perennial (long-lived) dicots grow bark instead of epidermis on old stems. so does cactus but only on very old stems. the reason has got to be because it needs the green epidermis to photosynthesise.

Edited by ThunderIdeal
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edit

Edited by tipz

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I would love to see a cut-through diagram of a cactus with an explanation of how its system works, but haven't been able to find one online. Anyone has a link for something like that?

I wish I could upload the illustration & explanation found on pg 26 of the Teratopia book.

But then again, what would be the purpose of even writing a book if the information was freely available on the web....

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I wish I could upload the illustration & explanation found on pg 26 of the Teratopia book.

But then again, what would be the purpose of even writing a book if the information was freely available on the web....

 

Well, it makes the web a much richer resource, especially when you can reference legitimate or scholarly material. As I'm sure we all realise, there is so much misinformation on the internet - it is its accessibility as a dynamic source that makes it so valuable. Although sometimes its like sifting through tonnes of sand to find a a small lump of gold. I will probably never buy or see that book, however I have good interest in learning certain material perhaps contained in it. The choice is yours, but like all good internet users know- 'if you liked the material - buy it'. Sometimes a freebie is what you need to make the choice. As a slightly related side note, another good thing about books is that they can last for hundreds or even thousands of years when stored correctly. I imagine it will be a real challenge to read a DVD in 100 years, little own 1000.

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