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This has been going on for a while (about a year), one of my cacti has turned purple. I've just replanted it after its pot fell and broke (no, it did not spend the year on the ground). What is the cause of this and what should I do about it?

I put the cacti out on the open balcony in the summer and put them on the windowsill for the rest of the year, so the usual temperature they are exposed to is 20-35C (as low as 10C during the night). I water them twice a month during the summer and once a month otherwise with tap water, about 100 ml per plant. The height of the cactus is about 6.5 cm. The cactus in question spends most of its time very close to my other cacti (~10 cm), and some of them don't look too hot, pictures below.

Cactus in question about to be replanted, click to open full image

Section of the offspring, the flesh is green with a white center and has no smell, click to open full image

Neighbouring plants showing possibly related symptoms (The other side of the one on the left is perfectly green), click to open full image

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can it be sunburn, red pigment is often a response to too much light, too quickly... another option is rot or disease... are the roots firm? –  Grady Player Sep 25 '12 at 18:12
    
@GradyPlayer I've taken one of the offspring, it's roots are soft and tear relatively easily, though they are thin little things and I don't know how much strength is normal for them. I've added a picture of a section, the flesh is a (healthy?) green with a white center and has no smell; also 2 pictures of neighbouring cacti. –  kotekzot Sep 25 '12 at 19:05
    
yeah doesn't look good to me... the green insides of that cut one look good though... sometimes the roots will clog the drains and you will get standing water in the bottom.. otherwise it could have gotten too hot in the pot and injured the roots.. maybe unpot them and remove any damaged bits and then leave them out for a couple of weeks to scab over and then try to root them... I guess you should investigate to see how bad the damage is... any roots that are spongy or crumbly are dead. –  Grady Player Sep 25 '12 at 19:08
    
also is that one on the bottom right a stapelia, or a euphorb? it doesn't seem like a cactus to me. –  Grady Player Sep 25 '12 at 19:11
    
@GradyPlayer I don't really know what it is, got the lot from relatives as a kid. I've tried the roots of the bigger ones (the first one was tiny, hardly 1.5 cm long) and they seem to be fairly supple and elastic. I've added some sharper pictures of the whole unpotted cluster. Do pictures of the other plants make rot/disease seem less or more likely? –  kotekzot Sep 25 '12 at 19:38
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Heaps of sun can turn parts or whole cacti a purple or burgundy hue. When the heat is on and you have cacti in outside pots, the roots and plant can also start getting damaged from overheated roots and frequent watering. Keep them in pots that don't overheat (avoid black plastic) and the media should be coarse but still retain some moisture without soggy bottom.

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Are you sure it's sun damage? Any thoughts about the 2 other plants I've posted pictures of? –  kotekzot Oct 2 '12 at 14:55
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