I got about a half dozen sections of a cactus from my housemate (he got them from someone else, doesn't know what they are). They remind me of some San Pedros I saw for sale earlier this year, but that's just a guess. What should I look for to identify them vs other similar looking cacti?

I have had a small mammillaria rhondantha for a few months. About a month ago I had to move it somewhere it had less sunlight, and within a week it developed this odd tilt. When I looked closer, the side that was facing brighter light had grown faster than the dark side. I tried turning it the other...

UPDATE: The cutting rooted!
It's growing a large mass of healthy looking roots

Also, the base has continued to grow roots, and the necrosis doesn't seem to be spreading. Which makes me wonder if cutting was necessary, but hey maybe I'll have two

I cut the stem earlier today and dusted it with rooting hormone. It is already starting to callous nicely (I live in a dry climate)

You can't see it in the pictures, but right above the dead part there are roots growing out of the main stem. I'm guessing I can't or it's not worth trying to save them? And in terms of maximizing my chance of success, is it better to try to root the whole thing or give up entirely on the main stem ...

I took some pictures with my computer's webcam, sorry for the very poor quality. It's a little hard to see the brown shriveled part I'm concerned about since it blends in with the dirt, I circled it in one picture

Any guesses as to what happened to that part? Too dry? Disease?

Sadly I still can't find my camera, but I'm 90% sure the base is dead. It's shriveled down to a quarter width of the healthier stem, and the stem above it getting pale, like it's cut off from water/nutrients I'm worried the dead part might rot? Should I be worried? Can I do anything to keep it from ...

I would not guess sunburn, because jade plant leaf tips turn red in the sun, and yours are still green

I have no idea if it's bugs. You could spritz the affected leaves with rubbing alcohol; this kills bugs and won't hurt the plant

I'm trying to save a sick cactus I got yesterday. It was unidentified, but I believe it is a Grusonia pulchella. The bottom half inch of the stem is brown and shriveled. Just above the dead part some roots are growing but have not yet reached the dirt. I'm not sure if the dead area is from rot (it's...

Actually, Brunoordino's Gymnocalycium does have chlorophyll. Although lighter colored (red, yellow, etc.) varieties lack chlorophyll, the dark ones do have it, enough even to be healthy without grafting http://hoorayplants.blogspot.com/2013/10/can-purple-moon-cactus-survive-on-its.html Is an example...