Most of mine rot off due to me over watering it. The top cactus can't live on it's own because it has no Chlorophyll to make food with. That's why they have those bright colors, just like fall leaves on the trees. There is a new one out there that has a rainbow of colors in them including green and black and does live on it's own. Expensive right now. They will come down. Found two Black/yellow ones at K-Mart of all places for only $3 each. And I bought only one. How stupid.
