I like cacti, and had a small collection at one time. They are adapted to dry climates, and have evolved a special adaptation of the photosynthesis reaction. Normally, plants undertaking photosynthesis open their pores for the gas exchange that is a part of the reaction. For cacti, open pores in the daytime would result in too much water loss, so they do the reaction in two stages; the second stage that requires open pores they do at night.
There is a cactus nursery not far from Carcasonne, at Montolieu, and we went to have a look. We were disappointed: the place looked to us like it had been neglected, and for some time. The website linked above promises an attractive display along a winding path in a greenhouse, but it was not to be found. There were the frameworks for greenhouses, but without polythene (or glass), there were many dead plants in pots, and the cactus garden outside looked unkempt. There were quite a few excellent specimen plants in pots, but I came away feeling a bit sad. The owner told us that the plants had suffered two weeks of -16 degrees a couple of Winters ago, which would have been devastating, and I got the impression that he might have lost heart.