THESE ARE sold as house plants, and, like every sad twig in a pot, they make ideal gifts. So, everybody has one tucked away somewhere - usually in the darkest corner of the spare room. This is absolutely the right place for the Christmas cactus (aka Schlumbergera) because it is one of the ugliest plants to stalk the planet, with its thick, lumpy leaves and dragon- jawed fleshy flowers. It's a fussy monstrosity, shedding flowers and buds alarmingly in a hail of brightly coloured floral dandruff if it is so much as breathed on. Also it is contrary - it rarely flowers at Christmas, as its name suggests, (file with Christmas rose), but rather, about now. If confined to the spare room, then fair enough, it may wither away and die. But, these days, in warmer areas, it is likely to be seen in borders outside, lovingly transplanted into a sunny spot, where it will terrorise everything in its vicinity. Even Alan Titchmarsh has got one in his greenhouse. If global warming is going to mean that this trend is taken up far and wide, throw away those aerosols and junk your fridge at once. Some things just do not bear thinking about.