The bad effects of heavy rainfall can be overcome largely by including in the soil a large proportion of sand or gravel or cobblestones and by growing the plants on ridges or raised borders. The effects of fog and extreme humidity can be corrected somewhat by growing them in dry conservatories. Where the temperatures fall below the lowest temperatures given in the section on “How to Grow” for each species, cacti must be grown in warm greenhouses in winter and preferably throughout the year—with the exception of the hedgehog cacti, which rarely grow successfully in greenhouses. With the specific information given under the heading “How to Grow” for each species, it is possible to grow cacti successfully in the tropics and over a large part of the temperate zone. The important things are: warm sunshine, protection from too low temperatures, the right kind of soil, and limited watering or irrigation. LOCATIONS FOR CULTIVATION Cacti may be grown out of doors in the entire southwestern section of the United States, in Mexico, Central America, and South America (except the southern part), where the temperatures are never colder than fifteen to twenty-five degrees below freezing. Also, they can be grown successfully out of doors in parts of Spain and Portugal, and in the region immediately bordering the Mediterranean Sea, over much of Africa lying at the lower altitudes, in Arabia, Persia, India, southern China, extreme southern Japan, and the northern half of Australia, in addition to the islands of the Pacific, nearly all of which lie between the 33° parallels, north and south (except where the temperatures are modified by mountains or other natural features). They can be grown indoors generally in the north and south temperate zones between the 34° and 54° parallels, north and south, where the temperatures reach as low as twenty to thirty degrees below zero. This includes the northern two-thirds of the United States, the lower half of the Dominion of Canada, Great Britain, Ireland, France, Germany, Austria, the northern half of the Chinese Republic, Japan, and the southern part of South America. The Cereus Group or Torch Flower Cacti (_Cereus_) How to identify and how they grow