Cleistocactus strausii
As a houseplant, the Silver Torch cactus is an easy to take care of so long as over-watering is avoided. To naturally simulate the porous effects of the sand in its native habitat, it's important to ensure that excess water will not pool in the soil, causing the roots to become water logged.
This plant’s native habitat does not have winter, but when temperatures drop to the low twenties, it can exhibit signs of tip frost. 
The Silver Torch cactus is most susceptible to 
mealybugs 
and spider mite.
Pereskiopsis
The shape of "Pereskiopsis" is not that of a typical cactus, it has large green leaves and grows like a bush. Only the presence of areoles, usually with spines and glochids, and the flowers, allow it to be identified as a cactus. Flowers open during the day. "Pereskiopsis" is often used as a grafting stock for other cacti, as it promotes quick growth of the scion. Its small diameter makes it an excellent grafting stock for small seedlings, but many species will outgrow the stock and need to be re-grafted or rooted. "Pereskiopsis" are tropical cactus and can not withstand any frost. Maintain a temperature of approximately 12 °C during winter and be careful with water. The stem tends to rot very easily, especially with low temperatures when the plant is not actively growing.