How has the stem of a cactus adapted to live in the desert?

Because leaves send out so much water in photosynthesis, The leaves of the cactus have evolved into little stickers. The Stems to further conserve energy have a waxy covering with few open areas to allow evaporation of the precious water. Further the stems have developed chloroplasts to perform the photosynthesis the leaves can no longer do to support the plant. In times of rain, the stems can take up and store huge amounts of water. If that isn't enough, the stems can be broken off and create new plants. The broken limbs or stems of the cacti harden at the broken part. It is called callousing. The callous sends out roots. The stored water in the stems sustain the cacti until it can get more water, all the while continuing to photosynthesize. The stems help shade other parts of the cacti helping it to remain cool in hot sun. This same shade from the stems protect the newly fallen limbs. In times of extreme cold the cacti stems can expand with the freezing liquids inside and not rupture. There are some more things but they are a little more time consuming to relate. Good luck.