Desert plants fall into two sorts according to the way they deal with the problem of surviving drought. There are the drought--avoiders--those which persist  only as seeds, ready to spring up when it rains, to flower quickly and produce another crop of seeds, and to die again. There are also the drought--resisters--those which have evolved various methods for strong water, locating underground water, or reducing their need for water by such devices as shedding their leaves. The drought--resisters are perennials ,they manage to live from one rainy season to another, slowly growing bigger and bigger. Of these, the succulents  is a small but interesting fraction . They may store water in their leaves, in their stems, or in underground containers.    In the American deserts the best--known succulents are the cacti . They come in a wide range of sizes, from 50--foot tall giant saguaros  to tiny round cacti about the size of a thumb--nail. They take thick, cylindrical  or even spherical forms, thereby exposing a minimum of evaporating surface to the air and light. They are leafless, except in youth, and then the leaves are small. Typically their surfaces are spiny, discouraging thirsty animals, and channeled like an accordion , so the fleshy stem may expand quickly when the plant drinks and contract slowly as it uses up the water. The root system is widespread and shallow, with good reason; only about 3 per cent of the rain that falls on the desert penetrates to any significant depth into soil. According to the passage, some desert plants drop their leaves   _   .
because the temperature is too high so that the plants can evaporate less water because they are drought resisters so that the plants can store more water