Cacti have a thick stem so that when water/nutrients are available they can be absorbed in mass amounts for storage for use for a long period of time
Plants in deserts have thick barks to store water.
Plants in deserts have thick waxy coating to reduce loss og water from the plant.
It keeps that plant from losing water to the air. This allows cacti to grow in climates that are too dry for most plants.
Thick leaves are found on succulent plants and they store water during times when water is plentiful. Waxy coverings of the leaves help prevent water loss during hot, dry seasons.
Cacti have a thick and succulent stem because the rainwater is stored in the stem. A thick coating also holds the water inside the cactus and prevents it from evaporating.
To help store water.
so they can hold more moisture
The vast majority of desert plants do have leaves and few have only spines.
Slows down evaporation of water from the pores of the leaves.
desert, xerophytic plants
The waxy coat helps to reduce water loss from the leaves.
Due to conditions of desert
Succulent plants have thick, fleshy stems and/or leaves. In the Americas that includes the cacti.
Yes, there are some plants in the desert with long leaves. A few examples:Palm treesYuccasSotol
The vast majority of desert plants do have leaves and few have only spines.
Camels do eat tumbleweeds. Aside from this, they also feed on grass, leaves, and plants. Their thick lips help them in eating hard and thorny plants in the desert.
Desert Spoons are found in hot deserts. They have leaves that store water for the plant and are tough and thick to reduce the loss of water. These plants are often eaten by white-cotton tailed rabbits.
Desert
Most desert plants evolved according to their environment, an environment in which conservation of moisture is paramount. The broad surface of a leaf would transpire too much moisture away from the plant in the hot, dry desert, so "leaves" are an almost unseen feature of most desert plants.
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Desert ecosystem have plants with wide spreading roots and spiny leaves.
Scale like leaves
Many desert plants have flat leaves so your question is invalid.
They are succulent plants, having thick fleshy leaves or stems to store water