Question

how are cactus adapted to survive in a desert

Answer 1

It can absorb and retain lots of water in a short time for later use 
It performs photosyntesis in a CAM system which saves lots of water 
It keeps the stomata closed during the day and absorbs CO2 at night 
It's leaves are thorns and the stem performs photosyntesis 
It's bloom is aimed at desert insects

Answer 2

cactus are adaptive as -
1. their leaves are modified to spines to reduce transpiration (water loss)
2. they are hollow to store water.
3. the have hard outer layer to protect from sand and wind.

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