The transpiration stream means: Plants are loosing water all the time by evapouration. When conditions are hot and dry the rate or transpiration increases. Therefore many plants that live in areas where there is a shortage of water have special adaptations to reduce the water loss. Plants with these special features are called XEROPHYTES.
For example: Cacti are plants which live in the desert where conditions are very hot and dry. They have special adaptations such as...
1) Their stomata are sunken at the bottom of pits allowing water vapour to build up
2) Ridges trap water vapour
3) Some cacti have long trap roots to collect water which is deep in the ground others are have shallow but extensive root systems
4) Thorns protect the plant from being eaten by herbivores and there is less surface area to loose water from.
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