Leaves are adapted to absorbing light, carbon dioxide and water so that photosynthesis can take place.
Carbon dioxide is taken into the leaf from the air through small pores in the leaf called the stomata. These pores also allow oxygen produced by photosyntheis to move out of the leaf.
Water is absorbed through the roots and moves through the xylem vessels. Some of the water is used in photosynthesis and some is lost by the leaves by transpiration. The water will evapourate from the spongy mesophyll cells in the leaf into the space below the stomata. As the humidity builds up in the space it will move by diffusion throught the stomata out of the leaf down a concentratioon gradient of high water concentration to an area of low water concentration gradient outside the leaf.
Light is absorbed by the chlorophyll molecules in the leaf and is used to convert water and carbon dioxide into glucose. The rate at which the plant looses water varies in different conditions.
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.