The Water Relations of Leaves 73 



cells. We have previously shown that water enters into the com- 

 position of all carbohydrates ; therefore water is necessary for 

 photosynthesis. 



Substances can enter plants only when they are in solution. 

 Both the gases and the mineral compounds that are used by the 

 plant in its various processes must be in solution in water before 

 they can be absorbed or pass from one cell to another within the 

 plant. Indirectly as well as directly, water is necessary to photo- 

 synthesis ; for water keeps the mesophyll cells wet and thus makes 

 it possible for the carbon dioxide to enter the cells. Water is 

 necessary for the absorption of minerals and gases and for the trans- 

 fer of materials within the plant. 



Growth and reproduction result from a series of many physical 

 and chemical changes within the cells. These changes can take 

 place only in the presence of water. Water is necessary for all 

 physical and chemical changes within the plant and consequently 

 for all plant activities. 



Transpiration. If we expose a wet cloth to the air, the water 

 evaporates ; that is, it changes from a hquid to a vapor and 

 passes off into the atmosphere. The same thing happens when 

 a plant is exposed to the air. The mesophyll cells of the leaf are 

 continually losing water vapor to the intercellular spaces, from 

 which, if the stoma ta are open, this vapor passes out into the 

 atmosphere. The epidermis of the leaf also allows some water 

 to pass through it, but in land plants this is a relatively small 

 amount, because the cuticle hinders the process. The loss of 

 water vapor from plants is called transpiration. 



The loss of water in the form of vapor is a process that takes 

 place in animals as well as in plants. If you hold your hand near 

 a window-pane on a cool day, a halo of minute water drops con- 

 denses on the glass. These water particles come from the moist 

 cells of your skin. If you blow on a glass, water collects even 

 more abundantly. The vapor in the breath is water that has 

 evaporated from the moist cells of the lungs. 



