The Water Relations of Leaves 49 



upon which the world depends for its supply of food, cannot 

 be carried on. 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 m 

 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 photosynthesis ; for water keeps the mesophyll 

 cells wet and thus makes it possible for the carbon dioxid to 

 enter the cells. Water is necessary for the absorption of min- 

 erals and gases and for the transfer of materials within the plant. 



Vacuoles the reservoirs of the cells. The vacuoles inside 

 the cytoplasm are minute reservoirs within the cells. They 

 contain the cell sap, which consists of water holding in solu- 

 tion sugars, mineral salts, and acids. The relation of the 

 vacuoles to the protoplasm is most important, for the pro- 

 toplasm can secrete excess substances into the vacuole or 

 remove substances from it as they are needed. In many 

 industrial establishments individual machines are provided 

 with small boxes or trays, some to hold raw materials and 

 others to receive the manufactured product. The vacuoles 

 have the same function as these storage boxes : they hold a 

 supply of the raw material for the use of the protoplasm, and 

 they receive some of the products that result from the ac- 

 tivities within the cell. 



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

 water evaporates; that is, it changes from a liquid 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 giving up water vapor to the 

 intercellular spaces, from which, if the stomata are open, 



