202 MOVEMENTS AND EXCHANGES OF FLUIDS 



from the walls of cells in contact with the air is compensated by 

 a current from the plasma, or from the walls of neighboring cells. 

 By this last method of replenishing the loss, water, or solutions, 

 may traverse the entire body of a plant without entering a proto- 

 plast or passing a single plasmatic membrane ; a small quantity 

 only may be conducted in this way because of the great friction. 



The outer membranes of the portions of plants exposed to the 

 atmosphere are generally so cutinized, or impregnated with sub- 

 stances impervious to water, that they contain a small proportion 

 of the fluid and hence evaporate but little into the air. Such 

 loss of water through external membranes may be designated as 

 cuticular transpiration. In leaves and other green parts of the 

 plant the stomata connect directly with spaces among the cells 

 containing air, and this air is in direct contact with thin-walled 

 cells usually turgid, and with walls completely saturated with 

 water. Evaporation goes on very rapidly and if the stomata are 

 open the moisture-laden air in the intercellular spaces diffuses 

 outward through the stomata and is replaced by air containing 

 less moisture. The closure of the stomata allows the air in the 

 intercellular spaces to become completely saturated, and hence 

 transpiration ceases or diminishes, according to the completeness 

 with which the pore is closed. This transmission of vapor 

 through the stomata may be designated as diastomatic trans- 

 piration. 



The loss of water from the cells exposed to the air either di- 

 rectly, or in the intercellular spaces, is replaced from the cells im- 

 mediately below, or contiguous, in such manner that the loss is 

 ultimately compensated by an equal amount taken in by the ab- 

 sorbing organs. This results in a more or less constant stream 

 of water from the absorbing to the transpiring surfaces, which 

 traverses the entire length of the body. The water absorbed 

 contains substances from the substratum, generally mineral salts, 

 necessary for the nutrition of the plant, which in this manner are 

 carried through the body and brought within the osmotic influ- 

 ence of all of the living cells. A second important function of 



