THE INTAKE AND UTILIZATION OF WATER 29 



The vessels may be considered as the path of the most rapid portion of the 

 transpiration stream. The tracheids transmit water more slowly but continue 

 to function when the water supply is limited. Though the internal thickenings 

 on the walls of the trachea? are essential for the transmission and stabiHty of a 

 stream under tension, the whole wall is not uniformly thickened because the per- 

 meability of the thinner portions is necessary. The flow of water is further 

 facilitated by the presence of bordered pits, which are themselves remarkably 

 adapted to permit a flow of water under proper conditions and to prevent the 

 expansion of bubbles beyond the Hmits of a single cell. The membrane and torus 

 of each bordered pit is able to take up three positions, a median position when 

 it is not acted upon by lateral forces and two lateral positions when the membrane 

 is deflected against one dome or the other. When adjacent cells are filled with 

 water, the membrane assumes the median position and permits a flow of water 

 through the delicate membrane which surrounds the torus. When a bubble 

 develops in the trachea and gradually distends until it fills it, the membranes of 

 the pits in the walls of the trachea become deflected away from the bubble, until 

 the torus hes over the perforation in the dome like a valve in its seat. In this 

 position the tension of the water on the one side and the pressure of the gas on 

 the other are withstood. 



The water after it reaches the tracheae of the leaf is drawn into the leaf cells 

 by osmosis. Thus the entire transpiration stream is raised by the activity of the 

 leaf cells. According to Dixon''^ these cells actually secrete the water and it is 

 removed by evaporation from their outer surfaces. The resistance encountered 

 by the current of water in passing through the wood at the velocity of the trans- 

 piration stream is probably equivalent to a head of water equal in length to the 

 wood traversed. Hence, the tension needed to raise the transpiration stream 

 in a tree must equal the pressure of a head of water twice as high as the tree. 

 In a tree 100 meters high, for example, a tension of 20 atmospheres is needed. 

 Dixon finds the cohesion of sap to be at least 200 atmospheres, so that it is in no 

 way taxed by the tension. He also finds that the osmotic concentration of the 

 mesophyll cells is adequate to resist the transpiration tension and is in many 

 cases much in excess of that required. Finally, he shows that the energy set 

 free by respiration in the leaves is sufficient to do the work of secretion against 

 the resistance of the transpiration stream. 



Summary. — The total system of water absorption, conduction and 

 transpiration is more or less a unit. As a rule water is absorbed only to 

 replace water lost by transpiration and in the long run the amounts of 

 each are equal. For short periods, however, transpiration may slightly 

 exceed absorption. The chief factors determining the flow of water 

 through the plant are the available water supply and transpiration. If 

 for any reason more water is transpired than is absorbed and this condi- 

 tion continues for any length of time, the cells lose their turgidity and the 

 plant wilts. If the supply of water is renewed in time, the plant recovers 

 its turgidity without ill effects, but prolonged wilting is attended by 

 serious disturbances and eventually results in death. 



Nursery stock loses its water-absorbing organs, the root hairs, upon 



