334 PLANT PHYSIOLOGY 



moves, contain no air. Air that enters the vessels as a result 

 of mechanical injury such as wounding, for instance, is not able 

 to move farther than through the first septum. Such injuries 

 will cause the exclusion of only small portions of the general 

 system of water supply. The tracheid walls in many trees such 

 as conifers are provided with bordered pits, which serve as check 

 valves to prevent the entrance of air into the vessels. 



The presence of bordered pits and other such structures in the 

 tracheids and vessels, which necessarily offer a considerable 

 resistance to the translocation of water in the wood, is neverthe- 

 less an indispensable protection to the regular functioning of the 

 whole water-conducting system. The air, which under high 

 tension may enter and clog the vessels, can be dissolved again 

 when the tension is relieved; and the vessels that have tempo- 

 rarily suspended their functions may then once more begin to 

 function in the conduction of water. 



77. The Rate of Water Movement in Plants and the Resist- 

 ance of Wood to the Ascent of Water. Participation of Inter- 

 mediate Motive Forces in the Translocation of Water. — As 

 wood consists of exceedingly narrow tubes, which moreover are 

 provided with septa, it shows a strong resistance to the ascending 

 water. Naturally, this resistance becomes an additional encum- 

 brance to the cells of the leaf parenchyma, which carry the weight 

 of the suspended water threads. 



This additional resistance was formerly rated very high, con- 

 sidering the pressure necessary to force water through pieces of 

 wood. The comparatively low rate at which the water moved 

 in the plant, however, was not taken into consideration. Exact 

 determinations by Ursprung have shown that with increase in 

 height in the trunk, the suction tension of the parenchyma cells 

 adjoining the vessels increases. But this increase constitutes 

 only 0.3 to 0.4 atmosphere per meter in height. This is a small 

 value indeed when compared with the increase in tension of water 

 moving through the parenchyma. In this movement from cell 

 to cell, the suction tension, as has been stated in Art. 74, increased 

 by 0.1 atmosphere. If the average dimensions of these cells are 

 assumed as 0.1 mm. since they are really one-third to one-half 

 as large, it will be found that the translocation of water in the 

 parenchyma for a distance of 1 m. would require the immense 



