TRANSLOCATION OF SUBSTANCES IN THE PLANT 325 



from the lower one, in spite of the fact that osmotic pressure is 

 greater in the second cell. 



Thus in order to make the water current, induced by evapo- 

 ration in the upper cell, move through a series of cells, suction 

 tension must be distributed in these cells in regularly decreasing 

 order. In the upper cell, suction tension must be the greatest; 

 in the following cell, it must be less; in the next, still less; etc. 

 The least suction tension must be in the lowest cell, which absorbs 

 water from the surrounding medium. 



The difference in suction tension of two neighboring cells is 

 determined by the resistance met by the water current in passing 

 from one cell into the other. This resistance, in its turn, depends 

 on and increases with the rate of the water flow. Ursprung was 

 able to trace the increase of suction tension in the cells of the 

 palisade parenchyma of the leaf, in proportion as these cells 

 were farther from the large fibrovascular bundles supplying 

 them with water. In the third cell from the bundle, he found a 

 suction tension of 12.1 atmospheres; in the two-hundred-and- 

 tenth cell, 32.6 atmospheres. In this case, in order to traverse 

 207 cells, a total difference in suction tension of 20.5 atmospheres 

 was required, or about 0.1 atmosphere per cell. In other 

 instances, somewhat different figures were obtained, but still 

 they were close to this value. 



The strong resistance met by water in mo\dng from cell to 

 cell shows that long distances cannot be covered in this way; 

 hence, the necessity for more rapid means of translocation of 

 water through dead, empty, elongated cells with numerous pores, 

 the tracheids, or still better through hollow tubes with very thin 

 septa, the tracheae, or vessels. Lower plants, such as the lichens 

 and mosses, have no true vascular systems. Therefore, they are 

 limited in size. The development of tracheids in ferns was an 

 important step forward in the evolution of plants. 



75. Translocation of Water in the Vessels. Forces Causing 

 the Movement of Water. The Pulling Action of Transpiration.— 

 The vessels and tracheids, being mere tubes filled with water, 

 possess of themselves no source of energy that can cause the 

 movement of water. On account of the very small dimensions 

 of these elements, capillarity might be considered to be the only 

 force operative in them. Confronted by the problem of the 

 upward movement of water through the vessels, early investi- 



