262 PLANT PHYSIOLOGY 



Similarly in leaves, as the water evaporates from cell Z, the salts 

 in the cell sap become more concentrated so that Z has a higher 

 osmotic pressure than Y and water will then pass from Y into Z. 

 This increases the osmotic pressure in Y as compared with X, and 

 water will pass from X into Y establishing here another osmotic 

 gradient. Ursprung found that in the ivy leaf the cells nearest 

 the fibrovascular bundles actually had a suction force only about 

 \i that of the cells farthest away, with a difference of about 

 20 atmospheres between them. This part of the water movement 

 where living cells are present is, therefore, not difficult to explain. 

 The difficult part has been to find the cause of the movement 

 through the xylem, where the water is passing through the dead 

 cells of the wood. 



Among the various factors which have been called in by divers 

 workers to explain the rise of sap through the wood have been 

 capillarity, atmospheric pressure, the osmotic action of living cells 

 in the root and along the stem, and the pull of transpiration as- 

 sisted by the cohesive forces within the water. Each of these 

 will be discussed in turn. 



Capillarity.— It is known that if a fine glass tube is placed in 

 water, the water will rise in the tube to a height which varies with 

 the diameter of the tube. Oil rises in a lamp-wick because of these 

 same surface tension forces. In the soil, the upward movement 

 of water is due to capillarity, and since there are fine tubes in the 

 wood, it was only natural to believe that capillarity might here 

 play an important role. The principal objections to this theory are 

 that surface tension forces must have a free surface or meniscus, 

 and in the wood such surfaces do not exist. To be sure, there are 

 free surfaces next the inclosed air bubbles, but for every meniscus 

 concave downwards there is one concave upwards in just the 

 opposite direction to balance it. There is also a free surface in the 

 leaf at the evaporating surface, but this is on the living cells and 

 beyond the region now in question. Another objection is the 

 presence of the numerous cross walls (especially in Gymnosperms 

 which lack vessels), through which the capillary force would have 

 to lift the stream. It is possible that these cross walls would offer 

 no unsurmountable objection (say the friends of capillarity) since 

 they would also imbibe water and thus act along with the capillary 

 tubes of the xylem. If this is the case, they might even be more 

 efficient than the vessels because their pores would be still finer! 



