TRANSLOCATION OF SUBSTANCES IN THE PLANT 331 



tude and osmotic in nature. This pressure in its turn induces 

 the movement of water by the cell from the vessels of the plant. 



It is not possible to raise mercury very high in a glass tube, 

 for the surface of the glass is always covered with a layer of 

 adsorbed air. To approach the conditions found in a plant, 

 Ursprung used, instead of a glass tube, the stem of a liana. He 

 obtained a rise of mercury twice the barometric height, since 

 the walls of the vessels in a plant stem are not merely moistened 

 but saturated with water. 



These experiments still do not give a true concept of the real 

 magnitude of cohesive force that holds the water molecules 



I 2 



Fig. 107. Fig. 108. 



Fig. 107. — Sporangium of a fern; (1) with curved, (2) with straightened annulus. 

 Fig. 108. — Cells of the annulus of a fern sporangium; above, saturated with 

 water; below, shrunken as a result of desiccation (after Walter). 



together and prevents the rupture of the water column in the 

 vessels of a plant. The cohesive force of water cannot be 

 expressed in a few atmospheres. The simultaneous experiments 

 of Renner and of Ursprung (1915) have shown that it may be 

 as high as 300 to 350 atmospheres. These experiments were 

 conducted on the annuli of the sporangia of ferns (Fig. 107). 

 These annuli are made up of dead cells, whose inner lateral walls 

 are thickened, while the outer walls are thin. In cross section, 

 such a cell looks like a horseshoe, the ends of which are connected 

 by a thin wire. When the sporangium matures, these cells, 

 originally filled with water, dry up. The volume of water in 

 them decreases, and the cells gradually shrink, drawing the thin 

 walls inward, so that the ends of the thick walls approach each 

 other (Fig. 108). Thus a highly strained spring is obtained, 

 tending to rupture the water within the cells, or at least to tear 

 it from the walls. The water column finally does rupture; the 

 strained springs straighten at once; the whole annulus rapidly 



