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TEXTBOOK OF PLANT PHYSIOLOGY 



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

 as 300 to 350 atmospheres. These experiments were conducted 

 on the so-called annuli of the sporangia of ferns (Fig. 85). 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. 86). 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 



I 2 



Fig. 85. — Sporangium of a fern; (1) with curved, (2) with straightened annulus. 

 Fig. 86. — Cells of the annulus of a fern sporangium; above, saturated with water; 

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



column finally does rupture, producing in the cell a Torricellian 

 vacuum. The strained springs straighten at once, the whole 

 annulus rapidly contracts, scattering spores as from a catapult. 

 The whole process may be readily observed under a microscope. 

 The force necessary for such a compression of the cell walls in the 

 annulus was determined by immersing them in highly concen- 

 trated solutions of various substances. It proved to be of the 

 magnitude of 350 atmospheres. 



This immense force of cohesion of the molecules is more than 

 sufficient to raise water to the top of the highest tree, as the 

 pressure of a water column 100 m. high is equal to but 10 atmos- 

 pheres. Hence the ascent of water in a tree may be represented in 

 the following way according to Dixon : Near the top is the evap- 

 orating leaf parenchyma in whose cells develops a suction tension 

 of many atmospheres. These cells draw water from the vessels of 



