TRANSLOCATION OF SUBSTANCES IN THE PLANT 333 



compression of the vessels is counteracted by the annular or 

 spiral thickenings of the vessels, which keep the walls apart. 

 Therefore on the whole, the compression is rather small. Never- 

 theless, it may be perceived under the microscope by observing 

 the vessels of plants that have a sufficiently transparent stem — 

 for instance, young plants of pumpkin or balsam, during rapid 

 wilting, when the suction tension of the leaves attains its maxi- 

 mum, while the roots are unable to supply water from the dry 

 soil. 



The elastic compression of the walls of vessels during periods 

 of increased transpiration is the cause of a phenomenon that has 

 been known for a long time but has usually been misinterpreted. 

 If on a hot day the stem of a rapidly transpiring plant is cut, the 

 air will quickly enter the vessels. When placed in water, such 

 a stem will soon wilt, as the air clogs the vessels, preventing water 

 from rising. If the stem is cut under mercury, the latter will 

 enter rapidly into the vessels and rise to a considerable height. 

 In order to keep plants fresh, cutting must be done under water. 



The reason for this phenomenon is that the vessels of the plant, 

 being compressed with the force of several atmospheres, widen 

 abruptly when the strained water threads are cut. Air, mercury, 

 or water, depending on the medium used, will rush into the empty 

 space. Yet for a long time this phenomenon was explained in a 

 different way. It was supposed that besides water, the vessels 

 contained highly rarefied air in the form of small bubbles. When 

 a cut was made, this air contracted and drew after it mercury or 

 water. The recent investigations by Renner and his coworkers 

 have shown, however, that vessels that conduct water normally 

 do not contain air but are filled with water throughout their 

 length. Air will enter only as a result of a mechanical injury. 

 If cutting is done carefully under raercury, the meniscus of 

 the latter will join the water, no air bubbles being present in the 

 vessels. The supposition of the presence of air bubbles in the 

 vessels, forming the so-called '^Jaminian chains," has prevented 

 for a long time the acceptance of the cohesion theory, as these 

 bubbles would interfere with the translocation of water and 

 would, moreover, rupture w^ater threads at such high tensions 

 as are observed in the trunks of trees. It is known now that air 

 is present only in the older wood, which has ceased to function 

 for water conduction. The younger parts, through which water 



