PATH OF SAP THROUGH STEMS 



211 



typical leaves. Active coniferous shoots have been found to exert 

 a force sufficient to raise a column of mercury 76 cm. (Fig. 107). 

 281. The Recovery of Wilted leaves by Aid of Increased Pressure 

 ia Stems. Cut a young shoot of Helianthus and allow it to be 

 supported in the air until the leaves have 

 wilted noticeably. Now fasten it to one 

 arm of a U-tube and fill the tube with 

 water. If recovery does not take place 

 pour several cc. of mercury in the free end 

 of the tube. This will drive the water up 

 into the stem with some force and will re- 

 sult in the recovery of the drooping leaves 



(Fig. 108). 



B 



107 108 



Fig. 107. Demonstration of lifting power of transpiration by a shoot attached to a 



tube filled with water and standing in a dish of mercury. 



Fig. 108. Shoot attached to short arm of U-tube, into which water and mercury 



are poured to illustrate influence of pressure upon absorption and consequent recovery 



from wilting. After Sachs. 



282. Path of Sap Through Stems. The function of conduction 

 of solutions, absorbed by the roots, in small annual plants, is car- 



