ROOTS: PROCESSES AND SOIL RELATIONS 



323 



[Chap. XXX 



two into the wood and a glass tube filled with water is quickly attached 

 by a tight-fitting rubber stopper, one may readily determine whether 

 the water in the trunk of the tree is under pressure or under tension. If 

 the lower end of the tube dips into a vessel of water and there is internal 

 pressure, water will flow into the vessel; if there is tension, water will 

 flow into the tree. The best time to set up such an experiment is when 

 it is raining, or in the early morning before sunrise. Gauges that register 

 both positive and negative pressures have been used in similar experi- 

 ments. 



Most of the experiments have shown that in late spring and early 

 summer, when transpiration is highest, there is tension, not pressure, and 

 that water may be drawn from the vessel into the tree. This would be 

 difficult to explain on the basis of root pressure. 



Another type of experiment to show root pressure is mentioned in a 

 preceding section. If a plant is cut off when transpiration is rapid, and a 



Fig. 135. Curves of hourly rates of absorption of water, and the simultaneous 

 rates of transpiration. Internal water tension, or pressure, results. Data from P. J. 

 Kramer. 



water-filled tube is immediately attached to the stump, the water level 

 in the tube falls. Under the most favorable conditions positive pressures 

 ( shown by a rise in the water level ) do not occur until after a lapse of 

 one-half to two hours' time. This experiment seems to indicate that in 

 spite of the osmotic absorption by the roots, the root cells were not fully 

 turgid. Some of the water evidently had been drawn out of the roots 

 by transpiration from the mesophyll cells above ( Fig. 135 ) . 



Additional evidence that the water content of tree trunks decreases 

 during periods of active transpiration is shown by the fact that tree 

 trunks have daily variations in diameter as indicated by exact measure- 

 ments. They are smallest in the afternoons of clear hot days, and largest 

 in the early morning hours ( Fig. 136 ) . 



The flow of maple sap. The flow of maple sap from trees in early spring 



