176 



Science of Plant Life 



Absorption and rise of sap in plants. Attention was called 

 to the fact that transpiration exerts a pull on the water in 



the conducting tissue of the 

 leaves (page 134). This pull is 

 transmitted to the water-con- 

 ducting tissues of the stem and 

 root. So a fourth factor enters 

 into the absorption of water by 

 the roots : the pull on the water 

 in the cells of the root is in- 

 directly due to transpiration 

 from the leaves. Large trees 

 have been kept alive for days 

 by placing the cut-off trunks in 

 water. This shows that suffi- 

 cient water to maintain the 

 water balance of the plant for 

 at least several days may be 

 lifted in a plant by the pull of 

 transpiration without the aid of 

 roots. It is of practical interest 



FIG. 102. Expenment to illustrate the 



water balance in a plant. The entire to knOW that CUt flowers Will last 



apparatus is filled with water, and A 

 and C are immersed in water. The 





j f 



ends Q f 



. 



water is absorbed by osmosis into the stems are bent over into a vessel 



porous cup A, and evaporated from the an( J cut un( ier the Water. If CUt 

 cup B. The rate of evaporation is 



faster than the rate of absorption, as is in the air, air bubbles get into 

 shown by the fall of the mercury in the ^he Water-Conducting tubes and 

 outer end of the tube C. 



prevent the subsequent move- 



ment of water into them. Air bubbles already in stems that 

 have been cut in the air may sometimes be removed by cutting 

 off an inch or two of the lower ends of the stems under water, 



