372 



LIFE ON THE EARTH 



stem by a rubber tube to a glass tube a meter long, of about the 

 same diameter as the stem. See that the rubber tube clings 

 strongly to both glass tube and stem. It may be best to tie it 

 tightly to these. Support the glass tube in a vertical position 

 above the stem and pour into it sufficient water to rise above the 

 rubber tube. (Figure 111.) Note the position of the 

 water column. Thoroughly water the soil about the 

 plant. Watch the height of the water column, marking 

 it every few hours. 



The water taken in by the roots passes on 

 from cell to cell by osmotic action and rises in 

 the stem in the same way that the water rose 

 in the tube attached to the stem of the growing 

 FIGURE in plant m Experiment 113. The root pressure, 

 together with capillarity, as seen in Experiment 97, will 

 account for the rise of the sap in lowly plants, but the cause 

 of the rise of the sap to 

 the top of lofty trees is 

 difficult to understand. 



Roots extend them- 

 selves through the soil 

 by growing at the tips. 

 Here the cells are rapidly 

 dividing, forming new 

 cells, and building root 

 tissue. As water is so 

 essential, they are always 

 seeking it and extending themselves in the direction where 

 it is to be found. This causes them to extend broadly and 

 to sink deeply (Figure 112). A single oat plant has been 

 found to have an entire root extension of over 150 feet. 

 This seeking of the roots for water sometimes causes the 

 roots of trees to grow into drain pipes and stop them up. 



FIGURE 112 



