146 



BOTANY FOR BEGINNERS 



CHAP. 



fixing on the cut end a manometer (Fig. 160). The pressure is 

 often sufficient to force the mercury up the tube to a height of 



several inches. In the Nettle 

 the root-pressure observed has 

 been found to be sufficient to sup- 

 port a column of mercury about 

 1 5 inches high. 



How Root-Pressure is Set 

 Up. In spring the root-hairs are 

 very active, taking in large quanti- 

 ties of water from the soil, which 

 passes by osmosis into the cells of 

 the cortex, and when these become 

 filled with water it is forced into 

 the vessels of the xylem. The force 

 with which the water is pumped 

 from the parenchyma cells of the 

 cortex into the vessels of the xylem 

 is produced by the activity of the 

 root-hairs in absorbing more water 

 than can be stored up in the cells 

 of the root. 



Thus the phenomenon of root- 

 pressure depends upon the tempera- 

 ture of the soil (p. 140), for it is 

 only when absorption is active that 

 it can take place. 



FIG. 160. Apparatus for measuring 

 root-pressure. The glass tube 

 g is joined to the cut stem j by 

 means of the rubber tubing c. 

 The mercury Q is forced up the 

 tube by the water IV, which is 

 given out by the cut stem. (S.) 



Ex FT. 138. Cut off the stem of a 

 Dahlia or Sunflower just above the soil, 

 and fix to the cut end a hollow glass 

 tube which contains a little coloured 

 water. The fixing can be done by 

 sliding the glass tube over the end of 



the stump and using rubber bands to hold it in place, and to pack 

 the base of the tube. Note 



The water is pushed higher and higher up the tube against the pres- 

 sure of the atmosphere. The weight of the water lifted will give the 

 amount of the root pressure. 



EXPT. 139. Jn spring, cut off a branch of the Barberry, 

 (i) A whitish fluid, the sap, oozes out of the cut end. 

 (iij The sap is forced out by the root-pressure. 



Note 



