6io 



MOLECULAR FORCES IN THE PLANT. 



pressure at the cut surface is at zero. A second root-stock from a plant of exactly 

 the same age and vigour and grown in a pot of the same size is provided with the 

 apparatus figured in Fig. 440, where the tube / through which the outflow takes 

 place reaches the vessel h through the cork g. This vessel contains water above, 

 mercury below. A tube k rises from the cork i to a certain height and is bent round 

 at the free end where it dips into a graduated tube. If the apparatus is so contrived 

 that, for example, the opening for the outflow o stands about 1 5 cm. above the level «, 

 then the column of mercury o?i exercises a pressure of 15 cms. on the water h, and 

 through it on the cut surface at b. When the water begins to flow out from the cut 

 surface at b, the quantity of water in h will be increased, and an equal volume of mer- 

 cury will flow out at 0. The mercury collects in the burette, and its level enables 



Fig. 440. — Apparatus for measuring; the root-pressure when considerable and constant. The cork I has a lateral incision 

 in order to allow of the escape of the air when the mercury is dropped in. 



the quantity of water which has flowed from the cut surface to be read off" from hour 

 to hour, and to be compared in the other apparatus where there is no pressure. After 

 a longer period of observation, the level n falls sensibly and the pressure on augments a 

 little. But it is easy to bring it again to the original amount if a fresh quantity of mercury 

 is poured in every twelve hours. 



I observed in this manner in the summer of 1870 for five days two equally strong 

 root-stocks of the sunflower^; and the result was that the diff"erence of the outflow was 

 but small, although the amount of pressure in one case was zero, in the other case 

 17 cm. of mercury. In the first thirty-three hours the outflow where there was no 

 pressure at the cut surface amounted to 26-45 cubic cm.; when the pressure was 17 cm. 



^ I cannot here describe the whole series of minute observations. 



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