THE MOLECULAR FORCES IN PLANTS. 



205 



In detailed researches concerning the periodicity of root pres- 

 sure, it is advisable to employ an arrangement for registering the 

 outflow of sap on the principle indicated by Baranetzky (Abhandl. 

 d. Naturf. Gesellschaft zu Halle, 1873). The apparatus, with thirty- 

 six glass tubes for collecting the sap, is to be obtained from 

 Albrecht in Tubingen, at a price of 100 marks. 



1 For the literature, see Detmer, Lehrbuch der Pflanzenpliysiologie, 1883, p. 

 122, and Wieler's treatise cited in 77. 



Kk 



K 



79. The Causes of Root Pressure and Related Phenomena. 



A glass tube about 80 mm. long and 40 mm. in diameter (see 

 Fig. 77) is covered at one end with a well- washed pig's bladder, 

 then completely filled with a concentrated solution of cane-sugar, 

 and closed at the upper 

 end with vegetable 

 parchment. The tube 

 passes so far through 

 the large cork K 

 closing the vessel Gl, 

 that its lower end dips 

 into the distilled water 

 contained in the vessel. 

 Over the upper, parch- 

 ment-covered end of 

 the glass tube is bound 

 a rubber cap, Kk, which 

 runs out into a rubber 

 tube, and into this is 

 introduced the twice 

 bent glass tube, Gr. 

 When the apparatus 

 has been put together, its action at once begins. Water pene- 

 trates osmotically into the tube, closed with membrane at both 

 ends (artificial cell), a pressure is set up in it, since the quantity 

 of water entering is greater than the quantity of sugar solution 

 passing out, and this pressure is after a time great enough to over- 

 come the resistance to filtration offered by the parchment paper. 

 When this is the case, fluid is driven into the tube Gr, and it can 

 be collected in the vessel F. It is true that the processes leading 



FIG. 77. Apparatus] for illustrating the processes 

 leading to root pressure. 



