TRANSPORT 169 



piration, is a matter of much practical importance in experi- 

 mentation, because, when the stem is cut in air, this rushes into 

 the ducts and interrupts .perfect conduction after the shoot is 

 placed in water. This difficulty is the greater the more rapidly 

 water is being removed from the plant, and disappears with greatly 

 reduced transpiration; but in practice it is safest for the student 

 to cut under water all shoots or leaves intended to be kept fresh 

 in water. 



Tension oj Water. Over the large opening of a thistle-tube tie tightly 

 a piece of soaked parchment paper; fill the tube with boiled and cooled 

 water, and support it upright with the small open end standing in mercury; 

 stand the whole arrangement in a bell jar which can be exhausted to a vacuum, 

 and observe any changes in the levels of the liquids. Also replace the paper 

 by a tightly inserted, stiff-leaved shoot, and observe as before. 



As commonly tried in the laboratory, without removal of atmospheric 

 pressure, this experiment is quite valueless for a test or demonstration of 

 the tension or cohesion power of water and of mercury, for the rise of the 

 mercury is due to nothing but the external atmospheric pressure as the water 

 is removed by evaporation. To demonstrate any proper tension, and hence 

 any real lifting power of evaporation or transpiration, it is necessary either 

 to use a tube which will show more than one atmosphere of mercurial pres- 

 sure, or, better, to make the test in a vacuum. Compare Vines in Annals 

 of Botany, 10, 1896, 291, and the practical direction for experimental study 

 of this subject given by Stetkbrinck in Flora, 94, 1905, 466, and a mode 

 of making it self-registering by Schouten in Flora, 97, 1907, 118. 



Participation oj Living Cells. Select a shrubby plant, such as a rose, and 

 from one branch held vertically remove a ring of bark for 1 cm. ; just below 

 this build on the stem by modelling-clay a close-fitting and deep funnel, 

 1 cm. deep, and into this pour hot water, renewed several times. On another 

 branch place in the funnel a concentrated solution of picric acid (very poi- 

 sonous to plants as well as to men), once or twice renewed. Note in both 

 cases the effect upon transfer, as shown by wilting of the leaves. These two 

 methods kill the living protoplasm, but in different ways. 



The student should now work out the present state of our 

 knowledge of this subject from the literature, giving special 

 attention to the structure of wood, the mechanical problem in- 

 volved in the ascent of sap, and the principal theories developed 

 to explain water transfer, with the energetics of each. As there 

 are many modifications of view of the subject, he may well 

 confine himself to the four principal theories: (a) the older capil- 

 lary; (b) Sachs' imbibition; • (c) the propulsion theory of God- 



