MOVEMENT OF MATERIALS IN THE PLANT 145 



Votchal has carried out a thorough investigation upon the transmission of 

 pressure by wood containing both air and water. [See note i, p. 131 for 

 reference]. Portions about 2 m. in length, from saplings or branches, were 

 placed in a horizontal position and water was forced through them from one end 

 to the other, by means of water or mercury pressure applied through glass tubes 

 suitably attached. The rate of entrance of water at one end and that of exit at 

 the other vary in a regular manner for a time after pressure is first applied. 

 Votchal's representation of these variations is reproduced in the diagram of 

 Fig. 82. The variation in the entrance rate, at the end where pressure is 

 applied, is shown by the line a. This rate first increases with remarkable 

 rapidity and soon attains a rather high value (a), but this high rate is maintained 

 only during several hundredths of a second after the pressure is applied. The 

 next stage (aß) shows a decreasing rate and is of longer duration, continuing 

 for from one-half to two minutes. In the third stage (ßy) the velocity continues 

 to fall, but more slowly and gradually, and it finally assumes a constant value. 

 In short pieces of stem the final constant rate is attained after five minutes, but 



■t 



Pig. 82. — Diagram showing variations in rates of entrance and exit of water moving under 

 pressure through a section of woody stem. (After Votchal.) 



with longer pieces this period may be prolonged. The simultaneous variation 

 in the rate of exit, at the opposite end of the piece of stem, is shown by the 

 line a'ß'. The velocity of movement here increases very slowly, gradually 

 attaining a value equal to that of the rate of entrance at the other end. 

 When the two rates become equal, the two curves become coincident, and water 



point can be true only with comparatively low transpiration rates, and with comparatively 

 ready entrance of water into the vessels below. The compound water column of the stem 

 is not broken in all vessels at the same level, however, and the transpiration stress is trans- 

 mitted laterally from the water of one vessel to that of adjoining ones, around the gas- filled 

 vessel segments. These matters have been very thoroughly treated by Dixon, and Overton 

 and Renner have each brought forward additional convincing arguments in favor of the 

 general interpretation adopted in the present note. See: Dixon, H. H., Transpiration and 

 the ascent of sap. Prog, rei bot. 3 : 1-66. 1909. Idem, Transpiration and the ascent of sap 

 in plants. London, 1914. Renner, 1910. [See note £, p 138.] Idem, 191 1, 1,2. [Se< 

 p. 137.] Idem, 1912, 1, 2. [See note k, p. 138.] Idem, Theoretisches und Experimentelles 

 zur Kohäsions-theorie der Wasserbewegung. Jahrb. wiss. Bot. 56 : 617-667. 1015. Holle, 

 H., Untersuchungen über Welken, Vertrocknen und Wider-straffwerden. Flora 108: 73-126. 

 1915. Overton, J. В., Studies on the relation of the living cells to the transpiration and 

 sap-flow in Cyperus. Bot. gaz. 51 : 28-63, 102-120. 191 1. — F.d. 

 in 



