/ 



t 



1902] /^/S£ OF THE TRANSPIRATION STREAM 265 



Janse suggests that the medullary rays will always pass water 

 in the same direction, being able to distinguish the upper end by 

 virtue of the stem's '' polarity." But the weight of evidence ever 

 since Hales has been that the water moves with equal readiness in 

 either direction (Hoffmann, 1850:842; Hartig, 1861:22; Unger, 

 1868:397; MacNab, 1874:363; and Bohm, Ber. Deut. Bot. 

 Gesell. 1890. On the other side Rauwenhoff, Reinhold, and 

 Meyer, 1897, the latter only contra Bohm's work). In view of 

 the fact that the transpiration stream does not become more 

 concentrated as it rises, the mechanism of its propulsion must be 

 different from that we have found most likely in root pressure. 

 Godlewski, therefore, embraces Pfeffer's suggestion that a stream 

 may be driven through a cell if a difference in internal concen- 

 tration be constantly maintained ; and he seeks the energy to 

 keep up this unequal distribution of solutes within the cell sap 

 in respiration. The expensiveness to the plant of this operation 

 is evident. 



Aside from the isolated instance reported by Janse (1887: 

 30-32), in which the path of eosin carried by the transpira- 

 tion stream seemed to traverse the medullary rays, the positive 

 experimental evidence used to prove the activity of living stem 

 cells is that "root" pressure is greater farther up than at the 

 base of a stem, and that rootless parts of plants bleed. Bara- 

 netzky (Abh. Naturf. Gesell. Halle 1873: 51-53) is authority 

 for the greater pressure higher up, but he experimented with 

 only four individual Helianthus and Ricinus plants, too few to 

 guarantee the results; and Detmer (1877. cf. Wieler, 1893) 

 reached the opposite conclusion. I have found the pressure of 

 Helianthus higher on the whole when the seedlings are cut near 

 the ground, but have tested only a dozen plants. 



Baranetzky (/. c. 53) also cut a piece fifteen feet long out 

 of a maple stem, closed both ends, cut a series of wounds in it, 

 and peeled the lower end and set it in water. After eight days 

 the wounds, especially the lower ones, began to bleed daily, from 

 U A. M. to 1:30 p. M. It was exposed to the sun from 10:30 

 until evening, and in spite of the fact that the temperature in the 



