too small to account for the volumes involved in the transpiration stream, and root pressure cannot usually be demonstrated in transpiring plants. Although this argument is frequently raised, it is an obvious non sequitur. Transpiration, when it does occur, provides a mechanism capable of moving large quantities ,of water, but this can quite well occur independently of other mechanisms; they are in no way mutually exclusive; and the common methods so far devised for demonstrating root pressure require the abolition of the transpiration mechanism during the test. Scholander has recently introduced a method which does permit simultaneous demonstration of the two and has measured them simultaneously in certain vines (7, 8).3) It has been stated, for example by Kramer (9), that "A . . . reason for doubting that . . . root pressure phenomena play an essential role in the intake of water, is the fact that it [root pressure] apparently never occurs in Gymnosperms. It would be very surprising if such a process were essential in one group of plants, yet not even occur in another [similar] group." And again, "Root pressure has never been observed in the Gymnosperms, and it is probable that active absorption never occurs in that group" (9, p. 790). This last argument, if based on incontrovertible evidence, would indeed seem to make untenable any idea of root pressure being of general importance in the movement of water to the tops of tall trees.On a suggestion from Scholander, made during the discussions at the Symposium on the Physiology of Forest Trees at the Cabot Foundation, Harvard Forest, Petersham, Mass. (Apr. 1957), we set out to examine the validity of this argument (10). Simple manometers were attached to the roots of three species of Gymnosperms: Pinus strobus, Picea glauca, and Picea rubens and, for comparison, on three Angiosperms: Betula lutea, Populus alba, and Fraxinus americana.The procedure was as follows. Trees of as uniform age and size as possible were chosen, 15 to 20 feet in height, 25 to 40 years old. Roots were traced away from the trunk until branches 0.5 to 1.0 cm in diameter were exposed. These were severed perpendicular to the axis. Since such tissues, in actively transpiring trees, sometimes show negative pressures in the vessels, the roots were left for about 15 minutes to allow any air which might be sucked into the vessels by this negative pressure to come into equilibrium. A second cut was then made about 15 cm acropetal to the first too small to account for the volumes involved in the transpiration stream, and root pressure cannot usually be demonstrated in transpiring plants. Although this argument is frequently raised, it is an obvious non sequitur. Transpiration, when it does occur, provides a mechanism capable of moving large quantities ,of water, but this can quite well occur independently of other mechanisms; they are in no way mutually exclusive; and the common methods so far devised for demonstrating root pressure require the abolition of the transpiration mechanism during the test...