We can investigate the mechanism of plant life as an individual, by means of rather simple techniques: measurement of the root xylem pressure by a pressure-transducer and that of the trans-root electric potential by a liquid junction and an operational amplifier with high enough input- impedance. Both indices are highly season-dependent, in relation to the transpiration activity in the leaves. Inhibition of transpiration by rain produces a pressure signal transmitted with a very high speed (sonic velocity in water) from leaf to root and activates the root activity pushing up water. The analysis of the further precise mechanism by means of intra-cellular micro-electrode technique is presented in Appendix. The insight into the evolution of vascular plants from the aquatic Chara plant is discussed.