SUMMARYThe aim was to vary the transpiration rate of water culture plants and to measure corresponding values of ISW, the leaf water potential depression ( = DPD = suction force), as well as relative water content (relative turgidity) and stomatal aperture. On the Ohm-law analogy tS.W and transpiration rate should be linearly related.Ricinus communis plants were placed in a wind tunnel and the transpiration rate was varied by changing the level of the relative humidity of the air stream. With increasing rate of transpiration, A [r rose steeply to a value of about 6 atmospheres at which it remained virtually constant (Fig. i). Thus from very low transpiration rates to the highest obtainable, far from AIF rising linearly with transpiration, there was no response in AW and similarly no response in relative water content and stomatal resistance. When transpiration was reduced to zero the leaves still maintained a ATFof about 3 atmospheres instead of zero, but reasons are put for\vard for considering this value to present water saturation.The main resistance to water movennent in the plant was shown to reside in the roots. If a leaf was detached from a transpiring plant and allowed to continue transpiration at a similar rate, water being freely supplied to the cut end of the petiole, its A IF fell from 6 to 3.5 atmospheres (Fig. 3). Thus the resistance must lie 'below' the petiole. Similarly it was shown to lie below the stem.The constancy of leaf APF in the face of large changes in flux of water through the roots, implies that the root resistance declines in step with the rise in flux. A simple hydraulic model is presented which would behave in this way.

SUMMARYTwelve-week-old Ricimis comiminis plants were transplanted to boxes of sand or soil in which the water table was maintained at various levels below the root system. The boxes were placed in a climatological wind tunnel and transpiration varied by altering the humidity of the air stream. Responses in the water potential depression (= suction force or D.P.D.) of the leaves (AH^i) and stomatal conductance were followed.With a fine sand it was found that when the water table was high (15 cm below the roots) AIF, was 6 atm. irrespective of the rate of transpiration and behaved therefore as if the roots were surrounded by f^ree water as previously shown. \\'hen the water table was 25 cm below the roots, A IF, was also 6 atm. provided the transpiration rate was low (0.4 g/dm^ leaf/hour) but when the transpiration rate was raised threefold, Afr, rose steeply to a value of 12 atm., a steady state value being approached in about 8 hours (see Fig. 3). On reducing the transpiration rate, A IF, returned to 6 atm. This rise in A IF, is interpreted as due to a rise in the AfF of the water in the sand at the root surface, this in turn resulting from the low water conductivity of the sand at this height above the water table. Varying the humidity continuously between saturation and about ^0°/^ relative humidity through a 24-hour cycle produced a regular 24-hour cycle of transpiration (see Figs. 4, 5 and 6). Responses in A IF, in plants rooted in fine sand, clay soil and moorland soil were explicable in terms of the above hypothesis of the development of perirhizal zones of soil water tension.These results suggest that much of the ecologically significant water stress in plants is a manifestation of the dynamic gradients of water potential developing in the perirhizal zones and that these can be considerable even when the water stress in the bulk of the soil is virtually zero.

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