The Supply of Water to Transpiring Leaves

Biological Flora of the British Isles: Ruscus aculeatus

“…Farmer () gives a figure relative to stem cross‐sectional area of 3.4 mL cm −2 h −1 at 0.04 Mpa; this compares to figures of 35 and 36 mL cm −2 h −1 in Ilex aquifolium and Ligustrum vulgare , respectively. Similarly, Warne () using identical conditions measured water conductivity as 0.172 ± 0.0116 mL 100 cm −2 h −1 of cladodes (SE, n = 20), which was the lowest of any of the 16 woody plants tested, and half that of Buxus sempervirens . This matched with low transpiration rates of 0.208 mL 100 cm −2 h −1 from cut stems of R. aculeatus placed in water in a laboratory in bright light with temperatures up to 28 °C (Warne ) and the very low maximum stomatal conductance of 33 ± 0.007 mmol m −2 s −1 (SE, n = 4), cladode cuticular conductance (0.379 ± 0.082 mmol m −2 s −1 , n = 10) and stem cuticular conductance (0.095 ± 0.025 mmol m −2 s −1 , n = 6; Pivovaroff et al .…”

“…). Warne () suggested that the low hydraulic conductivity is compensated for by the short distances over which this low‐stature plant needs to conduct water. Moreover, Pivovaroff et al .…”

An Analysis of Actual Evapotranspiration

“…The total resistance for liquid flow in the plant is mainly determined by the largest resistance in the various parts of the transport path, when such a resistance considerably exceeds the other ones. Various investigations (BREWIG, 1937;KRAMER, 1938;WARNE, 1942;BROUWER, 1954) show fairly conclusive that the flow resistance in the xylem vessels is relatively small. The main resistances in the plant are present when the transport of water goes from cell to cell, so in the leaf tissue and in the root cells.…”

“…Evapotranspiration can be calculated according to THORNTHWAITE and HOLZMAN (1939;1942) by assuming the following propositions: a. the transfer factor for momentum is identical with that for water vapour; b. the shearing stress is constant with height: where k(a 0.4) is a turbulence constant originally introduced by VON KARMAN, z 0 a constant for each surface, known as the roughness length, and d the displacement of the zero plane of wind velocity in relation to the earth surface.…”

“…Complete plants, however, can still withdraw water from the soil when the soil suction amounts from 10 to 15 atmosphere. The amount of liquid secreted by detopped plants due to root pressure, is less than 5% of the amount of water used by the complete plants under conditions of high transpiration (KRAMER, 1939;WARNE, 1942). It appeared that under conditions of high transpiration a suction was present in the xylem vessels.…”