Wind and transpiration

Bill Williams wewilliams at OSPREY.SMCM.EDU
Tue Feb 23 12:04:27 EST 1999

>I had a question today that I'm not quite sure I know the answer.  In
>connection with an assignment on the effects of various factors on
>transpiration a student suggested that at high wind velocities leaves would
>loose water rapidly due to the effect of low pressure above the stoma
>caused by the Venturi effect.  I have a feeling that such factors would not
>be too significant due to the small size of stoma and the movement of the
>leaves in wind.  Has anyone any thoughts on this.
>D. Haas

As mentioned, wind does have an effect (quite a large one), though it is
through reducing the boundary layer, rather than the Venturi effect.  I
suspect you are right about the small size of the stomata (particularly
their extremely small "length" -- the dimension along the path of
diffusion) making the Venturi effect negligible, but I know of no studies
about this specifically.  Perhaps your student would be interested in
exploring what the Venturi math would predict?

One responder mentioned the function of leaf hairs in increasing the
boundary-layer effect.  The conventional wisdom is that the boundary layer
is usually much, much thicker than the length of most leaf hairs, thus
making the hairs' effect on boundary-layer insignificant, but this might
change at high wind speed and under turbulent conditions.  In the one case
I know where it's been looked at closely, the hairs affect leaf reflectance
quite a bit and boundary-layer hardly at all (Ehleringer JR, Björkman O.
1978. A Comparison of Photosynthetic Characteristics of Encelia Species
Possessing Glabrous and Pubescent Leaves. Plant Physiology 62:185-190 and


William E. Williams
Biology Department, St. Mary's College of Maryland
St. Mary's City, MD 20686 USA
WEWilliams at

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