Abstract

Some dew-covered plants are strongly retroreflective. The bright glow seen when the antisolar point falls on grass is known as the heiligenschein. Its widely accepted explanation requires that the grass be covered with hair. The discovery of the sylvanshine, a closely related phenomenon best seen at night, revealed that strong retroreflection can occur on hairless dew-covered plants. A simple model shows that below a contact angle of 90°, the enhancement in the backscatter direction is virtually identical to that given by a diffusely reflecting surface, but as the contact angle climbs to 140°, enhancement increases by 2 orders of magnitude. Plants that exhibit such large contact angles are not the norm, but can be found chiefly among coniferous trees, a few of which display the sylvanshine with great brilliance.

© 1994 Optical Society of America

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References

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  1. The eyes of carnivores contain a mirrorlike surface made up of crystals of zinc cysteine. See A. Pirie, “The biochemistry of the eye,” Nature (London) 186, 352–354 (1960).
    [CrossRef]
  2. B. Cellini, Memoirs, Chap. XXVI (1562). The book is also sometimes known as The Autobiography of Benvenuto Cellini. It continues to be available in many translations and reprints from many publishers.
  3. Winterfeld, Gilbert's Annalen, Vol. XVIII, (1804), pp. 57ff. Gilbert's Annalen is also known as (and may be easier to find as) Annalen der Physik (Halle, Leipzig). Although 1804 would seem to be the birth of this idea, the Annalen paper was actually a reprint from the Deutschen Merkur (1795).
  4. E. Lommel, “Ueber den Lichtschein un den Schatten des Kopfes,” Annalen der Physik und Chemie (Leipzig, 1874), pp. 10–21.
  5. Although this is how Lommel's theory is usually presented, Lommel actually required the leaf to be slightly within the focal distance. This enabled the backscattering to be sufficiently divergent that a dewdrop located to the side of the observer's shadow would contribute to the heiligenschein.
  6. M. Minnaert, The Nature of Light and Colour in the Open Air (Dover, New York, 1954), Sec. 168.
  7. R. A. R. Tricker, Introduction to Meteorological Optics (Miles & Boon, London1970), Chap. 11.
  8. R. Greenler, Rainbows, Halos, and Glories (Cambridge, New York, 1980).
  9. H. W. Brandes, “Hof,” in Gehler's Physikalisches Wörterbuch (Schwickert, Leipzig, 1825), Vol. 5, pp. 439ff.
  10. J. M. Pernter, F. M. Exner, Meteorologische Optik (Braumüller, Vienna, 1922), Sec. 59.
  11. W. J. Humphreys, Physics of the Air, 3d ed. (McGraw-Hill, New York, 1940), p. 556.
  12. “Cellini's halo,” in The New Encyclopaedia Britannica, Micropaedia II, 15th ed., W. E. Preece, ed. (Encyclopedia Britannica, Benton, Chicago, Ill., 1976), p. 673.
  13. M. Minnaert, “Retro-reflexion,” Nature (London) 225, 718 (1970).
    [CrossRef]
  14. J. O. Mattsson, C. Cavallin, “Retroreflection of light from drop-covered surfaces and an image-producing device for registration of this light,” Oikos 23, 285–294 (1972).
    [CrossRef]
  15. See, for example, D. C. Hamilton, W. R. Morgan, “Radiant interchange configuration factors,” NACA TN2836 (1952); or E. R. G. Eckert, R. M. Drake, Heat and Mass Transfer (McGraw-Hill, New York, 1959).
  16. This claim seems to belie some casual observations that often reveal only an isolated drop sitting at the tip of a grass blade. Although they are often confused with dew, these are guttation drops, the liquid for which originates within the plant rather than by condensation from the atmosphere. For a discussion of the differences, one is referred to the classical essay, “On dew,” written by John Atken in 1885. It is to be found in the Collected Scientific Papers of John Aitken, edited for the Royal Society of Edinburgh by Cargill G. Knott. (Cambridge U. Press, Cambridge, 1923).For a recent discussion see C. F. Bohren, “All that glistens isn't dew,” Weatherwise 43, 284–287 (1990). In discussing the sylvanshine, the word dew is used to mean just that—not guttation.
    [CrossRef]
  17. See, for example, S. Perry, A. B. Fraser, D. Thomson, J. Norman, “Indirect sensing of plant canopy structure,” Agri. For. Meteorol. 42, 255–278, (1988).
    [CrossRef]
  18. J. Martin, B. Juniper, The Cuticle of Plants (St. Martin's, New York, 1970).
  19. V. S. Berg, “Plant cuticle as barrier to acid rain penetration,” in Effects of Atmospheric Pollutants on Forests, Wetlands and Agricultural Ecosystems, T. C. Hutchinson, K. M. Meena, eds.(Springer-Verlag, Berlin, 1987), pp. 145–154.
    [CrossRef]
  20. J. Schönherr, “Water permeability of isolated cuticular membranes: the effect of cuticular waxes on diffusion of water,” Planta 13, 159–164 (1976).
    [CrossRef]
  21. P. J. Holloway, “The effects of leaf wettability,” Ann. Appl. Biol. 63, 145–153 (1969).
    [CrossRef]
  22. H. N. Barber, “Adaptive gene substitutions in Tasmanian eucalypts: 1. Genes controlling the development of glaucousness,” Evolution 9, 1–14 (1955).
    [CrossRef]
  23. J. B. Clark, G. R. Lister, “Photosynthetic action spectra of trees,” Plant Physiol. 55, 407–413 (1975).
    [CrossRef] [PubMed]

1988 (1)

See, for example, S. Perry, A. B. Fraser, D. Thomson, J. Norman, “Indirect sensing of plant canopy structure,” Agri. For. Meteorol. 42, 255–278, (1988).
[CrossRef]

1976 (1)

J. Schönherr, “Water permeability of isolated cuticular membranes: the effect of cuticular waxes on diffusion of water,” Planta 13, 159–164 (1976).
[CrossRef]

1975 (1)

J. B. Clark, G. R. Lister, “Photosynthetic action spectra of trees,” Plant Physiol. 55, 407–413 (1975).
[CrossRef] [PubMed]

1972 (1)

J. O. Mattsson, C. Cavallin, “Retroreflection of light from drop-covered surfaces and an image-producing device for registration of this light,” Oikos 23, 285–294 (1972).
[CrossRef]

1970 (1)

M. Minnaert, “Retro-reflexion,” Nature (London) 225, 718 (1970).
[CrossRef]

1969 (1)

P. J. Holloway, “The effects of leaf wettability,” Ann. Appl. Biol. 63, 145–153 (1969).
[CrossRef]

1960 (1)

The eyes of carnivores contain a mirrorlike surface made up of crystals of zinc cysteine. See A. Pirie, “The biochemistry of the eye,” Nature (London) 186, 352–354 (1960).
[CrossRef]

1955 (1)

H. N. Barber, “Adaptive gene substitutions in Tasmanian eucalypts: 1. Genes controlling the development of glaucousness,” Evolution 9, 1–14 (1955).
[CrossRef]

1804 (1)

Winterfeld, Gilbert's Annalen, Vol. XVIII, (1804), pp. 57ff. Gilbert's Annalen is also known as (and may be easier to find as) Annalen der Physik (Halle, Leipzig). Although 1804 would seem to be the birth of this idea, the Annalen paper was actually a reprint from the Deutschen Merkur (1795).

Barber, H. N.

H. N. Barber, “Adaptive gene substitutions in Tasmanian eucalypts: 1. Genes controlling the development of glaucousness,” Evolution 9, 1–14 (1955).
[CrossRef]

Berg, V. S.

V. S. Berg, “Plant cuticle as barrier to acid rain penetration,” in Effects of Atmospheric Pollutants on Forests, Wetlands and Agricultural Ecosystems, T. C. Hutchinson, K. M. Meena, eds.(Springer-Verlag, Berlin, 1987), pp. 145–154.
[CrossRef]

Brandes, H. W.

H. W. Brandes, “Hof,” in Gehler's Physikalisches Wörterbuch (Schwickert, Leipzig, 1825), Vol. 5, pp. 439ff.

Cavallin, C.

J. O. Mattsson, C. Cavallin, “Retroreflection of light from drop-covered surfaces and an image-producing device for registration of this light,” Oikos 23, 285–294 (1972).
[CrossRef]

Cellini, B.

B. Cellini, Memoirs, Chap. XXVI (1562). The book is also sometimes known as The Autobiography of Benvenuto Cellini. It continues to be available in many translations and reprints from many publishers.

Clark, J. B.

J. B. Clark, G. R. Lister, “Photosynthetic action spectra of trees,” Plant Physiol. 55, 407–413 (1975).
[CrossRef] [PubMed]

Exner, F. M.

J. M. Pernter, F. M. Exner, Meteorologische Optik (Braumüller, Vienna, 1922), Sec. 59.

Fraser, A. B.

See, for example, S. Perry, A. B. Fraser, D. Thomson, J. Norman, “Indirect sensing of plant canopy structure,” Agri. For. Meteorol. 42, 255–278, (1988).
[CrossRef]

Greenler, R.

R. Greenler, Rainbows, Halos, and Glories (Cambridge, New York, 1980).

Hamilton, D. C.

See, for example, D. C. Hamilton, W. R. Morgan, “Radiant interchange configuration factors,” NACA TN2836 (1952); or E. R. G. Eckert, R. M. Drake, Heat and Mass Transfer (McGraw-Hill, New York, 1959).

Holloway, P. J.

P. J. Holloway, “The effects of leaf wettability,” Ann. Appl. Biol. 63, 145–153 (1969).
[CrossRef]

Humphreys, W. J.

W. J. Humphreys, Physics of the Air, 3d ed. (McGraw-Hill, New York, 1940), p. 556.

Juniper, B.

J. Martin, B. Juniper, The Cuticle of Plants (St. Martin's, New York, 1970).

Lister, G. R.

J. B. Clark, G. R. Lister, “Photosynthetic action spectra of trees,” Plant Physiol. 55, 407–413 (1975).
[CrossRef] [PubMed]

Lommel, E.

E. Lommel, “Ueber den Lichtschein un den Schatten des Kopfes,” Annalen der Physik und Chemie (Leipzig, 1874), pp. 10–21.

Martin, J.

J. Martin, B. Juniper, The Cuticle of Plants (St. Martin's, New York, 1970).

Mattsson, J. O.

J. O. Mattsson, C. Cavallin, “Retroreflection of light from drop-covered surfaces and an image-producing device for registration of this light,” Oikos 23, 285–294 (1972).
[CrossRef]

Minnaert, M.

M. Minnaert, “Retro-reflexion,” Nature (London) 225, 718 (1970).
[CrossRef]

M. Minnaert, The Nature of Light and Colour in the Open Air (Dover, New York, 1954), Sec. 168.

Morgan, W. R.

See, for example, D. C. Hamilton, W. R. Morgan, “Radiant interchange configuration factors,” NACA TN2836 (1952); or E. R. G. Eckert, R. M. Drake, Heat and Mass Transfer (McGraw-Hill, New York, 1959).

Norman, J.

See, for example, S. Perry, A. B. Fraser, D. Thomson, J. Norman, “Indirect sensing of plant canopy structure,” Agri. For. Meteorol. 42, 255–278, (1988).
[CrossRef]

Pernter, J. M.

J. M. Pernter, F. M. Exner, Meteorologische Optik (Braumüller, Vienna, 1922), Sec. 59.

Perry, S.

See, for example, S. Perry, A. B. Fraser, D. Thomson, J. Norman, “Indirect sensing of plant canopy structure,” Agri. For. Meteorol. 42, 255–278, (1988).
[CrossRef]

Pirie, A.

The eyes of carnivores contain a mirrorlike surface made up of crystals of zinc cysteine. See A. Pirie, “The biochemistry of the eye,” Nature (London) 186, 352–354 (1960).
[CrossRef]

Schönherr, J.

J. Schönherr, “Water permeability of isolated cuticular membranes: the effect of cuticular waxes on diffusion of water,” Planta 13, 159–164 (1976).
[CrossRef]

Thomson, D.

See, for example, S. Perry, A. B. Fraser, D. Thomson, J. Norman, “Indirect sensing of plant canopy structure,” Agri. For. Meteorol. 42, 255–278, (1988).
[CrossRef]

Tricker, R. A. R.

R. A. R. Tricker, Introduction to Meteorological Optics (Miles & Boon, London1970), Chap. 11.

Agri. For. Meteorol. (1)

See, for example, S. Perry, A. B. Fraser, D. Thomson, J. Norman, “Indirect sensing of plant canopy structure,” Agri. For. Meteorol. 42, 255–278, (1988).
[CrossRef]

Ann. Appl. Biol. (1)

P. J. Holloway, “The effects of leaf wettability,” Ann. Appl. Biol. 63, 145–153 (1969).
[CrossRef]

Evolution (1)

H. N. Barber, “Adaptive gene substitutions in Tasmanian eucalypts: 1. Genes controlling the development of glaucousness,” Evolution 9, 1–14 (1955).
[CrossRef]

Gilbert's Annalen (1)

Winterfeld, Gilbert's Annalen, Vol. XVIII, (1804), pp. 57ff. Gilbert's Annalen is also known as (and may be easier to find as) Annalen der Physik (Halle, Leipzig). Although 1804 would seem to be the birth of this idea, the Annalen paper was actually a reprint from the Deutschen Merkur (1795).

Nature (London) (1)

M. Minnaert, “Retro-reflexion,” Nature (London) 225, 718 (1970).
[CrossRef]

Nature (London) (1)

The eyes of carnivores contain a mirrorlike surface made up of crystals of zinc cysteine. See A. Pirie, “The biochemistry of the eye,” Nature (London) 186, 352–354 (1960).
[CrossRef]

Oikos (1)

J. O. Mattsson, C. Cavallin, “Retroreflection of light from drop-covered surfaces and an image-producing device for registration of this light,” Oikos 23, 285–294 (1972).
[CrossRef]

Plant Physiol. (1)

J. B. Clark, G. R. Lister, “Photosynthetic action spectra of trees,” Plant Physiol. 55, 407–413 (1975).
[CrossRef] [PubMed]

Planta (1)

J. Schönherr, “Water permeability of isolated cuticular membranes: the effect of cuticular waxes on diffusion of water,” Planta 13, 159–164 (1976).
[CrossRef]

Other (14)

See, for example, D. C. Hamilton, W. R. Morgan, “Radiant interchange configuration factors,” NACA TN2836 (1952); or E. R. G. Eckert, R. M. Drake, Heat and Mass Transfer (McGraw-Hill, New York, 1959).

This claim seems to belie some casual observations that often reveal only an isolated drop sitting at the tip of a grass blade. Although they are often confused with dew, these are guttation drops, the liquid for which originates within the plant rather than by condensation from the atmosphere. For a discussion of the differences, one is referred to the classical essay, “On dew,” written by John Atken in 1885. It is to be found in the Collected Scientific Papers of John Aitken, edited for the Royal Society of Edinburgh by Cargill G. Knott. (Cambridge U. Press, Cambridge, 1923).For a recent discussion see C. F. Bohren, “All that glistens isn't dew,” Weatherwise 43, 284–287 (1990). In discussing the sylvanshine, the word dew is used to mean just that—not guttation.
[CrossRef]

B. Cellini, Memoirs, Chap. XXVI (1562). The book is also sometimes known as The Autobiography of Benvenuto Cellini. It continues to be available in many translations and reprints from many publishers.

J. Martin, B. Juniper, The Cuticle of Plants (St. Martin's, New York, 1970).

V. S. Berg, “Plant cuticle as barrier to acid rain penetration,” in Effects of Atmospheric Pollutants on Forests, Wetlands and Agricultural Ecosystems, T. C. Hutchinson, K. M. Meena, eds.(Springer-Verlag, Berlin, 1987), pp. 145–154.
[CrossRef]

E. Lommel, “Ueber den Lichtschein un den Schatten des Kopfes,” Annalen der Physik und Chemie (Leipzig, 1874), pp. 10–21.

Although this is how Lommel's theory is usually presented, Lommel actually required the leaf to be slightly within the focal distance. This enabled the backscattering to be sufficiently divergent that a dewdrop located to the side of the observer's shadow would contribute to the heiligenschein.

M. Minnaert, The Nature of Light and Colour in the Open Air (Dover, New York, 1954), Sec. 168.

R. A. R. Tricker, Introduction to Meteorological Optics (Miles & Boon, London1970), Chap. 11.

R. Greenler, Rainbows, Halos, and Glories (Cambridge, New York, 1980).

H. W. Brandes, “Hof,” in Gehler's Physikalisches Wörterbuch (Schwickert, Leipzig, 1825), Vol. 5, pp. 439ff.

J. M. Pernter, F. M. Exner, Meteorologische Optik (Braumüller, Vienna, 1922), Sec. 59.

W. J. Humphreys, Physics of the Air, 3d ed. (McGraw-Hill, New York, 1940), p. 556.

“Cellini's halo,” in The New Encyclopaedia Britannica, Micropaedia II, 15th ed., W. E. Preece, ed. (Encyclopedia Britannica, Benton, Chicago, Ill., 1976), p. 673.

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Figures (6)

Fig. 1
Fig. 1

Lommel's model relied on hairy leaves to hold the dewdrop 1 focal length from the surface.

Fig. 2
Fig. 2

Mattsson's model relied on oblique illumination to enable the light to he focused on the leaf. The light enters at an angle of π/3 from the normal. This is obtained by observing that the drop center, the contact point with the leaf, and the focal point form a triangle with the focal distance for a water drop equal to twice its radius.

Fig. 3
Fig. 3

Half-drop of unit radius with the light entering from the left. The incident angle is i and the deviation of the light is D. The focal length f is the distance from the drop center to a point where the ray intersects the axis (and its mirror image from the other half of the drop).

Fig. 4
Fig. 4

When there is a lens, a larger fraction of the diffusely scattered light is sent back in the original direction.

Fig. 5
Fig. 5

Enhancement E on a log scale for the thin-lens model. It is plotted as a function of the displacement x0 of the lens (drop) center from the plane (leaf). Lommel's heiligenschein: all the normals point toward the source (Sun) (———). Two intermediate cases: the normals point within π/6 (--------) and π/3 (⋯⋯⋯⋯) of the source. The sylvanshine: the normals are randomly oriented (———).

Fig. 6
Fig. 6

Log plot of the enhancement E for the thin-lens model of the sylvanshine (———) and Lommel's heiligenschein (—), plotted as a function of the contact angle θc a drop makes with a leaf. The two intermediate cases are also shown: the normals point within π/6 (--------) and π/3 (⋯⋯⋯⋯) of the source.

Equations (14)

Equations on this page are rendered with MathJax. Learn more.

f = sin i sin D .
f i D = i 2 ( i r ) i 2 ( i i / n ) = n 2 ( n 1 ) 2 .
P = A ω S d ω L S cos θ = A L S ω S d ω cos θ ,
L D L S = ( PR ) / ( π A ) P / ( A ω S d ω cos θ ) = R F S ,
F = 1 π ω d ω cos θ
L D L S = ( R P ) / ( A P π ) P / ( A P π F L ) = R F L .
E 0 = F L / F S .
E θ = [ 1 E 0 + ( 1 x 0 f cos θ ) 2 ] 1 ,
θ c = arccos ( x 0 ) ,
E ̅ = 2 π d ω p ( ω ) E θ = 0 π / 2 d θ p ( θ ) E θ 2 π sin θ , where 1 = 2 π d ω p ( ω ) ,
A θ = 1 2 π ( ψ sin ψ ) , where ψ = 2 arccos ( | x 0 | sin θ ) .
A ̅ = 2 π d ω p ( ω ) A θ = arcsin | x 0 | π / 2 d θ p ( θ ) A θ 2 π sin θ ,
E = { 1 ( x 0 1 ) ( 1 A ̅ ) + A ̅ E ̅ ( x 0 0 ) A ̅ + ( 1 A ̅ ) E ̅ ( x 0 < 1 ) E ̅ ( 1 x 0 ) ,
p = { 1 2 π ( 1 cos θ ) ( θ θ ) 0 ( θ < θ ) .

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