A Spectrophotometric Study of Reflection of Light from Leaf Surfaces

1. The reflection of light from the surfaces of many kinds of leaves has been measured by means of a direct reading spectrophotometer, the measurements being made normal (that is, at an angle of 90⚬ to the leaf surface. The incident light, falling upon the leaf from the surface of a spherical housing, strikes the leaf at all angles. 2. The amount of reflection varies with the wave length, the maximum reflection falling usually at 540-560 mμ in green leaves. The value of the reflection in this region runs from 6-8 per cent in the darkest green leaves to 20-25 per cent in the lightest green specimens. 3. Hairiness or smoothness of cuticle does not necessarily mean high reflection. Leaves of Verbascum thapsus and Abutilon theophrasti show very little more reflection than leaves of non-hairy plants. The shiny cuticle of Morus rubra adds little to normal reflection. In some cases, however, as in the under surface reflection of Populus alba and Magnolia acuminata leaves, the hairs do increase the reflection. 4. The amount of reflection decreases with the age of the leaf. This is associated with the development of chlorophyll, which increases in amount very rapidly at first, and then more gradually for about two months, until the final depth of green color has been attained. The reflection then remains unchanged until the beginning of chlorophyll destruction at the close of summer. 5. White surfaces reflect almost equally across the spectrum, as seen in the under surface reflection of Populus alba; while albino leaves reflect mainly the longer radiations. There is absorption of the blue rays, corresponding to the yellowish color. The reflection from such surfaces is 40-50 per cent of the incident radiation. 6. The presence of mildew or other whitish superficial organisms increases the reflection of light very noticeably. In lilac the presence of Microsphaera alni increased the reflection more than 100 per cent. 7. Anthocyanin development is accompanied by a shift in the position of maximum reflection to the longer wave lengths. In Psedera the maximum reflection occurred at 640 mμ, while the normal maximum for green leaves is 540-560 mμ. 8. Yellow autumn coloration is much more brilliant than red. The reflection of yellow Betula leaves reached 42 per cent, with the maximum at 660 mμ. A leaf of Populus deltoides, less completely yellowed, reflected as much light, but the maximum reflection was in the yellow, at 580 mμ, instead of in the red. 9. In a considerable number of cases there is a depression of the reflection curve at 680 mμ. This obviously corresponds to the maximum absorption band of chlorophyll. The fact that it is evident is an argument for reflection from internal surfaces, in part. 10. The data presented are valuable in connection with the problem of income and outgo of energy in the leaf processes; but they must be supplemented with measurements in the invisible regions of the spectrum, particularly in the infra-red region, before a complete accounting of the energy utilization can be rendered.