A comprehensive treatment of light propagation through intact leaves based on the theories of radiative transfer and absorption statistics was used to calculate the theoretical absorption spectra of the chlorophyll-containing particles under conditions of multiple scattering and pigment spatial distribution equivalent to those in a leaf. These spectra were compared with the experimental in vivo spectra of leaves and in vitro spectra of chlorophyll-protein complexes extracted form these leaves. We conclude that the main discrepancies between the in vivo and in vitro spectra are apparently due to the optical artifacts specific for light propagation in leaves-multiple scattering and distributional error. Alterations of the pigment properties upon extraction significantly contribute to these discrepancies. The method has an estimated accuracy of about 10% and can be applied to derive the intrinsic optical properties of the photosynthetic mechanism in a leaf, as well as for the systematic study of their changes in the course of light adaptation.

Comparisons of photosynthetic rates were made on leaves of ten species of woody dicotyledons grown in the field under full sun or under a canopy which transmitted approxinately 18% of full light. Photosynthesis and dark respiration were measured and compared on various bases: area, chlorophyll, fresh weight of lamina, density thickness (fresh weight per unit area), and protein. It is well known that the photosynthetic characteristics of a broad range of plants are influenced by the light climate in which they are grown (for review: 2, 3, 19). Sun leaves are generally described as requiring a higher light saturation photon flux density, and having a higher light-saturated photosynthetic rate and light compensation point than corresponding shade leaves. However, the basis (5) on which the photosynthetic rate is expressed will make a difference in this comparison, because if sun leaves are thicker than shade leaves (2,3,11,12) (13), and illuminated from above through 4 cm of water with ten incandescent reflectorized 100-w bulbs. The air was humidified at 23°C and measured 26.5 ± 0.5°C (inlet) to 27 ± 0.5°C (outlet) in the cuvette. The light was varied by the use of screens, and measured at the leaf position using a sensor reading in ,Mmol m-2 s-' (Lambda Instruments, model LI-185 meter with LI-190-S sensor) of PAR (400-700 nm). Air used in the measurements was ambient air from the roof of a three-story building and had CO2 of 324 ± 10 ,ul/l which was similar to values reported by others (23). Each leaf was brought to the laboratory as rapidly as possible; measurement of the light curve began with the lowest flux density and worked up, with dark respiration being measured after the highest flux density. Measurements were completed between 9 AM and 3 PM. Transpiration rates or stomatal resistances were not measured, but care was taken to use leaves in good condition; if photosynthetic rates declined markedly, the leaf was discarded.Following photosynthetic analysis, the leaf area was measured and discs (30, total area 9.96 cm2) were punched from smooth, vein-free regions of the leaf. These were humidified and weighed for density thickness (mg cm-2; 11). Fresh weight of the leaf lamina was calculated from the leaf area and density thickness.The punches were homogenized in 80%o acetone and the Chl determined according to Arnon (1); the washed residue was air dried and subsequently extracted overnight with 20 ml of I N NaOH. The alkali-soluble extract was assayed for protein according to Lowry et al. (10), using a BSA standard.Errors are expressed as SD. The differences between sun and shade samples of the same species were statistically significant at the 0.1 level (t test) except where noted. McMillen (13) reported photosynthetic rates per fresh weight with the use of whole leaf weight, instead of the leaf lamina weight calculated as above.

MATERIALS AND METHODSIn a survey of over 300 nonevergreen dicots in 69 families, many species were found to have translucent patterns attributed to the presence of bundle-sheath extensions (BSE) on the small and ultimate veinlets. The BSE have been shown by others to inhibit transverse air movement within leaves, and it has been suggested that they are important passageways between vascular tissue and the palisade. The only characteristic found to be associated with prominent BSE is that more trees have such features than herbaceous plants. However, many important herbs have them also, including soybeans and sunflowers. Fresh leaves were photographed in transmitted light with a 35-mm single-lens reflex camera attached to a 19-cm bellows extension with a 20-mm, f/3.5 lens, giving about 13X magnification at the film. The leaf sample was pressed against a hole in a metal plate by a microscope slide held in place by clips. The upper (adaxial) surface of the leaf faced the lens.All leaves were from dicots, and were selected from broadleaf, deciduous trees, shrubs, or vines, or from annual or perennial herbs or vines. A few leaves had dark veins instead of bright ones (Amaranthus sp., Gomphrena sp.), which are presumed to be C-4 leaves. Sampling was of native, cultivated and weedy plants, mostly in southeast Nebraska.Two different methods were used to classify the contact prints. For the first, two lines were placed across the photograph (i.e. 48 mm on the film, or about 3.7 mm of the leaf) and the number of sharply visible veins that touched the lines were counted. For convenience in tabulation, these numbers were pooled into six 'vein density' groups that had the following designations and ranges: A (28-22 veins/3.7 mm), B (21-16), C (15-12), D (11-8), E (7-4), F (3-0).The second measure was a subjective assessment ('visual index,' ranging from 4 to 0) of the optical importance of the visible veins. This attempted to give credit to the optical contrast between the veins and leaf mesophyll, as well as to the extent of the network. The two measures were highly correlated (r = 0.85). RESULTS AND DISCUSSIONFigures 1 through 4 show representative photographs of the range of leaf pattems found. Wylie (7)

The sieve effect and scattering within leaves are analysed by the use of a simple model. By plotting the leaf transmittance (corrected for light not entering the leaf) vs the transmittance of an equivalent amount of homogeneous plastid pigments, an intercept is found where the latter is zero. This minimum transmittance represents the fraction of the leaf area devoted to the ray of the sieve effect which strikes no chloroplasts. It varied between 7"/0 and 0.2% in non-senescent leaves. When this was subtracted from the leaf spectrum, the peak absorbance was greater than that of the homogeneous leaf pigments in all cases. The ratio of the leaf absorbance to that of the homogeneous pigments, at the same wavelength, is the apparent optical pathlength, which increases with decreasing absorbance. By plotting this ratio vs the absorbance of the equivalent homogeneous pigment, an intercept is found where the latter is zero. This intercept is interpreted as an estimate of the true mean scattering pathlength. Leaves with high chlorophyll contents had low pathlengths (mean and SD = 2.30 ? 0.25); with moderate and low contents, the values were higher (2.75 ? 0.28, 3.95 ? 0.77). Another application of the model gave values between 3 and 4 for the true scattering pathlength.

Reflectance and transmittance spectra of leaves and their sum can be corrected to relate only to the light actually entering the leaf, if the reflectance of the epidermal surface is known. The latter is found if the leaf reflectances at several wavelengths near the transmittance minimum in the red are plotted vs the transmittances of a homogeneous suspension of the native pigment-proteins at the same chlorophyll content per unit area and at the same wavelengths. With non-senescent leaves, the relation is linear and the extrapolation of the pigment transmittance to zero gives the value for the surface reflection. Surface reflectance data (both adaxial and abaxial) are given for the leaves of a number of trees and a few herbs, plus examples of the raw and corrected spectra. With normal, glaucous leaves, the adaxial reflectance averaged 4.5% of the incident light ( n = 23, range = 3.7 -5.9, standard deviation = 0.4). The reflectances of the abaxial surfaces ranged between 7 and 13% since additional near-surface reflection occurred at the inside of the epidermis and in the spongy rnesophyll. Reflectance and transmittance data demonstrated strong absorption in the epidermis below 480 nm.