The spectral properties of plant leaves and stems have been obtained for ultraviolet, visible, and infrared frequencies. The spectral reflectance, transmittance, and absorptance for certain plants is given. The mechanism by which radiant energy interacts with a leaf is discussed, including the presence of plant pigments. Examples are given concerning the amount of absorbed solar radiation for clear sky and overcast conditions. The spectral properties of desert plants are compared with those of more mesic plants. The evolution of the spectral properties of plant leaves during the early growing season is given as well as the colorimetric behavior during the autumn.

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A set of fi ve spec ia lly selected colored-glass fil ters to identify va ri a bles of m a lfun ction of photoelectri c recordin g spectrop hoto lTl.ctcrs cq uipped wit h tristim ulus integrators have been standa rdi zed on a number of spectrop hoto meters corrected for all known errors (wavelen gth, ze ro, 100 p ercen t, sli t-w idth , ine rt ia, bac k-reflectance, and stray-energy). T o these standardi zed spectrop hoto metric d ata de fi nite a moun ts of t hese errors were deliberately introdu ced and co nve rted to t ri stimulus va lues a nd chromaticity coordinates of the International Com m ission of Illumin ation syste m of colorimetry for So urces A, E , a nd C. Simila r r eductions s how t he effects of slit widt hs of 1, 5, 10, a nd 15 millimi crons (nw) on compu ted r es ults both by t he selected-ordin ate method of 10, 30 , a nd 100 ordin ates, a nd by t he we ighted-ordinate met hods of 1-, 5-, 10-, a nd 15-ml" in ter vals. Duplicate se ts of these glasses have been evaluated by visual co mpa ri so n w it h t his set of m aster standards, and a re availa ble as pa rt of t he Stand ard M a teri a ls P rogra m of the N atio nal B ureau of Standa rds. By co mpa ring t he ce rt ified values of lumin ous tra nsmi ttance a nd chro maticity coord inates for a set of t hese glasses wi t h t he values obtained on a par ticular in teg rator-spectrop hotometer co mbinatio n, t he type an d extent of instru mental errors ma.v be evaluated .

Spectral transmittancy values at room temperatures approximating 25° C were obtained for 1.25 X 10-5 M metacresolsulfonphthalein in water and in 0.001-to 8-M hydrochloric acid. In water the indicator exists in the yellow form (Y), and hydrogen ions (H) from acids convert it into the red form (R). The molar concentrations of each form of the indicator in various concentrations of hydrochloric acid were computed from the spectrophotometric data. The dissociation constant (Kb) for the reaction of the indicator with hydrochloric acid is given by the equation in which M and j, with appropriate subscripts, represent the molarities and activity coefficients, respectively. The value of Kb is 1.98 X 10-2 and that of -log Kb or pK2 is 1.703 ± 0.005. The indicator is useful over the pH range 0 to 3. CONTENTS Page

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Measure men ts of t he spectral transmi ttance and i nternal transmit tan ce of fi ve optical glasses fOT the wavelength region 200 to 1,000 millim icro ns are reported .

Contract No* 33 (6l6) £2-21. It is coordinated with Air Force Contract No. 33 (6l6) -262 under Dr. High T. O'Neill O'Neill Associates, Annapolis, Maryland, who requested the NBS to perform this year-long test of leaves of white oak trees.