COLORS OF AUTUMN LEAVES. 297 



connected to its plant, a narrow strip of flexible lead or tin foil, 

 and expose the leaf to bright sunlight. After a quarter or half an 

 hour remove the strip, and the spot which has been kept shaded 

 by it will be seen to be distinctly deeper in color than the part 

 which has been exposed to the sun's rays. 



791. Chlorosis, or blanching of plants from lack of iron. 

 Although iron has not been detected as a constant component 

 of the pure pigment of chlorophj-ll, this element has been shown 

 in many ways, especially by water-culture, to be essential to 

 the green color and even to the normal formation of the 

 granules. When a seedling of Indian corn is grown with 

 its roots abundantly supplied with a nutrient solution from 

 which all salts of iron are absent, and it has all other condi- 

 tions favorable to rapid and healthy development, the leaves 

 are pale yellow, or even whitish, and the whole plant sooner 

 or later appears sickly and ill-nourished. When, however, a 

 salt of iron is supplied to the nutrient liquid, a normal green 

 color is at once imparted to the leaves and the plant becomes 

 healthy and vigorous. The effect of the local application of a 

 salt of iron is thus described : When a weak solution of ferric 

 chloride, ferric nitrate, or ferrous sulphate is applied to a leaf 

 blanched by want of iron, the part moistened assumes a nor- 

 mal green color in a few days, and sometimes in a much shorter 

 period. Neither cobalt nor nickel salts have similar relations 

 to chlorophyll. 1 



792. Autumnal changes in color. The leaves of many decidu- 

 ous plants undergo changes of color at some period before they 

 fall. In not a few instances these changes occur early in the 

 season after full development of the leaf; for example, during 

 the first days of summer it is not unusual to find on the swamp 

 maple bright red and yellow leaves. The colors, however, be- 

 come most striking in temperate climates at the approach of 

 autumn. 



The change of color in autumn leaves is due to changes which 

 take place in the chlorophyll pigment. This breaks up into 

 various matters of unknown composition, but classed in a gen- 

 eral way with the erythrophyll (the reddish coloring-matter) and 

 xanthophyll (the yellowish), obtainable artificially from chloro- 

 phyll. Comparison of the spectra of these substances exhibits 

 certain very striking features of similar! t} r . 



1 Eusebe Gris, 1844, and Arthur Gris, in Ann. des Sc. nat., ser. 4, 

 tome vii., 1857, p. 179. 



