THE CHLOROPLAST PIGMENTS 307 



present it is impossible to reconcile the corpuscular with the undulatory mani- 

 festations of light. All we can say with certainty is that in some of its effects 

 light behaves as if it travelled in waves, in others as if it is propagated across 

 space as a shower of photons. 



The energy value of radiations can be expressed in terms of their equiva- 

 lent in other forms of energ}\ On a clear summer's day in the mid-temperate 

 zones the energy value of the impinging solar radiation at noon is usually 

 between 1.2 and 1.5 g.-cal. per square centimeter per minute. This corre- 

 sponds to an illumination value of about 8,000 — 10,000 foot candles. 



Light and all other radiant energy varies in several different waj'S, the 

 most important of which are : (i) intensity, (2) quality, and (3) duration. 

 The term "intensity" is often considered to refer to the "brightness" of light, 

 but the two are not the same, as brightness is a measure of intensit}' only 

 insofar as changes in intensity are registered by the human eye. On the basis 

 of the quantum theory light intensity is considered to depend upon the num- 

 ber of quanta impinging upon a surface of given area per second regardless 

 of the energy content of the quanta. "Quality" refers to the composition of 

 the light in terms of its constituent wave lengths. The quality of the light 

 coming from a tungsten bulb, for example, is relatively richer in infrared 

 radiations and relatively poorer in blue light than sunlight. "Duration" as 

 applied to the light-relations of plants generally refers to the number of hours 

 per day to which a plant is exposed to illumination. 



The Chloroplast Pigments. — Green is the distinctive color of the plant 

 kingdom. With only negligible exceptions all leaves are green in color as 

 are also many other plant organs such as herbaceous and young woody stems, 

 young fruits, and the sepals of flowers. The green coloring of plants is 

 olten termed simply chlorophjll, although actually, as we shall see shortly, 

 two chemically different chlorophylls can be extracted from plants. 



Less evident is the fact that the leaves and many other green organs 

 of plants also contain yellow pigments. These are seldom apparent except 

 in leaves in which the chlorophyll fails to develop or in which it is destroyed 

 as a result of senescence or internal physiological disturbances. Corn plants 

 that have grown from seed in a dark room, for example, do not synthesize 

 chlorophyll but are yellow in color because of the presence of 3'ellow pigments. 

 Similarly the leaves of many woody species become yellow in the autumn. 

 This autumnal change of leaf coloration is due to the disintegration of the 

 chlorophyll which results in an unmasking of the yellow pigments already 

 present. The yellow pigments found in foliage leaves are apparently all either 

 carotene or xanthophylls. Collectively these pigments, together with certain 

 others, closely related chemically, are called the carotinoids. 



