422 THE PIGMENT SYSTEM CHAP. 15 



pigments with increasing depth — a typical ontogenetic intensity adapta- 

 tion; while the comparison of species usually found at different levels 

 shows an increase in the ratio of [phycoerythrin]: [chlorophyll] with 

 increasing depth of their habitats — a typical example of phylogenetic 

 chromatic adaptation. 



Among land plants, intensity adaptation is more important than 

 chromatic adaptation because variations in the intensity of light are 

 more pronounced than those in its spectral composition. Correspond- 

 ingly, the land plants have not much means to change their color — 

 variations in the ratio of [a] : [b] or of [carotenoids] : [chlorophyll] can 

 cause a minor change in the absorption spectrum of the leaves, but can 

 have no color effects comparable with those caused by phycocyanin, 

 phycoerythrin, or even fucoxanthol in algae. 



The light intensity adaptation of land plants reveals itself in the 

 existence of shade and sun species ("ombrophihc" and "heliophiUc" 

 plants). Lubimenko (1905, 1907, 1908, 1928) first pointed out the dis- 

 tinction between these two types, both in the structure of their leaves 

 and in the kinetic properties of their photosynthetic apparatus. Ombro- 

 phihc leaves are thinner and their chloroplasts are larger and richer in 

 chlorophyll, so that the average concentration of chlorophyll is higher 

 despite the more numerous chloroplasts of the heliophiUc plants. Ex- 

 amples of typical ombrophihc plants are Aspidistra elatior, with a chloro- 

 phyll content of 0.40% of the fresh weight of the leaves, Tilia parvifolia 

 with 0.44%, and Theohroma cacao with 0.79%; typical heliophihc plants 

 are Larix europea with 0.12%, and Pinus silvestris with 0.11% chloro- 

 phyll. In table 15.1 Lubimenko's figures were quoted for the average 

 chlorophyll content of several hundred species in the tropics, subtropics 

 and temperate zones and the wider spread of the individual values in 

 the tropics was pointed out. Lubimenko ascribed this to the extreme 

 differences in the intensity of illumination to which plants are exposed 

 in direct tropical sunlight and on the floor of the tropical forest. 



The pigment systems of plants associated with sun-exposed and 

 sheltered sites were investigated by Harder (1933), Harder, Simonis, and 

 Bode (1938), Seybold and Egle (1937, 1938^), and Egle (1937). These 

 workers found that the leaves of "shade plants" contain not only more 

 total chlorophyll but also relatively more chlorophyll h, and interpreted 

 the latter phenomenon as a chromatic adaptation. 



It will be shown in volume II, chapter 22 that chlorophyll b can 

 improve the utilization of light between 450 and 480 mn; light of these 

 wave lengths is comparatively abundant in the ''blue-green shade" of 

 overhanging fohage. Egle (1937) called chlorophyll b "the typical 

 shadow pigment." Seybold and Egle (1938, 1939) found the average 

 values of the ratio of [a]: [b] given in table 15. VIII. 



