THE ADAPTATION PHENOMENA 419 



in contradiction with the relative prominence of the absorption maxima 

 of the two pigments in living algae, as illustrated by a figure in chapter 22 

 (Vol. II). (This figure refers to phycocyanin, rather than phycoerythrin, 

 but the pure red color of many Florideae indicates that these algae, too, 

 must contain much more than two molecules of the red pigment for 100 

 molecules of the green.) 



E. Influence of External Factors* 

 1. The Adaptation Phenomena 



Various external factors may affect the composition of the pigment 

 system. The two kinds of relationship which fall under this heading 

 can be called phylogenetic and ontogenetic, respectively. On the one hand, 

 the composition of the pigment system of a certain class or species is 

 related to the conditions under which it usually lives. For example, 

 red algae abound in the "blue-green shadow" deep under the sea, whereas 

 green algae predominate near the surface. Species with a high content 

 of chlorophyll prefer shady sites, while species with comparatively pale 

 leaves thrive in direct sunlight. These differences are usually treated as 

 "adaptation phenomena" — it is assumed that each class or species of 

 plants has acquired in its phylogenetic development the pigments which 

 are most suitable for its needs, in particular, for the most efficient ab- 

 sorption of light for photosynthesis. 



On the other hand, many plants are capable of individual variations 

 of the pigment system. Unicellular green algae, for example, Chlorella, 

 become deep green if grown in dim light, and light green if grown in 

 strong light. Red Florideae become olive-brown; or even green, when 

 exposed to direct sunshine. In some cases, these "ontogenetic" adapta- 

 tions are slow and permanent, in others, they are comparatively rapid 

 and reversible. The most striking case is that of certain blue-green 

 algae which change their color, chameleon-like, in response to variations 

 in the color of light (cf. pages 424-427). 



The occurrence of similarly rapid changes in the concentration of 

 chlorophyll in the higher plants is a matter of controversy. Willstatter 

 and Stoll (1918) found, in a much quoted experiment, that the chlorophyll 

 concentration and the [a] : [b] ratio remain unchanged after an exposure 

 of leaves to intense light for several hours (cf. Table 19.11). It is gener- 

 ally assumed, mainly on the strength of this result, that the pigment 

 system of the higher plants is invariable, except for the periods of rapid 

 growth in spring and decomposition in fall. Recently, however, Bukatsch 

 (1939, 1940) and Wendel (1940) have claimed that large diurnal variations 



* Bibliography, page 435. 



