420 THE PIGMENT SYSTEM CHAP. 15 



in the concentration of chlorophyll (doubling or trebling in the course 

 of 2-3 hours!) may occur in many species, particularly those of alpine 

 plants (e. g., Rumex alpinus, and Adenostyles alhifrons). The alpine 

 species showed a minimum of chlorophyll at midday and a maximum in 

 the morning, while lowland plants contained more chlorophyll in the 

 middle of the day. Seybold (1941) doubted the correctness of Bukatsch's 

 measurements because of their contradiction with the above-mentioned 

 result of Willstatter and Stoll. Wendel has denied that such a contra- 

 diction exists. He asserted that the "diurnal rhythm" can be observed 

 also if the plants are kept in artificial darkness, so that this phenomenon 

 (if at all real) is not directly related to photosynthesis but represents 

 an example of diurnal periodicity in biochemical processes which defies 

 explanation in terms of simple photochemical effects. (A similar example 

 was encountered in chapter 10, when the periodic acidification of succu- 

 lents was mentioned; and another will be met in volume II, chapter 26, 

 when dealing with the so-called "midday depression" of photosynthesis). 

 All adaptation phenomena — whether phylogenetic or ontogenetic — 

 must be based on the effects of external factors on the rates of formation 

 and decomposition of the pigments. The stationary concentration of a 

 pigment — and of any other component of the living cell — is the result 

 of a balance between formative and destructive processes. If a "sun 

 plant" contains less chlorophyll than a "shade plant," this means that, 

 in the first organism, this pigment is formed more slowly (or destroyed 

 more rapidly) than in the second one. However, we know as yet very 

 little about the chemical mechanism by which the pigments are synthe- 

 sized in the plants (c/. page 404) and next to nothing about the mechanism 

 by which they are decomposed. We shall therefore treat the adaptation 

 phenomena from the usual " teleological " point of view, that is, we shall 

 consider that we have "explained" a certain variation in pigmentation 

 if we can point out the advantage to the plant which may accrue from it. 



2. Phylogenetic Adaptation of the Pigment System 



The concept of phylogenetic adaptation of plants to the prevailing 

 intensity and color of light has its origin in observations of the vertical 

 distribution of marine algae, which is characterized by the predominance 

 of the green Chlorophyceae in shallow waters and of the red Florideae 

 in deep waters, with the brown Phaeophyceae in an intermediate position. 

 Engelmann (1883, 1884) attributed this distribution to the adaptation 

 of the algae to the predominant color of the light. Sunlight becomes blu- 

 ish green after passage through several meters of water (cf. Vol. II, Chap- 

 ter 22). Consequently, plants living deep under the sea do not receive 

 much Ught which could be absorbed by green chlorophyll (or by yellow 



