CHAP, v LIGHT 21 



high latitudes ; x while a whole series of heliophytes (Myrtus bullata, 

 Perilla nankinensis, Prunus Pissardi and others) growing naturally possess 

 the dark-red to blackish-red tints characteristic of the familiar copper- 

 varieties of the beech, hazel, and other trees. 



The features described above will be treated in greater detail in 

 subsequent chapters that deal with xerophytes. 



That light is of great significance in influencing the external and 

 internal construction of plants is beyond doubt. This follows, not only 

 from what has been already said, but also from the fact that many, 

 perhaps most, plants can adjust their anatomical structure, especially of 

 their leaves (' plastic leaves '), according to the intensity of the light. 

 A beech-leaf exposed to sunlight is structurally different from a beech-leaf 

 in the shade. 2 The arrangement and movements of chloroplasts in cells, 

 and therefore the tint of foliage, depend upon the light ; 3 stronger light 

 causes the leaf to be paler in tint, weaker light causes it to become darker 

 green. As to the exact method in which light acts physiologically our 

 notions are very hazy. Some (Stahl, Pick, Mer, and Dufour) opine that 

 it is light itself which determines, according to its intensity, the above- 

 mentioned structural differences in the chlorenchyma ; but these investi- 

 gators fail to explain how light acts. Others (Areschoug, Vesque and 

 Viet, Kohl, and Lesage) suggest that the cause may be increased transpira- 

 tion due to increased light. Still others (Wagner and Mer) are inclined 

 to lay chief stress upon the strong assimilation following upon more 

 intense light. 



The action of light of different composition upon the activity of proto- 

 plasm and the arrangement of chloroplasts is treated in papers by Sachs 

 and Kissling. 4 



It is scarcely open to doubt that the structural differences between 

 heliophytes and sciophytes must be regarded as affording an example 

 of self-regulation (direct adaptation 5 ) on the part of the plant. We see 

 this taking place before our eyes in plastic plants which adjust their 

 structure to light ; opposed to this are other cases in which the structure 

 probably has been modified during the course of phyletic development 

 and become fixed by heredity in successive generations. Among the 

 uses of the various structural features are the following : Protection 

 of the chlorophyll from decomposition by intense light, 6 protection of 

 the protoplasm itself (intense light can injure protoplasm, as is demon- 

 strated by its destructive action on bacteria, its use as a means of disin- 

 fection, and so forth), protection against excessive transpiration, and 

 regulation of assimilation. When we consider that the volume of the 

 palisade tissue is increased not only by more intense illumination, but 

 also, as research has proved, by stronger transpiration, as well as by 

 various factors (salts in the substratum, injury to the roots, and so forth) 

 that influence the absorption of water from the soil and consequently 

 affect transpiration ; and when we further consider that the palisade 

 tissue increases in all stations where great atmospheric aridity prevails, 

 then we shall be inclined to regard regulation of transpiration as the most 



1 Bonnier et Flahault, 1894 ; Schiibeler, 1886. 



2 Stahl, 1880, 1883 ; Hesselman, 1904 ; Woodhead, 1906. 



3 Stahl, 1880, and others. * Kissling, 1895. 

 6 See Section XVII, Chapter C. Wiesner, 1876. 



