^7Z 



find heliotropic curvature in young cress seedlings at a distance of one cen- 

 timeter, from a glass tube containing 5 g. of radium bromid. 



In bright sunlight, we find that parts of the plant often not only become 

 yellow but even turn brown and die^. That this dying is a specific light 

 action and not a result of too great an increase in temperature is proved by 

 the fact that the chlorophyll remains unchanged^ in temperatures var}dng 

 ■from 30 degrees below zero to 100 degrees above zero and, on the other 

 hand, that the destruction takes place with rays of shorter wave length 

 which influences most of all the processes of growth and protoplasmic 

 movement. 



The rays of a concentrated sun image, which have passed through 

 ammoniacal copper oxid often cause death after a few minutes, while the 

 same amount of light, after passing through a solution of iodine in carbon 

 disulphid (which lets only the outermost red rays pass through) scarcely 

 causes any destruction, or only a \tvy tardy one^. In this red light, how- 

 ever, an extensive warming takes place, but not in the blue light. 



Among the phenomena arising from an excess of light belongs also the 

 production of shadow pictures, i. e., intensive green pictures of overshad- 

 owing organs on a strongly lighted leaf surface. No destruction of the 

 chlorophyll apparatus necessarily takes place here, only a change in the 

 position of the chloroplasts is produced. 



Observations, made by Bohm, Famintzin, Borodin, Stahl and Frank,, 

 proved that, in sunlight too high for the special need of the plants, the 

 chlorophyll grains begin to move from the cell walls, parallel to the upper 

 surface of the leaf, towards the walls at right angles to them. The chloro- 

 plasts pass from the episfrophe to the apostrophe and thereby bring about 

 the lighter color of the too strongly lighted part. 



A further observation which can be made easily is the appearance of a 

 red color with too strong lighting in the green leaves of plants which turn 

 red in the autumn, as, for example, when the under sides of sweet cherry 

 leaves are turned uppermost. In the same way, a decided brownish red 

 color may be found in many plants, especially in those with fleshy leaves, 

 when brought in spring from the shaded conservatories into an open, sunny 

 place. Molisch* has investigated such cases. He proved in Aloe and 

 Selaginella that anthocyanin is not formed in the cells but that the chloro- 

 plasts themselves turn red and become green again when put in the dark. 

 In some varieties of Selaginella, red or brownish red chloroplasts were 

 observed, colored by carotin, especially above a place where the stem had 

 broken. 



The process most important agriculturally and most significant 

 hygienically, however, consists in the destructive action of the sunlight on 



1 Bohm, Versuchsstationen 1877, p. 463. 



2 Wiesner, D'e naturlichen Einrichtungen zum Schutze, des Chlorophylls. 

 Festschrift: cit. Bot. .Jahresber. 1876, p. 728. 



3 Pring-sheim. Jahrb. f. wiss. Bot. 1879, Vol. 12, p. 336. 



4 Molisch, H., tJber voruberg-ehende Rotfarbung der Chlorophyllkorner in 

 Laubblattern. B. der Deutsch. Bot. Ges. 1902, XX, p. 442. 



