l6 PHYSIOLOGY OF NUTRITION 



nies the formation of chlorophyll. But another explanation is also possible: 

 strong light may not act directly upon chlorophyll that has already been formed 

 but may, somehow, have a harmful effect upon some process antecedent to 

 chlorophyll formation; this might explain why less chlorophyll accumulates in 

 strong light. 



Plants do not become green under the non-luminous heat rays. In order to 

 separate this portion of the spectrum, TyndalPs solution is used, iodine in carbon 

 bisulphide; in low concentrations the rays between Fraunhofer lines A and B 

 are transmitted, but these, produce no green color. In ultra-violet light green- 

 ing is very slight. 



The production of chlorophyll is dependent upon temperature. Medium 

 temperatures are most favorable, and no greening occurs at very low or at very 

 high temperatures. Wiesner obtained the following results from experiments 

 with etiolated barley seedlings. 



Time Required 



Temperature for Greening 



Deg.C. Hours 



2-4 (No greening) 



4-5 7 25 



10 3-5° 



18-19 l °7 



3° r S8 



37-38 400 



40 (No greening) 



The autumn coloration of leaves is dependent upon light and upon the tem- 

 perature of the air; chlorophyll is decomposed by sunlight in autumn, while its 

 re-formation is hindered by the low temperatures then prevailing. According 

 to Batalin, 1 the conifer Chamacy parts obtusa is especially interesting in this 

 connection. Branches in sunshine have a golden-yellow color in the cold sea- 

 son, while shaded ones remain green ; h at the margin between the shaded and un- 

 shaded regions the different colors may often be seen in neighboring cells. 



The products of chlorophyll decomposition do not remain in the leaf but dif- 

 fuse away. 2 This is shown by the following experiment: if an incision is made 

 in a leaf in the autumn, while it is still green, so that the chlorophyll decomposi- 

 tion-products are prevented from diffusing away, the part of the leaf above the 

 cut remains green while the other parts turn yellow (Fig. 9). 



The presence of iron is a third condition necessary for the formation of 

 chlorophyll. 3 Without iron, plants remain bright yellow, thus suffering from 

 chlorosis. 



1 Batalin, A., Ueber die Zerstorung des Chlorophylls in lebenden Organen. Bot. Zeitg. 32 : 433-439- 1874. 



2 Stahl, Ernst, Zur Biologie des Chlorophylls; Laubfarbe und Himmelslicht, Vergilbung und Etiole- 

 ment. Jena, 1909. 



3 Gris, Eusebe, Nouvelles experiences sur Taction des composes ferrungineux solubles, appliques a la 

 vegetation, et specialement au traitement de la chlorose et de la debilite des plantes. Compt. rend. Paris 

 19:1118-1119. 1844. Molisch, Hans, Die pflanze in ihren Beziehungen zum Eisen. Eine physiologische 

 Studie. Jena, 1892. 



h This may also be seen in the arbor vitse (Thuja occidentalis) of the northeastern United 

 States in very cold, bright winter weather. — Ed. 



