142 



RELATIONS OF PLANTS TO LIGHT 



FIG. 73. 



directly concerned with low tempera- 

 ture, and rapid radiation of heat from 

 the tissues of the plant. Plants have 

 been subjected to the diurnal fluctua- 

 tions of light and temperature so long 

 that a rhythm of action is set up that 

 persists for several days, even when 

 Mimosa pudica. the plant is placed in continuous dark- 



Normal position of leaves. neSS Or light. 1 



216. Formative Influence of Light. 



Light has a most notable influence 

 in the determination of the external 

 form of a large number of plants. 

 One phase of this action has al- 

 ready been discussed under Etiola- 

 tion in which the reactions of the 

 plant in total darkness are discus- 

 sed. The dorsiventrality of prothal- 

 lia, and shoots in general is due to the 

 action of this agency. The develop- 

 pudica. ment of certain tissues or organs on 

 one side of the axis of a shoot, and 

 their suppression on other parts of 

 the body, may be regulated by illu- 

 mination which, in consequence, gen- 

 erally flattens and increases the sur- 

 faces devoted to the exposure of 

 chlorophyl. Different developmental 

 stages of an organism find their opti- 



FIG. 75. Mimosa pudica. mum in different intensities of illu- 

 Position of leaves at night. mination, and if these are not fur- 



1 Jost, L. Ueber die Abhangigkeit des Laubblattes von seiner Assimilations- 

 thatigkeit. Jahrb. Wiss. Bot. 27 : 403. 1895. 



FIG. 74. Mimosa 

 Noonday position of leaves. 



Jost, L. Beitrage zur Kenntniss der nyctitropischen Bewegungen. 

 Bot. 31 : 345- 



Jahrb. Wiss. 



