ETIOLATION 113 



could be of advantage to living matter. The maximum would be 

 reached when the oxidizing effect of light exceeds the capacity of 

 the organism to repair the damage thus caused, or compensate 

 the material broken down. No actual maximal intensities have 

 been determined so far as this phase of the action of light is 

 concerned, although it is known that the blue-violet rays are 

 operative in producing such effects. 1 



173. So-called Rigor of Darkness. A number of lower forms 

 are known to become rigid and inactive when placed in darkness, 

 but actual observation of this phenomenon is mostly confined to 

 such forms as Oscillaria, and Bacterium photometricum. The 

 manifestations generally classed under the effects of darkness- 

 rigor in higher plants include a number of separate reactions. 

 Thus, for example, when a mature green plant is placed in a dark 

 chamber the periodic movements of the leaves soon cease, and 

 the tissues assume a pathological condition and die. Similar be- 

 havior is manifested by the same plants in an atmosphere free 

 from carbon dioxide in light, and hence may not be ascribed 

 directly to darkness-rigor. The death of leaves under both cir- 

 cumstances is due primarily to the destruction of chlorophyl, 

 causing a pathological condition of the mesophyl cells of the 

 leaves. Mimosa has been cited so much in this connection, that 

 it is proper to say that when a branch of this plant is allowed to 

 develop in darkness not only do the leaves assume a fairly nor- 

 mal stature but they also exhibit periodic motility and irritability 

 to shock and other stimuli. 2 



174. Etiolation. The development of plants in darkness is 

 characterized by alterations in form and structure, as well as in 



1 Ward, H. M. The action of light upon bacteria. Proc. Roy. Soc. 54 : 472- 



1894. 



2 Jost, L. Ueber die periodischen Bewegungen der Blatter von Mimosa pudica im 

 <3unkeln Raume. Bot. Zeitung. 55: 17. 1897. 



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

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



MacDougal. Relation of the growth of foliage leaves and the chlorophyl func- 

 tion. Jour. Linn. Soc. 31 : 526. 1896. 



