CHAP, x The Problems of Growth 453 



In 1892 Marshall Ward found that such exposure 

 is fatal to many species of bacteria, a discovery since 

 confirmed by Fliigge in 1896, and by Arloing, who ascer- 

 tained that the vegetative cells have a greater power of 

 resistance than the spores. The present writer found, in 1897, 

 that the more refrangible rays of the spectrum, together 

 with the ultra violet, cause the decomposition of diastase. 

 Klemm showed that the more refrangible rays stop the 

 streaming of protoplasm and tend to bleach the chloro- 

 plasts, besides destroying various pigments in the cells. 

 On the other hand some plants are quite unaffected by 

 brilliant light ; such is the case according to Tammes, 

 (1900), with mosses and lichens growing on rocks. 



The periodic movements of certain plants, known as 

 nyctitropic movements, which are associated with the 

 alternation of day and night, were studied by many 

 observers after 1860. The assumption of the nocturnal 

 position by the leaves of Mimosa was shown as long ago 

 as 1848 by Briicke, and even earlier by Dutrochet, to be 

 of different origin from the movements carried out by 

 them in response to mechanical stimulation. In 1869 

 Millardet ascertained that the movement leading to the 

 nocturnal position and recovery therefrom is not a simple 

 rise and fall, but is the expression of a periodicity or rhythm. 

 After its first fall, when it reaches the nocturnal position, 

 the leaf slowly rises to an acute angle with the stem, and 

 then as light dawns it sinks slowly to the day position. He 

 correlated this periodicity with a daily rhythm of tension 

 in the plant, and inferred that the primary petiole is not 

 nyctitropic, the fall being due to the nyctitropic change 

 in position of the secondary petioles and leaflets. Pfeffer 

 investigated the two kinds of movement in 1875, and showed 

 that the periodic change can be distinguished from the 

 response to stimulation, the mechanism being different in 

 the two cases. 



