468 The Physiology of Plants BOOK in 



carbon disulphide, which is absolutely opaque to the rays 

 of the visible spectrum. 



Wiesner examined also the dependence of the effect upon 

 the intensity of the illumination, and determined the upper, 

 lower, and optimum degrees of the latter. He said that its 

 lower limit coincides with the lower limit of light-intensity 

 which causes the stoppage of growth in length, while its 

 upper limit is only occasionally the same as that which 

 affects the latter process. He found, too, that the relative 

 efficiency of the several rays is not the same for all plants. 



In 1876 Kraus showed that the yellow and blue rays 

 have almost equal effects on the stalks of the perithecial 

 heads of Claviceps. Brefeld confirmed this observation in 

 1881. Sachs stated in 1882 that he found the exciting rays 

 extend from the ultra-violet to the blue. 



The position assumed by the various organs of the plant 

 was examined by Frank in 1870 and by Sachs in 1873. 

 They agreed that the final result of the stimulation is that 

 plants take up a definite position which has been called 

 a fixed light position. Wiesner showed that this position 

 is determined in the case of most leaves by the direction 

 of the incidence of the brightest diffuse daylight and not 

 by that of direct sunlight. In 1880 Darwin ascribed the 

 attainment of that position to a modification of the move- 

 ment of circumnutation. Indeed, the view that this move- 

 ment is universal in all growing organs and serves as a 

 basis for the movements of curvature under the various 

 conditions of stimulation which such organs encounter is 

 the main thesis of the Power of Movement in Plants, pub- 

 lished in that year. This hypothesis met with considerable 

 opposition from Wiesner. 



The influence of direct sunlight in causing certain plants 

 to take up a fixed light-position was shown by Stahl in 

 1881 in the case of the so-called Compass plants. 



Certain investigations into some of the less conspicuous 



