594 LECTURE XXXIV. 



days ; rigor due to darkness sets in. If then a plant which has passed into the 

 state of rigor due to darkness is again placed in the light, the motile condition 

 reappears after several hours, or, according to circumstances, only after several days' 

 exposure to the light. 



Very profound darkness is by no means necessary for inducing this condition 

 of rigor, however; on the contrary, it sets in when a plant which requires much 

 light, as Mimosa, remains exposed for several days to deficient illumination, such as 

 prevails at a distance from the windows in the interior of an ordinary dwelling-room. 



In contrast to the rigor due to darkness I have designated the normal condition 

 of motility caused by the alternation of day and night as Phototonus. From what has 

 been said, then, such a plant after it has been placed in the dark is found for some 

 time longer (several hours or even days) in the condition of phototonus, which then 

 gradually disappears ; in the same way the plant under normal vital conditions is in 

 the condition of phototonus during the night. A plant which has passed into the 

 state of rigor due to darkness, on the other hand, retains this rigor for some time 

 (hours or even days) after being placed in the light. Both conditions of the plant 

 therefore pass over into one another only slowly. 



Even on the setting-in of rigor due to darkness, in the case of jSIimosa the 

 irritability for shaking disappears first, then the periodically spontaneous movement. 

 In like manner a Mimosa which has passed quite into the state of darkness-rigor 

 regains first its periodic movements, and then the irritability. 



The position of the various parts of the leaves of Mimosa in rigor due to darkness 

 is different from that produced by darkness on phototonic plants, and even diff"erent 

 from that in heat-rigor : in rigor due to darkness the leaves are fully expanded, the 

 secondary petioles directed downwards, and the primary stalk almost horizontal. 



Alterations in the intensity of the light act as stimuli to movement only on healthy 

 plants which are in a state of phototonus: leaves in a state of rigor due to darkness do 

 not react to variations in the intensity of the light, until owing to long-continued illu- 

 mination they have regained the phototonus, when they then become stimulated to move- 

 ments by changes in the intensity of the light. I convinced myself of this in the case 

 of Acacia lophaniha. A specimen had been left for five days in the dark, where 

 for forty-eight hours it had given up nearly every trace of its spontaneous periodic 

 movements. It was then placed in a window where, the sky being cloudy, it 

 placed its leaflets decidedly downwards within two hours, and then small changes of 

 position took place in the secondary petioles also ; in this condition, however, the 

 plant was nevertheless in a state of darkness-rigor, for when it was placed in the 

 dark about 12 o'clock (noon) with another plant of the same kind which was in 

 phototonus, its leaves did not change their position, and its leaflets remained open, 

 whereas the other one within an hour assumed the most pronounced nocturnal 

 position and its leaflets closed. Both were then placed in the window, where the 

 plant in a state of dark-rigor opened its leaves in one hour, the sky being cloudy. 

 In the evening of this day the lower six leaves remained still rigid and open, but the 

 upper eight or nine leaves were closed : next morning, however, all the leaves again 

 expanded to the normal diurnal position. Trifolium iticarnaium behaves in a similar 

 manner, although differing in details. 



It is to be noticed that in the case of the plants which I have observed, the 



