8oO PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



rigidity in the nocturnal position. Conversely the diurnal position caused by the admission 

 of light or an increase in its intensity is the result of a diminution of the rigidity or tur- 

 gidity ; but it is then the side of the organ which becomes concave by day (/ e. the upper 

 side in the large organs of the petiole of Mimosa, the under side in Phaseolus) that loses 

 water and contracts. The causes of this phenomena are however unknown. Increase 

 of temperature, on the contrary, which affects the motile part directly, is, according to 

 Pfeffer, associated, in Oxalis, and in a less degree in Phaseolus, with increase of rigidity, 

 and therefore also of turgidity, and causes a movement towards the nocturnal position, 

 and hence a stronger turgidity of the upper side. 



When, on the other hand, an increase in the intensity of the light and a rise of 

 temperature act on a contractile organ at the same time, its curvature is a resultant of 

 the two changes ; according as the one or the other preponderates, the leaf approaches 

 more nearly the diurnal on the nocturnal position. 



But the quantity of water contained in the whole plant, and therefore to a certain 

 extent that contained in the motile part, depends in addition on the relation between 

 transpiration and the activity of the roots. If, for example, the transpiration in the night 

 is small but the roots very active in moist and warm soil, the quantity of water in 

 the plant will gradually increase and its motile organs will be able to become more 

 strongly turgid, but at the same time more rigid ; and when one side of the organ (in 

 the petiole of INIimosa the under side) thus becomes the longer one, a curvature results 

 towards the opposite side (in Mimosa therefore upwards) ^ Conversely the increase of 

 transpiration when the roots are not sufficiently active will tend to diminish the 

 quantity of water in the contractile organ, and therefore in general to induce the 

 nocturnal position. These processes must act in combination with the direct effects 

 of light and temperature on the contractile organs ; and thus, under normal conditions, 

 where these external agencies are subject to continual variation, the motile parts seldom 

 come to rest, even independently of the internal causes which induce spontaneous 

 periodic motion. 



The movements of Mimosa pud'ica which take place under the combined conditions 

 of the normal alternation of day and night have been more accurately investigated. Its 

 leaflets remain closed during the whole night, but are usually open in the day ; but the 

 primary petioles are in perpetual motion day and night. To the exhaustive observations 

 of Bert, and especially to those of IMillardet, we are indebted for a knowledge of the fact 

 that the contractile organ of the petiole, after it has bent strongly downwards in the 

 evening, begins to become again erect before midnight, and continues to become more 

 and more so until the most erect position has been reached before sunrise. At sunrise 

 it begins to be suddenly depressed, while the other parts of the leaf assume their ex- 

 panded diurnal position. This depression of the petiole continues to increase till even- 

 ing, the lowest position being reached when complete darkness commences ; and the 

 other parts then also assume their nocturnal position. The depression of the primary 

 petiole during the day, and the corresponding movements of the other parts of the leaf, 

 are interrupted by a slight rise both in the forenoon and afternoon. 



In this diurnal and nocturnal periodicity a depression of the petiole also follows the 

 access of light, the same phenomenon taking place also in the middle of the day if the 

 light is suddenly withdrawn. The rising of the petiole does not proceed with equal 

 rapidity as the intensity of the light increases and diminishes, as might be expected from 

 what takes place when light is admitted. Finally, we still need an explanation of the 



1 Millardet (Z. c. p. 46 et seq.) has already pointed out what has here to be especially kept in 

 view, that in Mimosa any change in the tension of the tissues {i. e. every alteration of turgidity) in 

 the contractile organ of the primary petiole takes place more strongly on the irritable under side 

 than on the upper side, and that the periodic movements depend mainly on this. Where the 

 position during sleep is erect, as in the leaflets of Mimosa and Trifolium, this must be supposed to 

 be the case on the upper side. 



