CHANGES TN THE AMOUNT OF WATER CONTAINED. 635 



in the intensity of" the hght. There is still, however, a very important point to be 

 observed here, namely that in darkness an increase of rigidity of the whole organ and 

 therefore an increase in the amount of water which it contains occurs, while on illumin- 

 ation a diminution of the expansive tendency of the parenchyma and thus a decrease 

 in the quantity of the water and rigidity of the whole organ takes place ; in the nocturnal 

 position, the entire organ is more rigid than in the diurnal position, as Brücke, and 

 subsequently Pfeffer, proved. Moreover, Millardet showed, and Pfeffer confirmed, that 

 in darkness an increase of turgescence appears simultaneously in the upper and lower 

 half of the organ, and in hke manner, on illumination, diminution of rigidity occurs in 

 both simultaneously; but although these changes occur simultaneously they do not occur 

 with equal rapidity on both sides of the organ. That side in which the increase of tur- 

 gescence results more rapidly thus first becomes convex, and the other side is passively 

 compressed by its extension ; meanwhile, however, the same change takes place, though 

 slowly, in the now concave other half, and this begins to extend and then presses the 

 organ over towards the other side. This process requires from a few to as many as 

 twelve hours. Then, according to Pfeffer's description, the reverse change begins 

 spontaneously : the side of the organ which has at last become concave, again becomes 

 convex, and this process also is then reversed, and this goes on, the movements be- 

 coming less and less marked, till after a few days the organ comes to a stand-still. 



This result, i.e. this long-continued after-effect, only occurs in an undisguised form, 

 however, when the plant remains for several days in the dark, after having been exposed 

 to the normal alternation of day and night, and has assumed its nocturnal position ; or 

 when, normally vegetating, it has assumed its diurnal position and is then constantly 

 illuminated for some days. In both cases the after-effect, swinging to and fro like a 

 pendulum, dies away, but with a different final effect in each case; for if the plant re- 

 mains in constant darkness until the after-effect ceases, it is then in a state of darkness- 

 rigor, and is no longer sensitive even to sudden strong illumination, though if con- 

 tinuously illuminated it may again return to the condition of phototonus ; if, on the 

 contrary, the movement ceases during constant illumination, the motile organs are at 

 once irritable for a subsequent darkening, and assume the nocturnal position. 



From the behaviour here described, as was first shown by Pfeffer, the ordinary 

 daily periodicity now arises, the sleeping and waking, the daily recurrent variations of 

 light combining with the after-effects — i.e. when the leaves have assumed their nocturnal 

 position in the evening, there still follows during the darkness of the night, as an 

 opposite effect, a tendency to assume the diurnal position. If then in the morning 

 the light falls on such a leaf, this also induces a tendency to the diurnal position: after- 

 effect and the influence of direct light thus combine, and since these changes in the 

 organs, which have produced the diurnal position, are now followed in their turn 

 again by the tendency to attain the nocturnal position coming in spontaneously, the 

 latter is aided by the darkness which comes on in the evening. It is clear that the 

 periodic variations caused by the after-effect need not always exactly coincide with 

 the daily variations in brightness, and that therefore several kinds of differences 

 in the combined actions must result. 



The above, then, embraces in the main the theory of the sleeping and waking 

 of periodically motile leaves which possess special motile organs. I may still add that 

 Pfeffer, by means of ingenious researches, has also measured the magnitude of the 



