OF MOVEMENTS IN PLANTS. 81 
noted in those leaves which are highest, so in the awakening the same fact is conspicuous. 
Important as these facts are to the general question of nyctitropie movement, we can 
only introduce them incidentally at this time, since it is not our present purpose to 
determine the precise influence of external causes upon the processes of growth whereby 
these changes are effected, but simply to determine the mechanism of movement through 
the various tissues involved. Weare, therefore, more intimately concerned in considering 
the various changes which occur in the leaf and leaflet during the process of sleeping and 
waking. 
As the period of sleep approaches, the most conspicuous indication is to be found in 
the change of position which the leaflets assume. From a horizontal or slightly elevated 
position, they gradually droop, until they assume a position at right angles to their 
normal diurnal position ; thus, assuming the leaf as a whole to be horizontal, each leaflet 
becomes vertical. Two important facts are here to be noted, viz., the relation of these 
changes to gravitation, and the indications they give of the operation of an active force. 
Whatever the position of the leaf as a whole may be, the leaflets are found to be 
influenced in certain directions by the action of gravitation upon their mass. Thus, if the 
leaf as a whole be horizontal, the drooping leaflets will finally assume a position perpen- 
dicular toits length. If it be raised or depressed above or below the horizontal, the leaf- 
lets no longer hang perpendicular to the leaf—with reference to its length—but fall 
vertically. Thus each leaflet is seen to turn laterally upon its petiolule as an axis, in 
direct response to the influence of gravitation upon its mass, and in this respect it is inde- 
pendent of the position of the leaf as a whole. In harmony with this, it will always be 
found that, in petiolules of a depressed leaf, there is a distinct torsion conforming to the 
relation which the leaflet bears to its main rachis. In leaves which hang almost vertically 
from drooping limbs, the leaflets thus often come to lie nearly parallel with the rachis. 
When the leaflets fall into the sleep position, there is always a strong tendency for 
them to pass by the vertical plane passing longitudinally through the leaf; or in other 
words, the movement of the leaflets, with respect to the width of the leaf, is mdependent of 
gravitation. As the sleep movement progresses, the leaflets of each pair hang quite parallel, 
being separated throughout by a distance of 5 or 8 mm., representing the combined 
width of the rachis and length of the two petiolules. Soon, however, each leaflet bends 
in at the tip toward the other, so that they finally touch. This is effected in part by 
a curvature throughout the entire length of each leaflet, but much more by a continued 
curvature of the pulvinus of the petiolule, the result being that each tip is carried several 
millimetres beyond that point which would be determined by gravitation alone. Remov- 
ing the opposite leaflet of each pair, does not seem to affect the movement of that remain- 
ing. In the terminal leaflet, the sleep position is assumed precisely as if it were a lateral 
leaflet, with the difference that its change of position is much greater, and it is carried 
much farther past the vertical. Without any regard to the position of the leaf as a whole, 
the terminal leaflet drops until it forms an acute angle with the rachis on its lower side. 
If the leaf be horizontal, the terminal leaflet bends several degrees past the vertical. If, 
however, the leaf be drooping, then the leaflet still establishes the same relation to the 
rachis, and thus often becomes horizontal or even turns up past the horizontal. In the case 
of leaves which were hanging vertically, this reflex position of the terminal leaflet was 
often found to be 10° above the horizontal, or about 100° from the position which would 
Sec. IV., 1886. 11. 
