82 PENHALLOW ON MECHANISM 
be established by gravitation alone. These facts, therefore, show that the true sleep move- 
ment is not passive, similarly to that previously discussed, but that it is due to an active 
force, the measure of which is partly expressed in the degree to which it overcomes 
gravitation. 
In assuming the sleep position, each leaflet droops: first, by curvature of the petiolule 
through its whole length ; secondly, by a sharper bending at the junction of petiolule and 
leaflet ; thirdly, by a slight curvature through the entire length of the leaflet itself. In 
this connection it is important to note that, when leaflets are removed by cutting away 
at the extreme base of the petiolules, the latter almost immediately curve, the curvature 
conforming to that which is produced during the normal sleep movement. There is, in 
all this, a strong indication that the change is due to release of tension in the tissue of the 
pulvinus. 
In the leaf as a whole, there is comparatively little movement. Darwin’ has shown 
that there may be an actual elevation during sleep, to the extent of 3° or 4. Our own 
observations show, and probably with greater frequency, a depression of the whole leaf to 
the extent of 35° or 50°. The same change may sometimes be induced by irritation, 
occasionally in a more marked degree. In all such changes of position, they appear to be 
accomplished at the extreme base of the pulvinus which thus acts asahinge. As the leaf 
drops, the central ridge on the lower side of the pulvinus recedes slightly, the cushion 
around it becomes somewhat wrinkled, but on the upper side the pulvinus is drawn 
quite tense and smooth. It is important here, to note the difference between the pulvinus 
of the leaf and of the leaflet during the sleep movement, as it will be found to be 
correlated in a most significant manner, to the internal structure in each case. 
CONCLUSION. 
The deductions which can reasonably be based upon the foregoing facts, may be briefly 
stated. 
By comparison with Cucurbita and Vitis, the absence of any marked sensitiveness in 
Robinia, would imply the absence of a tissue in which variation of tension under external 
irritation is a special function. This we find quite in accord with the presence of 
collenchyma in the former, its absence in the latter, and the relation which it bears to the 
sensitiveness of the organ itself. Whatever transmission of impulse there may be, can be 
readily determined as in the previous cases through the continuity of protoplasm. 
In the leaf, the soft tissue of the pulvinus proper is that in which the variations of 
tension under external influences is determined. Moreover, the fact that this tissue is 
greater below than above the centre, points to its serving as the true erectile tissue 
wherever its internal tension is augmented sufficiently—becoming simply passive when its 
tension is reduced below a certain point. This is a more important factor in the pulvinus 
of the leaflet than in the large pulvinus, since the changes in the leaflet are greater, 
and require a relatively greater erectile force. As the pulvinus determines the upward 
movement, the included fibrous elements determine the downward and reflex movements. 

2 Movements of Plants, 355, 
