OF MOVEMENT IN PLANTS. 75 
bita are the same, and that they may be regarded as representing a particular class of 
movements, so far as their mode of production, as well as their general external features, 
are concerned. It is not as yet possible to say how far each of these may represent the 
type for the family ; but from the similarity of the structure and circumnutation presented 
by Sicyos and} other Cucurbitaceous vines, it is perhaps safe to infer that, in that case, 
Cucurbita is the type of the family. 
The deductions which the preceding facts justify are as follows :— 
1. Movements of circumnutation arise through unequal growth of the tissues, which 
is chiefly represented by the vibrogen bands. 
2. The bands of more active growth are strictly localized. 
3. Movements due to irritation depend upon continued elongation of the opposite 
side, together with cessation of growth and contraction in the irritated parts. 
4. The collenchyma tissue is that which is chiefly concerned in variations of tension 
under mechanical stimuli. 
III—ROBINIA PSEUDACACIA, L. 
In the Robinia, there is not only an entirely distinct variety of motion, but also a 
motile organ which differs widely in many respects from the plants that we have 
previously considered. In this case, the special organ endowed with motion, is the leaf, 
which, instead of serving as a prehensile organ, is invested with the power of movement, 
for reasons directly connected with its own preservation against sudden and extreme 
atmospheric changes. Unlike tendrils, therefore, such motile leaves are found to present 
certain periodic changes of a most conspicuous character. They are, moreover, in most 
cases, supplied with æspecial cushion or pulvinus, through which the motion is primarily 
determined. As a whole, such movements present a certain relation to those already 
discussed, in that they may be regarded as modified circumnutations.' 

PULVINUS OF THE LEAF.—Each pulvinus surrounds the base of its corresponding 
petiole as a cushion, conspicuously larger below than above. It extends upward from 
the point of insertion of the petiole, for a distance of 4.5 mm. to 7.0 mm. Its diameter is 
variable, increasing with age of the leaf, but apparently much more dependent upon the 
rankness of growth in the plant as a whole, since the largest pulvini are invariably found 
upon rank growing suckers. Under these circumstances, the diameter has been found 
to vary from 3.0 mm. to 5.0 mm., the mean size being not far from 4.0 mm. Externally, 
with a smooth and shining surface and very firm throughout, it possesses all the features of 
hightension. Of uniform size throughout, its strongest development is on the lower side 
of the petiole, while above it often but slightly exceeds the petiole itself. At the base on 
the lower side, just at the point of insertion, there are two triangular depressions in the 
pulvinus, formed by three ridges, one of which is central and strongly developed, while 
two are lateral and less strongly defined. All these ridges extend downward from the 
base of the pulvinus for some distance on the stem, and serve an important mechanical 
purpose, as braces or supports to the leaf. Directly interior and corresponding to the 

1 Darwin, Movement of Plants, 280, etc. 
