OF MOVEMENT IN PLANTS. GL 
the parts by means of the camera, under an amplification of 20 diameters. A line 
representing the minor axis was then passed transversely through the true centre, and all 
tissues lying above and below measured by means of a planimeter. The results were as 
follows :— 
UPPER Sipe. Lower Sie. 
JES) popage Looneo CCDponec 1.50 Sq. em. 1.50 Sq. cm. 
WoodandiBastien-reeerec-cc rte 16:50) 5 a 28 00 
Parenchyma of Pulvinus........... 80.65 “ “ 11725 le GG 
From this it appears that, leaving the pith altogether out of consideration, as of no 
mechanical importance, whatever tension is produced in the vascular bundles, as opposed 
to the tension in the surrounding pulvinus, must be developed above and below the true 
centre in the proportion of 1 : 1.39. Since the vascular elements are of the nature of per- 
manent structure, their tension in relation to surrounding parts must be positive, and any 
general release of tension must result in a contraction of the organ through the vascular 
region. It therefore follows that this contraction must be stronger along the lower side 
of the pulvinus in the ratio given, and hence a tendency to curvature of the pulvinus 
downward. This, though slight, may often be noticed. 
The tension in the tissue of the pulvinus, as opposed to that of the vascular structure, 
is developed above and below the true centre in the proportion of 1: 2.12. The paren- 
chyma tissue of the pulvinus is that capable of the greatest and most continued growth, 
as also that in which the greatest variations of tension must occur through variable turges- 
cence. Its tension, with relation to the vascular structure, must be negative; hence any 
release of tension must permit contraction of the whole organ, while increase of tension 
must tend to elongate the pulvinus, and this action will be developed above and below 
in the ratio given. From this it is obvious that elevation and depression of the leaf, as a 
whole, depend respectively upon the pulvinus proper and the enclosed vascular structure. 
From what has thus far appeared, we are doubtless prepared to gain a true explana- 
tion of the large intercellular cavity and its external braces, which occur at the base of 
the pulvinus. The three braces, of which the central has been seen to be the largest, must 
doubtless be regarded, first of all, as means of mechanical support through the firmness of 
their structure ; while the pulvinus proper, which still surrounds each, seems to control 
changes of position to a certain extent, by its variable tension. The depression of the leaf, 
under any circumstances, however, must cause a much stronger compression of the 
structure on the under side of the pulvinus, where the flexure occurs, than elsewhere ; 
and this is at once compensated for by the large intercellular cavity, which permits the 
central brace to bend into it, the leaf thereby hinging chiefly upon the upper side of the 
pulvinus. This view gains additional weight from the fact that, while very slight 
curvature may arise through the whole length of the pulvinus, the depression of the leaf 
is chiefly accomplished by sharp bending at the extreme base of the pulyinus, which thus 
becomes the true joint or hinge. 
PULVINUS OF THE LEAFLET.—The pulvinus of the leaflet bears but little external 
resemblance to the main pulvinus. It is of uniform width, and extends the entire length 
of the petiolule, being but slightly flattened along the upper side. The length varies— 
between leaves just unfolding and in their mature state—from 5.0 mm. to 6.0 mm., 
