78 PENHALLOW ON MECHANISM 
while for the same period the diameter varies from 1.0 mm. to 1.5 mm. These dimen- 
sions are subject to much less variation, as dependent upon conditions of growth, than in 
the main pulvinus. Externally, each little pulvinus is minutely and somewhat densely 
pubescent, a character which at once distinguishes it from the large pulvinus. Both the 
superior and inferior terminations are devoid of lateral ridges or depressions, nor is there 
any intercellular cavity. In these facts, there appears a very strong argument in support 
of the supposed mechanical importance of these structural features. The structure, in 
all its parts is continuous throughout, so that in most respects relating to their grosser 
anatomy, the larger and smaller pulvini are quite distinct. 
In its internal structural features, the epidermis is simple and the cuticle thin. The 
tissue of the pulvinus is the same as that in the large pulvinus, though not so strongly 
developed as a whole. (Plate V. Fig. 5.) The important feature of the organ, as a whole, 
is the peculiar form of aggregation of the tissues and their mutual relations. The 
transverse section is nearly round, its vertical diameter exceeding its transverse in the 
ratio of 1: 1.06. The wood and bast, instead of forming closed zones, are open along the 
upper side, where the pith blends with the tissue of the surrounding pulvinus. This 
peculiar arrangement bears an important relation to the flexibility of the petiolule as a 
whole, and corresponds with the fact that curvatures of this organ are downward and 
not upward. Determining the distribution of similar tissues, as in the previous case, with 
reference to the true centre, we find the following :— 
UrPPer Sipe. Lower Sins. 
MWoodhand iBast-------- "rec 1.9 Sq. cm. 6.6 Sq. em. 
Parenchyma of Pulvinus ........... Ges) r pairs}, | 
Assigning the same function to these tissues in their relations of mutual tension, as 
in the case of the main pulvinus, it would appear that the influence of the special tissues 
concerned, in promoting elevation or depression of the leaflet, must be exerted between 
the upper and the lower side of the pulvinus in the following ratios :— 
Urrrr. Lower. 
Woodtand Bastercmataseticeetireistelsiensielsisrelorratterers ih RB" 87 
Parenchyma------c---h-e-e----..--re-béeert-e LC TE? 
Comparing these values for those obtained in the previous case, it appears that in the 
lower side of the pulvinus the vascular elements exert a much stronger influence asa © 
contractile tissue, while the parenchyma exerts a much weaker influence as an erectile 
tissue. Inharmonious with our previously expressed views as this at first sight appears 
to be, it really offers no ground of conflict if we bear in mind the relative size of leaf 
and leaflet, and thus realize the very inferior influence of gravitation upon the latter. 
The intercellular cavity which appeared in the large pulvinus is, in the smaller organ, 
replaced by the peculiar development of the vascular structure. The open pith which 
appears along the upper side permits a greater extension of parts in that region, as the 
pulvinus curves downward. 
During the period when the leaflets are folded, just as they emerge from the bud 
and for a short time afterward, they manifest no nyctitropic movement. During that 
