1894. ] Comparative Histology of Pulvini. 487 
vinus and pass through that organ as a single cylinder, sepa- 
rating again as soon as beyond the limits of the motor organ, 
one is again impressed by the mechanical advantage which is 
gained by this course. As shown in the cross section the 
central meristele is kidney shaped in agreement with the form 
of the organ and is entirely surrounded by a cylinder of bast 
fibers. (The outline is shown in fig. 4.) The cells with, the 
thickened walls at the hilum, which are considered to repre- 
sent the remnant of the pith, have very much thicker walls 
than the remainder of the bast fiber layer which is made up 
of the bast fibers belonging to the phloem of the bundles. As 
seen in the longitudinal section the cells at the hilum are 
found to be short fusiform cells, while the latter are much 
longer and contain straight cross partitions which are much 
thinner than the longitudinal walls. The phloem forms a con- 
tinuous ring around the xylem except at the hilum. The 
xylem vessels, spiral, pitted and reticulated, radiate from the 
hilum and between them are well marked pith rays the cells 
of which are filled with granular protoplasmic contents. 
From the short longitudinal dimensions of the cells of the motor 
organ proper, it might be thought that on account of the ten- 
sions set up, the cells of the vascular bundles would likewise 
be shorter than elsewhere. This supposition is confirmed by 
observation. The pitted and spiral vessels show cross walls 
much more frequently than in the general bundles of the 
plant. 
Secondary organ.—The secondary pulvinus is nearly circu- 
lar in cross section, 0. 7™™ in diameter. The length is 1.4™",1. €., 
twice as great as its diameter. As seen-in the cross abet 
the epidermal cells are of the same size as the hypoderma 
collenchyma cells but their longitudinal dimensions are he 
than the same of the collenchyma. The external epiderma 
Wall is very thin and covered with a thin cuticle which facet 
longitudinal ridges as already described for the sel eae 
organ. The radial diameter of the raeaees ae eh 
al, or if any differ- 
ence is to be detected the longitudinal ts eenthe epidermis to 
ize and 
orm; they are arranged in regu 
Not fit together in the manner 0 
