76 PENHALLOW ON MECHANISM 
depressions noted, and thus occupying the extreme base of the pulvinus, is a large inter- 
cellular space, into which project numerous coarse, straight, sharp-pointed and thick-walled 
intercellular hairs. As seen in transverse section, this space lies within the lower half of 
the pulvinus, while the vascular structure is divided into three bundles, which traverse 
the projecting ridges referred to above, and thus it passes by the intercellular space on its 
lower side, in the extreme peripheral portion of the pulvinus. That portion of the 
pulvinus which lies on the upper side of the intercellular space, is quite uniform both 
externally and internally, and presents only those internal modifications of the normal 
tissues which are essential to its character as a pulvinus. In its structural details, other 
than these, the pulvinus may be characterized as follows :— 
The epidermis is simple, the cells of equal diameter or slightly elongated tangentially ; 
the cuticle is thin. The hypodermal tissue, which constitutes the pulvinus proper, and 
extends, without modification, to the bast zone, is of the same kind throughout, and 
consists of simple, round-celled parenchyma, with moderately thin cell walls. The cells 
show no essential variation in form, though in size they are usually much the largest in 
the central region. Throughout this tissue, continuity of protoplasm may be readily 
determined by the methods already stated. In the centre of the pulvinus and completely 
surrounded by it, is the fibro-vascular structure which forms the base of the petiole. In 
this vascular axis, the various tissues of the stem—pith, wood, cambium and bast— 
may be readily distinguished. .The pith has the outline of an equilateral triangle with 
its base facing the upper side of the pulvinus, thus conforming to the external configura- 
tion of the pulvinus as a whole, as well as to the general outline of the other tissues. 
The cells of the pith are small and usually with medium-thick cell walls, though in some 
cases, especially toward the base of the pulvinus, they become very thick. The wood 
zone is well defined, and completely encloses the pith. The medullary rays are very 
prominent, but the most striking feature is the presence of numerous pitted ducts and 
vessels, which, from their long diameter, conspicuously thick walls and regular radial 
arrangement, at once attract notice. Among them, there appear, in much less conspicuous 
manner, the wood cells, which are small both in length and breadth. Surrounding the 
wood is a somewhat narrow zone, which, in its earlier periods of growth, is meristematic, 
and provides for radial extension of the wood zone. It possesses the usual characteristics 
of such tissue. The bast zone forms a continuous tissue. The cells are of small diameter, 
but very long and fusiform. The walls are of medium thickness and traversed by 
numerous pits, which terminate at the intercellular substance. During the activity of the 
pulvinus these cells are all filled with protoplasm, avd from the facility with which the 
walls swell under the influence of strong sulphuric acid, the tissue presents one of the 
best opportunities for observing the continuity of protoplasm. The bast, as a whole, is 
probably to be regarded as one of the most important mechanical elements present. 
From what has previously been stated with reference to the sectional outline of the 
pulvinus and its included vascular structure, it will be seen that the latter is not concen- 
tric with the former, and that the minor axis, which passes transversely through the true 
structural centre, lies considerably above the centre of the section, and since this is a 
constant feature of the pulvinus, it will be seen that the lower half of the transverse 
section and the lower side of the pulvinus always exceed the upper half or side. The 
relations of parts in these respects were determined by making an outline drawing of all 
