MECHANISM OF THE MOVEMENTS, 
889 
another from the fibro-vascular bundle to the middle layers of tissue ; the very small ones 
of the outer layers have the appearance of isolated triangular cavities, and when 
cut (therefore in the irritated state) seem to be full of water. The cells of the 
under side of the 'pulvinus' are thin-walled, those of the upper side have much thicker 
walls (about three times as thick) of pure cellulose. Together with a moderate quantity 
of protoplasm (including a nucleus) and small chlorophyll-granules and starch, each of 
the cells contains in its cavity a large globular drop, consisting, according to Pfeffer, 
of a concentrated solution of tannin surrounded by a delicate pellicle^. The young organs 
however manifest sensitiveness when the cell-walls of the upper side are not thicker than 
those of the other side and the globular drop has not yet made its appearance. 
A slight concussion of the whole plant causes the contractile organs of all the 
primary petioles to curve downwards, those of the petiolules forwards, those of the 
leaflets forwards and upwards. The primary petioles which were previously turned 
obliquely upwards then become horizontal or turn obliquely downwards, while the petio- 
lules and leaflets close. This condition is identical externally with the nocturnal position 
of the leaves, but differs internally, inasmuch as a concussion still acts as a stimulus 
in the latter case and causes especially a stronger depression of the primary petiole. 
The irritated contractile organ is also flaccid, as Brücke has shown, and is more flexible 
than before the irritation, the weight being the same ; in the nocturnal position, on 
the contrary, the organ is more rigid and less flexible than in the diurnal. In the 
contractile organs of the primary petioles and petiolules a light touch on the hairs 
on the under side is sufficient to produce the movement, and in those of the leaflets 
the lightest touch on the glabrous upper side. When the temperature is high and 
the air very damp, the irritability is much greater, and any local irritation incites 
movements in the neighbouring organs, often in all the leaves of a plant, a phenomenon 
which has been termed Conduction of the irritation. If, for example, one of the 
anterior leaflets is cut off by a pair of scissors, or its contractile organ is touched, or 
if it is placed in the focus of a burning-glass, it takes the position which is normally 
the result of irritation, the next lower pair of leaflets then follows, and in succession 
those at a greater distance; after a short time the leaflets of an adjoining petiolule 
begin to fold together from above upwards, and the same with the other petiolules. 
Finally, often after a considerable time, the primary petiole bends downwards; the 
phenomenon is then transferred to the primary petiole of the next leaf below, and 
also probably to that of the next one above; their petiolules and leaflets taking also 
similar positions. Thus in the course of a few minutes all the leaves are set in motion ; 
sometimes particular parts are passed by which only begin to move subsequently. The 
conduction of the irritation appears to take place more easily from above downwards 
than from below upwards, both in the leaves and the stem. If the plant is left to 
itself the leaflets and petiolules again expand after a few minutes, the primary petioles 
become erect, and the leaves are again irritable. 
If the parenchyma of the upper side is cut away as far as the central fibro-vascular 
bundle from the large contractile organ of a primary petiole, the petiole afterwards 
again becomes erect, and more so than would otherwise be the case; and the organ thus 
treated retains a smaller degree of irritability. If, on the other hand, the parenchyma 
is removed from the under side, the petiole turns sharply downwards, and its irritability 
is destroyed. It follows from this that it is only the under side that is irritable; the 
parenchyma of the upper side takes only a subordinate part in the movement, as will be 
shown more clearly presently. 
If one of the large contractile organs is cut away close to the stem, it curves down- 
wards, and a drop of water escapes from it. If it is now split lengthwise through the 
central fibro-vascular bundle into an upper and lower half, the former bends still more 
^ Similar globular drops are stated by Unger to occur also in Glycyrrhiza and Desmodium 
gyrans. 
