794 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION. 



of a concentrated solution of tannin surrounded by a delicate pellicle ^ The yomig organs 

 however manifest sensitiveness when the cell-w^alls of the upper side are not thicker than 

 those of the other side and the globular drop has not yet made its appearance. 



A somewhat 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, a concussion still acting as an irritation in this position 

 and causing especially a stronger depression of the primary petiole. The irritated con- 

 tractile organ is also flaccid, as Briicke 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 leaf- 

 lets 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 downw^ards ; 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 of the leaf is cut away as far as the central fibro- 

 vascular bundle from the large contractile organ of a primary petiole, the petiole after- 

 wards again becomes erect, and more so than would otherwise be the case ; and the part 

 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 

 show-n 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 m.ore 

 strongly downwards, while the lower bends only slightly or becomes nearly straight. 

 These curvatures are still more clearly seen if the two halves are again divided length- 

 wise by a cut at right angles to the previous one ; the four pieces then also manifest a 

 small lateral curvature inwards. If the upper and lower parenchyma are again separated 

 from the central fibro-vascular bundle by two cuts lengthwise, the former bends strongly 

 downwards, the latter slightly upwards ; they also increase so much in length as con- 

 siderably to exceed the central bundle. These and other experiments show that a con- 

 siderable tension of the parenchyma exists with reference to the central bundle even 



^ Similar globular drops are stated by linger to occur also in Glycynhi/a and De^moditan 

 gyrans. 



