MECHANISM OF THE MOVEMENTS. 795 



when the contractile organ has been irritated and has lost its water, and that in this 

 condition the tension is greater betw^een the parenchyma of the upper side of the fibro- 

 vascular bundle than between the parenchyma of the under side and the bundle. 



If one of the contractile organs of the petiole which has been treated in this way is 

 placed in water, in order to replace the loss of water which has taken place during the 

 operation, and thus to produce a condition similar to the normal one, the downward 

 curvature of the upper half becomes still stronger, but the under side also curves strongly 

 upwards, and its tissue, previously flaccid, becomes very tense and almost cartilaginous, 

 as in the other half. This shows that the turgidity of the parenchyma of the under side 

 had decreased more than that of the upper side from the loss of water resulting from the 

 operation, and that it increases also more rapidly when re-absorbing water. In other 

 words, the irritable under side both gives off and re-absorbs w^ater more readily than the 

 upper side. The upper parenchyma always has a tendency to press the central bundle 

 downwards ; but the lower parenchyma tends to press it upwards only when it contains 

 much water ; when therefore the organ contains but little water, it must be bent down- 

 wards, and can only be bent upw^ards when the quantity of water in it is larger. Con- 

 versely, in the case of a leaf springing from the stem it may be concluded that when 

 the contractile organ is erect it contains abundance of water, when it bends downw^ards 

 in consequence of irritation but little. This conclusion is still further confirmed by the 

 fact that if an incision is made with a very sharp knife in the stem of a very irritable 

 plant at a distance from a leaf and without shaking it, a large drop of water escapes 

 when the knife enters, and the large contractile organ of the next leaf bends downwards 

 or takes the position caused by irritation, evidently in consequence of loss of water; the 

 tension of the sap in the interior of the plant being suddenly diminished and the organ 

 losing water^ This conclusion is also in harmony with the fact already mentioned and 

 first established by Briicke, that the organ which bends downwards in consequence of 

 irritation is more flaccid and flexible than before, the only cause of which, under the 

 circumstances, can be diminished tr.rgidity, or loss of water. This further conclusion 

 may also be drawn from these experiments, — that the loss of water in the irritated organ 

 takes place only or at least chiefly in the parenchyma of the under side ; and this is 

 again in harmony with the fact that an organ from which the upper parenchyma has 

 been removed still remains sensitive, while the removal of the lower parenchyma de- 

 stroys all irritability. Pfeff"er obtained in his recent investigations a clearer insight into 

 these processes : the following is the result communicated to me by letter. 



Pfefter first of all determined, by careful measurements of the same organ in the two 

 conditions, that the mass of the lower parenchyma which contracts from the irritation 

 decreases, while that of the upper parenchyma which lengthens increases ; but the in- 

 crease in mass of the upper half is much less than the decrease in mass of the lower. It 

 follows that the w^hole organ decreases in mass, since irritation causes it to bend up- 

 wards. This decrease in mass of the lower parenchyma is the result of loss of water, as 

 is shown by the following experiment. After cutting through the contractile organ at 

 the base of the petiole where the central fibro-'vascular bundle is still undivided, the 

 organ is at first not sensitive (and bent downwards) ; but if the plant is placed in air 

 saturated with moisture, it again becomes sensitive after a shorter or longer time ; when 

 irritated, w^ater escapes each time very rapidly from the incision in considerable quantity 

 when the tissues of the plant are abundantly supplied with water. This water (as Pfeff'er 

 has shown can be clearly demonstrated by experiment) comes from the parenchyma, 

 and almost entirely from that portion which surrounds the central bundle and contains 

 large intercellular spaces. It is apparent sometimes only beneath and at the sides of 

 the fibro-vascular bundle, sometimes also above it. Sometimes PfefFer observed also 

 the section of the fibro-vascular bundle to exude moisture. When a powerful irritation 



' Fur further observations on the result of this experiment see Sachs, Handbuch der Exp.-Phys. 

 p. 481 et :eq. 



