890 PERIODIC MOVEMENTS AND THOSE DUE TO IRRITATION, 



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. Again, if the upper and lower parenchyma are 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 

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

 condition the tension is greater between 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 oft' and re-absorbs water 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 upwards when the quantity of water in it is larger. It 

 has already been mentioned that Briicke first discovered the important fact that the 

 organ becomes flaccid on stimulation ; he pointed out that the angle which a petiole 

 makes with the stem varies much more, when the position of the stem with reference 

 to the horizon is altered, if its contractile organ has been stimulated than if the organ 

 is still irritable and at rest. I drew attention to the fact which had been previously 

 established by Dutrochet and by Meyen, that the leaves will move and that their 

 contractile organs will become flaccid if an incision is made so deeply into the stem 

 that the wood is penetrated, provided that an escape of water is produced by the 

 wound. This and other facts seemed to indicate that the movement produced by 

 stimulation is associated with an escape of water from the irritable organ. Pfeffer, 

 however, first brought forward a complete proof that this was the case, and obtained 

 a clearer insight into the processes attendant upon stimulation. 



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

 conditions, that the volume of the lower parenchyma which contracts in consequence of 

 stimulation decreases, while that of the upper parenchyma which lengthens increases ; 

 but the increase of the upper half is much less than the decrease of the lower. It 

 follows that the whole organ decreases in volume when irritation causes it to bend 

 downwards. This decrease 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, water escapes each time very rapidly from the incision in considerable quantity 

 when the tissues of the plant are abundantly supphed with water. This water (Pfeffer 

 has shown that this can be clearly demonstrated by experiment) comes from the paren- 

 chyma, 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 

 stimulation is applied to the under side of an organ from the upper side of which the 



