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 off 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 Brücke 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 low^er 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 supplied 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 
