Oe EN ae ee ee ee a ee 
1905] DARBISHIRE—APPARATUS FOR TRANSPIRATION STREAM 363 
we get readings which at least show that the pressure may vary very 
much in even two or three inches of stem, is a proof, I think, that the 
“atmospheric pressure” theory of the rise of water is not correct. 
When we get away from the root we find the phenomenon of shoot 
suction manifesting itself. In fact, as already pointed out, shoot 
suction generally seems to be stronger than the pressure from the 
root at any point except just above the root. That is to say, the 
shoot is able at any time to take in more water than can be supplied 
by the root. Numerous experiments show that this is the case in 
winter and summer, day and night, in the plants I experimented upon. 
Which part of the plant is exerting this suction? I have spoken 
here of shoot suction, but it is possible to split up this part of the 
transpiration stream into two distinct processes, namely the leaf 
suction and the wood lift. There is not much mystery about the 
ormer. We can understand how the water is removed from the 
finest endings of the vascular system. If, however, we remove the 
leaves from a shoot which has been attached to a pinometer or even 
cut off the upper part bearing leaves, leaving only a short leafless 
shoot stump, we still get water rising in the wood and exerting a pull 
on the gauge of the pinometer. The activity of the leaves simply 
removes the-water from the top of the vascular system, and _ this 
water is replaced by a process going on in the wood. 
I am mentioning this simply to show that in the experiments with 
the pinometer the taking in of water by the shoot is not a phenomenon 
of leaf suction but one of wood lift. The force of the wood lift is very 
great; it is generally greater than the root push. But, as our experi- 
ments show, it cannot act efficiently for any length of time if the 
lowest end of the vascular system is exposed toa pull from below. It 
is as yet not known how the wood lift acts. It is therefore all the more 
necessary to make as many observations as possible on this process. 
I think the pinometer does make it possible to observe at least one 
property of this water current. It is continually in a state of what is 
known as ‘negative pressure.” A natural result of this is that the 
air, which the water absorbed by the plant contains under ordinary 
atmospheric pressure conditions, escapes from the water when inside 
the plant. Over and over again my experiments have been brought - 
to an end by air collecting, generally under the shoot. Air-bubbles 
