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Notice of Book . 
cell- walls should be in a state of imbibition, (2) that the cavities of the 
tracheae should be to a certain extent filled with water, and (3) that 
they should be isolated, so as to exclude the entrance of air. Atmo- 
spheric pressure helps to keep the water suspended, but does not 
cause its ascent. Transpiration is only important in so far as it 
makes room for the ascending water. Should the supply of water be 
deficient, certain of the tracheal channels are emptied and closed. 
In such closed tracheae a very low pressure prevails, which is main- 
tained until they can be refilled with water. The difference between 
the atmospheric pressure and that in the tracheae, great as it is, is not 
usually sufficient to force air into the emptied elements. Root-pressure 
is not immediately concerned in the ascent of water. 
As the insufficiency of capillarity has long been established, we are 
led, as the net result of the most elaborate investigation of the question 
which has yet been made, to a purely negative result; the cause of 
the ascent of water in trees is still unknown. A great point is how- 
ever gained, if we may take it as proved that the process is a 
purely physical one. The protoplasm, which is responsible for so 
much, has at least been relieved of one strictly mechanical function, 
and the problem of the sap is thus brought within a measurable distance 
of solution. The author’s work does indeed afford some indications 
(e. g. the movement of the film of water between the air-bubbles and 
the walls of the tracheae) which, if taken up by physicists, may, 
he hopes, lead to the long-sought explanation. 
The details of the experiments must be read in the original. A 
summary of them would far exceed the limits of this review. In 
every case the minute acquaintance which the author has gained with 
the anatomy of the plants investigated has proved to be of the greatest 
value, and gives a special character to his investigations. 
The following passage, bearing on some of the most crucial ex- 
periments, may be quoted (p. 623): — ‘The fact that trees up to 
20 metres in height, can, without the assistance of root-pressure, take 
up for weeks together a substance so poisonous as a 5-10 % solution 
of copper sulphate, and conduct it through their stems, which were of 
necessity killed within the first days of the experiment, shows clearly 
that the living elements of the wood can have nothing to do with the 
raising of water, and that this process is a purely physical one. On 
the other hand it may be assumed, on physical grounds, that atmo- 
spheric pressure and capillarity, even if taken together, are insufficient 
