642 Report of a Discussion on 
validity of Schwendener’s theoretic objections, and must leave this 
point. It is a question of great importance whether it is possible that 
on the breaking of a column of water a film of water remains sur- 
rounding the air bubble, and capable of holding the two columns 
together. If this is impossible, we must suspend our judgment until 
we know more of the contents of the tracheals. 
To sum up this part of the subject, we may believe that the tracheals 
in their youngest condition may contain water in continous columns, 
since the cambium cells from which they arise certainly contain fluid. 
But we know also that this condition is not absolutely maintained, 
since Strasburger has shown that the young wood contains air, though 
in small quantity. We must therefore believe either (1) that the 
transpiration-current is able to travel past the air bubbles, or (2) that 
tracheals partly filled with air may again become continuous waterways 
by solution of the air. If we adopt the first alternative, we must 
believe that the film of water between the bubble and the wall of the 
vessel is able to bear such a tensile stress that it can serve to link 
the column above with the column below the bubble. But this is 
analogous to trusting a rope so nearly cut through that only a few 
threads remain intact. With regard to the second alternative, we 
have at least indications from Strasburger’s work that a tracheal, 
partly filled with air, does not necessarily remain permanently function- 
less (see Leitungsbahnen, p. 692). 
The isolation of the tracheal . — There are a number of points con- 
nected with the structure and properties of wood which ought to be 
considered in relation to the modern theories. Want of space forbids 
my doing more than referring to two of them. 
The resistance which the wetted cell-wall offers to the passage 
of undissolved air is a point on which many writers have laid stress. 
It is clear that on any theory of the movement of water in the tracheals 
it is essential that air should not filter into the waterway. This 
necessity is not, however, stronger in the case of the modern theories 
we are considering. The pressure tending to fill the tracheals with 
air from outside cannot be greater than atmospheric pressure, and 
since the wetted cell-walls of gymnospermous wood can resist the 
passage of air under a pressure of about an atmosphere 1 , we need not 
fear criticism of the theory on this ground. The above remarks 
1 Leitungsbahnen, p. 722. Nageli and Schwendener, Das Mikroskop, 2nd edit,, 
p. 367, give 225 cm, of mercury. 
