682 REPORT—1896. 
He concludes! that the path of the transpiration current is not absolutely 
free from air. The younger wood, which especially functions as the water- 
carrier, is the most free. 
Dixon and Joly quote Strasburger’s results, which they consider 
sufficiently favourable to their views. They rely, in addition, on the 
impermeability of wet cell-walls to air isolating the conduits in which 
air has appeared ; and on the possibility that the air may be redissolved 
under root pressure,” an idea well worth testing. 
I think Strasburger’s facts are not so favourable to their theory as 
these authors believe ; in the same way it seems to me that Askenasy is 
rash in saying? that the tracheals in many cases contain continuous 
columns of water. It is true that this statement does not affect the 
validity of his general argument, since he faces the undoubted occurrence 
of air bubbles in many cases. This is undoubtedly necessary, and fortu- 
nately we can once more turn to the Leitungsbahnen. Strasburger states 
that he has seen water creep past the air bubbles‘ in coniferous tracheids. 
The best evidence for this seems to be the fact mentioned * that the part 
of a single tracheid in front of an air bubble gets red with absorbed 
eosin, though the neighbouring tracheids are colourless. This clearly 
suggests the creeping round the bubble which Strasburger believes in. 
Schwendener® has been unable to confirm Strasburger’s microscopic obser- 
vations, and, moreover, denies the physical possibility of the phenomena. 
I am unable to judge of the 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 surrounding 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 continuous 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 functionless (see Leitungsbahnen, p. 692). 
The isolation of the tracheals.—There are a number of points connected 
with the structure and properties of wood which ought to be considered 
' Loc. cit. p. 688. 
2 Phil. Trans. p. 572. 
% Verhand. naturhist. med. Vereins Heidelberg, 1895, p. 15. 
* Leitungsbahnen, pp. 704, 709. See also Hist. Beitr. v. p. 76. 
5 Ibid. p. 79. 
5 Zur Kritih, &e., p. 921. 
