ON THE ASCENT OF WATER IN TREES. 683 
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 un- 
dissolved 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 consider- 
ing. 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,! we need not fear criticism of the theory on this 
ground. The above remarks seem, however, to be needed in face of the 
frequently recurring statement that wet wood membranes are impermeable 
to free air. Schwendener has some good remarks on this head.* 
Strasburger has called attention to the important subject of the 
localisation or isolation of vessels or of certain lines of tracheids. When 
this is possible we may have one set of tracheals containing continuous 
water columns, while neighbouring ones contain air at negative pressure.* 
This is especially important in connection with the Dixon-Joly-Askenasy 
theory, since, if there were no such isolation, a functioning tracheal con- 
taining a continuous column of water would give up its water to one which 
was not functioning. In other words, the inactive tracheals would, by nega- 
tive pressure, suck water from the active ones. In the coniferous trees the 
young wood is cut off by the absence of pits in the tangential walls * from 
free communication with the older wood, where air is more frequent. 
In the same way the valve-like closure of the pits by the aspiration 
of the pit membrane comes to be a subject of much importance. 
At present I merely wish to show by a couple of examples the necessity 
of a complete study of the minute structure of wood in relation to the 
modern theories. It is at least a hopeful fact for Messrs. Dixon, Joly, and 
Askenasy that we cannot point to anything in the anatomy of wood which 
is absolutely inconsistent with their views. Finally, with regard to the 
question at large, whether we are friends or opponents of Messrs. 
Dixon, Joly, and Askenasy’s theory, the broad facts remain, that water 
has the power of resisting tensile stress, and that this fact must hence- 
forth be a factor in the problem. There are difficulties in the way of our 
authors’ theory, but it is especially deserving of notice that many of these 
_ difficulties are equally serious in the case of any theory which excludes 
the help of the living elements of the wood, and assumes a flow of water 
in the tracheals. The authors have not only suggested a vera causa, but 
have done so without multiplying difficulties. There is therefore a dis- 
tinct balance in their favour. 
Huxley, quoting from Goethe, makes use of the expression thdtge 
Skepsis. 1t is a frame of mind highly appropriate to us in the present 
> juncture if we interpret it to mean a state of doubt whose fruit is activity, 
_ and if we translate activity by experiment. 
_ en 
1 Leitungsbahnen, p. 722. Niigeli and Schwendener, Das Mikroskop, 2nd edit., 
'p. 367, give 225 cm. of mercury. 
” Zur Kritik, p. 9438. 
3 See Histolog. Beitrige, v. p. 87. 
4 Strasburger discusses in this connection the existence of tangential pits in the 
autumnal wood (see Leitungsbahnen, p. 713). 
