643 
the Ascent of Water in Trees. 
seem, however, to be needed in face of the frequently recurring 
statement that wetwood membranes are impermeable to free air. 
Schwendener has some good remarks on this head h 
Strasburger has called attention to the important subject of the 
localization 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 2 . This is especially important in connexion with 
the theory of Dixon and Joly and of Askenasy, since, if there were no 
such isolation, a functioning tracheal containing a continuous column 
of water would give up its water to one which was not functioning. 
In other words, the inactive tracheals would, by negative pressure, 
suck water from the active ones. In the coniferous trees the young 
wood is cut olf by the absence of pits in the tangential walls 3 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 
the authors of these views 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 the new theory, the broad facts remain that water has 
the power of resisting tensile stress, and that this fact must henceforth 
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 distinct balance in their favour. 
Huxley, quoting from Goethe, makes use of the expression thatige 
Skepsis . It 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. 
1 Zur Kritik, p. 943. 2 See Histolog. Beitrage, v. p. 87. 
3 Strasburger discusses, in this connexion, the existence of tangential pits in the 
autumnal wood : see Leitungsbahnen, p. 713. 
