640 Report of a Disctission on 
increased turgescence to various stimuli* Whether they can re- 
spond sufficiently to account for the ascent of water is another 
question. 
My own opinion is that the question of resistance to the flow of 
water is a difficulty which the authors of the modern theory have not 
sufficiently met. Unless it can be shown that the resistance to the flow 
of water in wood is less than that indicated by existing researches, 
we must face the fact that we do not at present know of osmotic 
forces which we can suppose capable of raising water to a greater 
height than 40 metres. 
Continuity of the water in the tracheals . — The theory we are 
considering apparently requires that there shall be continuous columns 
of water from leaf to root, because a break in the column means 
a collapse of the machinery* This seems at first sight a fair 
assumption, though I doubt its complete correctness. It is in 
any case worthy of discussion. It has been constantly insisted on 
by Sachs and others that at the time of most active transpiration 
the vessels contain air, and not water. It is therefore a violent 
disturbance of our current views to believe in continuous columns of 
water. 
For evidence on this point we are chiefly indebted to Strasburger, 
It is a remarkable fact that he should, without any theory to en- 
courage such a view, have come to the conclusion that approximate 
continuity of water columns is a condition of primary importance, 
and that he should have made out the cognate fact that the whole of 
the aihurnu?n need not be simultaneously occupied by a transpiration- 
current ; parts of it may be so occupied, while parts of it are filled 
with air, and do not function as waterways. This is a valuable con- 
tribution to knowledge, and to the adherents of the new theory it is 
priceless; the very existence of their hypothesis may depend on it. 
Strasburger’s statements and reasoning are by no means accepted 
by every one ; for instance, Schwendener refuses to take them 
seriously \ 
Strasburger has microscopically examined the condition of the 
tracheals as regards air 2 . He found in the Spruce Fir in July £ almost 
no air bubbles * in the wood of the current year, but air in considerable 
1 K. Preuss. Akad., 1891, p. 931. 
2 Leitungsbahnen, p. 683 et seq. ; Russow in 1882 (Bot. Centr., Vol. xiii. 1883) 
observed similar facts in the distribution of water and air. 
