ON THE ASCENT OF WATER IN TREES. 681 
leaves for high trees is wanted ; also investigations of the variation in 
osmotic power produced by varying resistances in the flow of the current. 
The experiments of Pfeffer and others | show that the osmotic strength of 
cell-sap is capable of great adaptation to circumstances—cells respond 
by increased turgescence to various stimuli, Whether they can respond 
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 encourage 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 alburnwm need not be 
simultaneously occupied by a transpiration current ; parts of it may be so 
eccupied, while parts of it are filled with air, and do not function as water 
ways. This is a valuable contribution 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 
everyone ; for instance, Schwendener refuses to take them seriously.” 
Strasburger has microscopically examined the condition of the tracheals 
as regards air.? He found in the spruce fir in July ‘almost no air bubbles’ 
in the wood of the current year, but air in considerable quantity in four- 
year-old wood. In the same month Pinus Salzmanni (Laricio) showed 
scattered bubbles in the spring wood of last year, and more in the autumn 
wood. In a larch there were only very occasional bubbles in the two 
last years’ wood. In the silver fir the current year’s wood was practically 
free from air: the air increased in the inner rings. TZsuga canadensis 
had no air in this year’s wood, only a little in last year’s, and an in- 
creasing quantity in the older rings, the fifth being very rich in air. In 
February Pinws strobus had hardly any air in this year’s wood, and the 
silver fir was all but free from it in the youngest ring. Robinia in July 
had the youngest wood almost air-free. Ficus elastica and spuria, various 
Acacias, and willows gave vessels not entirely free from air, but nearly so. 
1 Pfeffer, Ahhand. der hk. Sachs. Ges. xx. p. 300; Eschenhagen, Untersuchungen 
aus d. Bot. Inst. z. Tubingen, 1889; Stange, Bot. Zeit., 1892. 
2 K. Preuss. Akad., 1892, p. 931. 
3 Leitungsbahnen, p. 683 et seg.; Russow in 1882 (Bot. Centr., Bd. xiii. 1883) 
observed similar facts in the distribution of water and air. 
