6 3 8 
Report of a Discussion on 
not only the lifting of a given column, but the overcoming of the 
resistance to its flow. 
The resistance to the flow of the transpiration-current is in want of 
further investigation. Janse 1 has discussed the question, and points 
out (loc. cit. p. 36) that two kinds of resistance must be reckoned with. 
The first (which he calls statical) is illustrated by means of a cylinder 
of Pinns wood fixed to the short arm of a J-tube filled with water, 
when it was found that in five days the level of water in the long arm 
was only one mm. above that in the short arm 2 . That is to say, 
when time enough is given, the resistance is practically nothing. Janse 
has also investigated the resistance to the passage of water flowing 
through wood at the rate of an ordinary transpiration-current. His 
method seems to me open to criticism, but this is not the place to 
give my reasons. His experiments give a wide range of results. 
With Pinus Strobus a pressure of water equal to ten times the length 
of the wood was required to force water through at a pace equal to 
the transpiration-current. In Ginkgo the pressure was twenty-one 
times the length of the wood. Strasburger 3 has repeated Janse’s 
experiment, and finds a column ‘ several times the length of the 
object’ necessary. Nageli 4 found that 760 mm. of mercury were 
needed to force water through fresh coniferous wood at the rate of 
mm. per second, i.e. at 180 mm. per hour. If we allow one metre 
per hour as a fair transpiration rate 5 , we get a pressure of 5 atmo- 
spheres required to produce such a flow. To return to Janse’s 
experiments : even if we assume that the resistance (expressed in 
water) = 5 times the length, it is clear that with a tree 40 m. in height, 
the resistance of 20 atmospheres has to be overcome. This would 
not be a pressure greater than that which osmotic forces are able to 
exert, but when we come to a tree of 80 m. in height, and a resistance 
of 40 atmospheres, the thing becomes serious 6 . A great difficulty in 
the question of resistance is that the results hitherto obtained are 
(though here I speak doubtfully) much greater than those obtained by 
physicists for the resistance of water flowing in glass capillaries. 
1 Pringsheim’s Jahrb., xviii. 1887, p. 1. 
2 Strasburger (Leitungsbabnen, p. 777) observed equilibrium established a good 
deal quicker. 
3 Leitungsbahnen, p. 779. 4 Das Mikroskop, 2nd edit., p. 385. 
5 Sachs, Arbeiten, ii. p. 182. 
* Schwendener’s experiments, K. Preuss. Akad. 1886, p. 579, do not particularly 
bear on this question. 
