656 Report of a Discussion on 
The experiments are still in progress; but we have sufficient in 
these results to indicate an enormous potential osmotic tractive force 
in the case of tall trees such as Tilia. In this case the limit of this 
force was not reached at 20 atmospheres. 
Following Mr. Darwin in assuming that the resistance to motion 
at the transpiration velocity is given by a hydrostatic pressure equi- 
valent to five times the length of the branch, this minor limit of 
osmotic pull suffices for a lime tree 100 feet high. The limit 
of height recorded for Tilia americana is sixty feet. In the case 
of the Laburnum, this tree might, so far as Dixon’s results indicate, 
flourish to a height of 130 feet. Its recorded limits are between thirty 
and forty feet. Similarly the Sunflower might attain a height of 
eighty feet or thereabout. It appears therefore as if it was not osmotic 
conditions which limited the heights of trees; although the limit, in 
the case of the lower growing plant, is the lowest observed. 
I may mention here that, so far, conditions of sunshine or shade 
have not been observed to notably alter the results. If not freely 
supplied with water, however, the branches might indicate differently. 
Dixon hopes yet to make an observation upon the branch in situ. 
The following table shows the weights of water transpired by 
similar branches exposed to like conditions, as far as possible, save 
that one was immersed in carbon dioxide and the other surrounded 
with air. The pressure on each was the normal atmospheric 
pressure : — 
Helianthus multiflorus. 
Normal pressure. 
Surrounding 
medium. 
No. of leaves. 
Time. 
Weight 
transpired. 
A. Air. 
10 
90 mins. 
0*302 
a. Co 2 . 
10 
90 mins. 
0*257 
B. Air. 
10 
60 mins. 
0*372 
b. Co 2 . 
IO 
60 mins. 
0-108 
B. Air. 
10 
180 mins. 
0*598 
b. Co 2 . 
10 
180 mins. 
0*236 
B. Air. 
10 
24 hrs. 
2*862 
b. Co 2 . 
10 
24 hrs. 
1 *979 
C. Air. 
12 
24 hrs. 
5*230 
c. Co 2 . 
12 
24 hrs. 
2*240 
