ioo Ewart and Rees .— Transpiration and the Ascent of 
which do not represent any such consistency in the plant. The resistance 
to flow in transpiring plants varies in fact along the path of the transpira¬ 
tion current, and all we need to determine is the maximal total resistance 
and the approximate limits of variation. In any case the results suffice to 
show that the presence of air not only lowers the conductivity but causes 
increasing heads to produce less and less of the expected flow. The dis¬ 
crepancy may become remarkable. Thus in the following series of 
experiments with Eucalyptus amygdalina the segments 1-7 were taken 
20 40 60 80 100 
Head in crus of coater. 
Figure 4. Acaci a moll is 51m a. 
from two slender transpiring shoots with short and narrow vessels, whereas 
No. 8 was a large main stem of a tree and was saturated with water. 
Even in this case, in which the resistance to flow is probably nearly 
minimal, it requires a head of twice the length of stem to produce a rate 
of flow of 3 metres an hour, which is a fair average rate for the active 
transpiration current in Eucalyptus , and to produce the maximal rate 
observed of 12 metres per hour would need a correspondingly increased 
head. 
