Water in Trees under Australian Conditions. 
IOI 
Eucalyptus amygdalina. 
Length of stem 50 cm. Temperature 20°. 
Heads in cm. of water. 
1. 12.5 cm. 
2. 25 cm. 
3- 37-5 cm. 
4. 50 cm. 
5* 63 cm. 
6. 75 cm. 
7. 100 cm. 
8. 100 cm. 
Rate of flow per hour. 
13 6 cm. 
15-8 cm. 
18-6 cm. 
20-2 cm. 
25.4 cm. 
32-2 cm. 
41*8 cm. 
324 cm. (branch of tree saturated with 
water). 
With exceptionally favourable material of Acacia mollissima , Miss Rees 
obtained a maximal rate of flow of 1,080 cm. per hour for a 50 cm. length 
of stem under a head of 50 cm. of water at 25 0 C. Assuming that this was 
Fig 5 Eucalyptus amygdalina 
through an open vessel of 001 cm. diameter, the rate of flow would, 
according to a theoretical calculation from Poisseuille’s formula, have been 
thirty times greater, and in a vessel of 0*05 cm. diameter seven times 
greater. Similarly, with a head of 100 cm., the rate of flow was only 
1,560 cm. per hour, or less than one-thirtieth the theoretical value. The 
difference is due to the facts that the vessels are not perfectly circular, that 
their walls are not smooth, that they are longer than the stem bearing 
them, and that at every sharp bend with a high rate of flow, eddy currents 
will form which increase the resistance to flow, and finally that whenever 
one vessel is conducting under pressure more rapidly than neighbouring 
ones, lateral exudation through the permeable walls will tend to level up, 
or rather level down, the rate of flow more or less to the average one for 
