Water in Trees tinder Australian Conditions. 105 
may lose 396 grams of water per sq. metre of transpiring leaf-surface per 
hour, whereas the maximum rate for Dracaena Draco was 17*6 grams. 
Cut trees always absorb water at a less rate than rooted ones 
evaporate it. The maximum rate of ascent of sap noted was 12*3 metres 
per hour ( Eucalyptus viminalis) and 6*5 metres per hour (E. amygdalina ), 
whereas in cut branches of Eucalyptus and in cut trees of Acacia mollissima 
it rarely exceeds 1 to 2 metres an hour, and is often less than 1 metre. 
Single vessels may run nearly from end to end of the main trunk in 
young Eucalyptus and Acacia trees several metres high, but only a very 
small fraction exceed half the main trunk in length. In the branches the 
vessels are shorter and narrower, but the sap will usually not pass any 
more transverse partitions in the wood-vessels than it does in the process 
of passing from root-hair to wood, and from the wood to the transpiring 
surface. The existence of a rapid transpiration current appears to favour 
the development of broad vessels, but not to affect their length. 
Branches containing air taken from actively transpiring trees show 
a much greater resistance to flow than when saturated with water; and 
with increasing heads the rate of flow does not increase proportionately. 
To produce the transpiration rate of flow heads of two to ten times the 
length of stem may be required, but in unblocked fully saturated stems 
with large long vessels a head of one-fifth the length of stem may be 
sufficient. During prolonged active transpiration, however, the total 
resistance to the upward flow of sap in a Eucalyptus tree may amount to 
a head of two to ten times the height of the tree, which would therefore 
equal in the tallest trees a maximal pressure lying between 20 and 100 
atmospheres. 
A coloured liquid will rise slowly in a saturated stem kept in 
a saturated atmosphere, but a somewhat slower ascent is shown after the 
stem has been killed, so that the phenomenon is not the result of any vital 
pumping action, and must be capable of a physical explanation, although 
in a saturated stem it cannot be due to capillarity or imbibition, and is too 
rapid to be the result of diffusion. 
No appreciable rise of sap took place in a tree deprived of its leaves, 
but a pumping action may only be excited when the leaves are exerting 
suction on the water in the wood. 
