380 
Fig. 2. Trunks of small-crowned trees in closed 
stands, normally free of persistent adventitious roots. 
This stand is in Glenwood, Hawaii. 
crowned, fine-limbed trees typical of dense 
forests usually have trunks free of all but the 
finest roots (Fig. 2). 
Persistent adventitious roots generally emerge 
from the upper trunk, crotches of large limbs, 
and the underside of large limbs. No large roots 
have been found emerging from the upper sur- 
face of a limb, or from a point low on the un- 
branched portion of a trunk. 
DISCUSSION 
Adventitious rooting is controlled by mois- 
ture level of the bark. E. robusta bark is fibrous, 
absorbent, and more than 7 inches thick on 
some large trees. It is an efficient reservoir for 
PACIFIC SCIENCE, Vol. XX, July 1966 
water. In some areas it is almost continuously 
saturated. Both rainfall and fog drip are effec- 
tive in maintaining high bark moisture content. 
Some parts of a tree receive more water, in 
the form of stemflow, than others. Stemflow is 
concentrated in large crotches and on the main 
stem. Because E. robusta bark is absorbent, 
water drains to the lower surface of a near- 
horizontal limb as well as down the limb toward 
its junction with the trunk. The bark on the 
lower side of the limb may be a more effective 
reservoir, being about twice as thick as the 
upper-side bark (e.g., 1/2 vs. 1/4 inch) and 
less subject to drying during periods of clear 
weather. Penfound and Mackaness (1940:168) 
also found distinct "ffowways” running down 
the spreading limbs of live oaks and down the 
trunks. Most of the live oak stemflow coursed 
along the upper surface of the limbs, probably 
because the bark was not highly absorbent. 
By examining trees after showers, an observer 
can discriminate wet and dry zones of the bark 
by their appearance or by feel. He can see that 
trees with high small crowns often conduct only 
enough stemflow to saturate the limbs. But 
spreading-crowned trees with massive branch 
systems channel enough stemflow down the 
trunk to saturate the bark for a considerable 
distance below the crown. Thus, the location of 
wet bark is partly governed by crown size and 
configuration. Inasmuch as these attributes are 
strongly influenced by spacing, adventitious 
roots are unlikely to become prominent on trees 
in closed stands. 
A somewhat analogous situation is the layer- 
ing of woody plants into pads of moist humus 
accumulated by epiphytes (Herbert, 1958:23). 
But in E. robusta the bark itself is the medium 
that supplies moisture to the adventitious roots. 
E. robust a\d.s been planted in other tropical 
countries (Penfold and Willis, 1961), and ad- 
ventitious rooting will probably be encountered 
wherever moisture conditions are satisfactory. 
M. R. Jacobs, in a letter to the author in 1963, 
said that he observed such rooting in Uganda 
and Argentina. There is also a recent report 
of adventitious roots of E. robusta and E. 
camaldulensis Dehnh. in a greenhouse in Russia, 
where relative humidity was maintained at 80- 
90% (Gerasimov, 1962:1531). In Australia, 
E. camaldulensis puts forth adventitious roots 
