6 
Transactions of the Royal Society of South Africa. 
capable of transmitting a limited supply of water, the leaves are fitted just 
to utilise this amount with very little margin ; in other words, though the 
plant requires little, it can hardly do with less." 
In May of 1919 the present writer began, at the suggestion of 
Prof. D. Thoday, a series of measurements of the specific conductivity of 
the wood of certain South African trees with the object of ascertaining 
whether this property of the wood played any part in deciding the usual 
habitat of the various trees. It was soon realised, however, that this factor 
could not be considered independently, but that the transpiring power of the 
leaves must also be taken into account. For instance, a tree with wood of 
high specific conductivity and mesophytic leaves, which use up nearly all 
the water they can obtain from the stem, will probably be quite as much 
restricted as to its habitat as a tree with wood of low conductivity and 
xerophytic leaves. Accordingly it was decided to compare the transpiration 
rates and the water-conducting power of the wood of two trees, and see 
whether any indication could thus be obtained of the drought-resisting 
power of the trees. 
For the preliminary work the two trees selected were Pinus pinaster 
and Leucadendron argenteum, both of which grow within practicable range 
of the University of Cape Town. The leaves of each of these evergreen 
trees are distinctly xerophytic. That of the Pine is so well known as to 
need no description. The leaves of the Silver Tree are sessile, 2 in. long, 
tr-lj in. broad, and lanceolate. They are densely pubescent on both 
surfaces with adpressed, silvery, silky hairs. According to Marloth (2) 
these hairs are closely pressed to the surface of the leaf in dry weather, 
and stand out at an angle of about 30° from the surface during damp 
weather. With the naked eye, however, the present writer could observe no 
difference in the position of the hairs in dry and damp weather, though no 
critical experiments were conducted to test the statement. As in the case 
of other Proteaceae the stomata of Leucadendron argenteum are protected 
by a small raised pore. 
II. Methods. 
a. Measurement of conductivity. — Lengths of 15 cm. were cut under 
water from the centre of the twigs chosen for the experiments, and placed 
in jars of water, over which the pressure of air was reduced by means of a 
suction pump. Half an hour to an hour was deemed sufficient to remove 
all the air from the stem, and to inject it with water. 
To obtain the necessary pressure for the actual conductivity measure- 
ments an aspirator was placed on top of a cupboard and filled with distilled 
water. In order that the pressure might remain constant throughout the 
series of experiments, a glass tube was inserted through a cork in the top 
