23S Aspects of Ecology. 
for measuring separately, water content, humidity of air, and light 
and give some critical comments on each. 
For measuring water-content no continuous recording method 
is satisfactory and the procedure is to take, at proper times, samples 
of soil with a borer and determine the water content directly. 
Dr. Clements lays great and proper emphasis on the fact that all 
the water in a soil is not available for the plant. Wilting and death 
by dessication take place before all the water present is withdrawn 
by the roots from a drying soil. The whole water-content of a soil 
(holard) is divisible then into the physiological water (chresard)and 
the non-available residue (echard). The chresard alone concerns 
the plant and as the echard varies greatly in different types of soil 
from in gravel to 12% in clay it follows that knowledge of the 
holard alone is insufficient. The echard of a given habitat is 
experimentally ascertained by isolating a blockof soil by impermeable 
plates, allowing it to dry slowly and determining its water-content 
at the time when the plants growing on it are wilting irrecoverably; 
a point of time which in practice it is, however, not easy to be 
certain about. 
While the chresard of the soil affects the roots and absorption 
by them, the humidity of the air (Dr. Clements’ second factor) 
directly affects transpiration. The more water-vapour there is in 
the air, the more of these molecules impinge upon the evaporating 
surfaces of the plant and are reabsorbed by them, so diminishing 
the net loss of water. 
This factor is to be measured by direct observations with a 
“wet and dry bulb psychrometer,” or by a recording hair-pyschro- 
meter. Dr. Clements discredits the stationary instrument and 
recommends the use of the “sling” psychrometer or the“cog- 
pyschrometer,” an ingenious portable instrument of his own 
invention, on the same principle, made out of two thermometers 
and an egg-whisk. In both these the thermometers are kept in 
rapid (it should also be standard) movement through the air. By 
this procedure a uniform rate of movement of air over the wet bulb 
is substituted for the variability of natural wind. Now the faster the 
wind, the quicker the water-vapour is swept away from the 
evaporating surface, and the fewer molecules pass back again, so 
that the water-loss is increased, even though the humidity of the 
distant air in general remains unchanged. In still air the evapo¬ 
ration is much diminished and the thermometer readings indicate a 
humidity number which is too high. From a meteorological point 
