470 
Notes. 
The energy relations of the leaf with its surroundings, on the 
assumption that evaporation at capillary water-surfaces is mainly 
responsible for the elevation of sap, may be illustrated by the well- 
known power of the water-filled porous pot to draw up mercury in 
a tube to which it is sealed. The authors describe an engine in which 
the energy entering in the form of heat at the capillary surfaces may 
be in part utilized to do mechanical work : a battery of twelve small 
porous pots, freely exposed to the air, keeping up the continuous 
rotation of a fly-wheel. Replacing the porous pots by a transpiring 
branch, this too maintains the wheel in rotation. This is, in fact, 
a vegetable engine. In short, the transpiration effects going on at the 
leaf are, in so far as they are the result of spontaneous evaporation 
and uninfluenced by other physiological phenomena, of the ‘ sorting 
demon’ class, in which the evaporating surface plays the part of a sink 
of thermal energy. 
If the tensile stress in the sap is transmitted to the root, the authors 
suggest that this will establish in the capillaries of the root-surface 
meniscuses competent to condense water rapidly from the surrounding 
soil. They show by experiment the power possessed even by a root 
injured by lifting from the soil, of condensing water vapour from 
a damp atmosphere. Such a state of things may be illustrated by 
a system (which the authors realised) consisting of two porous pots 
connected by a tube and all filled with water ; one, the ‘ leaf/ exposed 
to the air gives out vapour, the other, the ‘ root/ buried in damp earth 
supplies the demand of the 4 leaf/ and an upward current in the con- 
necting tube is established. 
